Fluoride sintered body for neutron moderator and method for producing the same

Abstract

A fluoride sintered body suitable for a moderator which moderates high-energy neutrons so as to generate neutrons for medical care with which an affected part of the deep part of the body is irradiated to make a tumor extinct comprises MgF.sub.2 of a compact polycrystalline structure having a bulk density of 2.90 g/cm.sup.3 or more and as regards mechanical strengths, a bending strength of 10 MPa or more and a Vickers hardness of 71 or more.

Claims

1. A neutron moderator comprising a fluoride sintered body, wherein the fluoride sintered body has a thickness of 60 mm or more and 64 mm or less and comprises MgF.sub.2 of a compact polycrystalline structure having a bulk density of 2.90 g/cm.sup.3 or more and 3.07 g/cm.sup.3 or less and a bending strength of 10 MPa or more and 25 MPa or less as regards mechanical strengths.

2. The neutron moderator according to claim 1, wherein the sintered body has a Vickers hardness of 71 or more and 120 or less as regards mechanical strengths.

3. The neutron moderator according to claim 1, wherein the sintered body has volume of 2,183 cm.sup.3 or more and 2,421 cm.sup.3 or less.

4. A neutron moderator comprising a fluoride sintered body, wherein the fluoride sintered body has a thickness of 60 mm or more and 64 mm or less and comprises MgF.sub.2 of a compact polycrystalline structure having a bulk density of 2.90 g/cm.sup.3 or more and 3.07 g/cm.sup.3 or less and a bending strength of 10 MPa or more and 25 MPa or less as regards mechanical strengths, wherein the sintered body is produced by a process comprising the steps of: pulverizing a high-purity MgF.sub.2 raw material and mixing it with 0.03-0.5% by weight of a sintering aid: molding the resulting mixture obtained after the mixing step at a molding pressure of 5 MPa or more using a uniaxial press device and subsequently at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device to obtain a molded article; conducting preliminary sintering of the molded article in the temperature range of 550 C.-600 C. for 4-10 hours in an air atmosphere; heating in the temperature range of 750 C.-840 C. for 5-12 hours in an inert gas atmosphere; and conducting main sintering by heating in the temperature range of 900 C.-1100 C. for 0.5-3 hours in the same atmosphere as the preceding step so as to form MgF.sub.2 sintered body having a compact structure.

5. The neutron moderator according to claim 4, wherein the sintered body has a Vickers hardness of 71 or more and 120 or less as regards mechanical strengths.

6. The neutron moderator according to claim 4, wherein the sintering aid is selected from the group consisting of carboxymethyl cellulose and calcium stearate.

7. The neutron moderator according to claim 4, wherein the sintered body has volume of 2,183 cm.sup.3 and 2,421 cm.sup.3 or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a phase diagram of the MgF.sub.2CaF.sub.2 binary system:

(2) FIG. 2 is a diagram showing the relationship between the heating conditions in the preliminary sintering step and the shrinkage rates of preliminary sintered bodies:

(3) FIG. 3 is a diagram showing the relationship between the heating conditions in the sintering step in a nitrogen gas atmosphere and the formation states of sintered bodies;

(4) FIG. 4 is a diagram showing alterations of the neutron type after moderation (alterations of the mixed fast neutron dose and the epithermal neutron dose suitable for therapy, after moderation) when a MgF.sub.2 sintered body and a CaF.sub.2 sintered body are superposed on each other as a moderator:

(5) FIG. 5 is a table showing the relative densities of MgF.sub.2 sintered bodies and alterations of the neutron type after moderation; and

(6) FIG. 6 is a table showing measured data of examples and comparative examples.

MODE FOR CARRYING OUT THE INVENTION

(7) The preferred embodiments of the fluoride sintered body for a neutron moderator and the method for producing the same according to the present invention are described below by reference to the Figures.

(8) In order to produce a fluoride sintered body suitable for a neutron moderator according to the preferred embodiments, a high-purity (purity of 99.9% by weight or more) MgF.sub.2 powder was used, and as a sintering aid, for example, a carboxymethyl cellulose (CMC) solution was added in the proportion of 0.03-0.5% by weight (not included in 100) to 100 of the powder. The mixture was used as a starting raw material.

(9) After filling the raw material into a mold form of a prescribed size, it was compressed at a molding pressure of 5 MPa or more using a uniaxial press molding device, and the molded article was further molded at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device.

(10) Preliminary sintering was conducted by heating this CIP molded article in the temperature range between 550 C. and 600 C. in an air atmosphere, and the preliminary sintered article was heated in the temperature range just below the starting temperature of foaming (the temperature defined through the measurement using a differential thermal analyzer, about 850 C.) (750 C.-840 C.) for 4-16 hours in an air atmosphere or an inert gas atmosphere. By this heating, sintering was more uniformly promoted, and thereafter, the same was heated in the vicinity of the temperature limits in which a solid solution starts to be formed, that is, in the temperature range of 900 C.-1100 C. for 0.5-3 hours, and then cooled so as to produce a MgF.sub.2 sintered body having a compact structure.

(11) As described above, in the phase diagram of the MgF.sub.2CaF.sub.2 binary system shown in FIG. 1, the temperature at which a solid solution starts to be formed is in the temperature limits in the vicinity of 980 C. However, the present inventors presumed from the observation of the sections of actually sintered bodies, that there was a high possibility that a solid solution would be formed at a temperature dozens lower than 980 C., the temperature indicated in this phase diagram in the case of MgF.sub.2 simple. Therefore, they considered that the vicinity of the temperature limits in which a solid solution starts to be formed should be 900 C. or more, and guessed that a solid solution would be formed even in the case of heating at a temperature less than 980 C.

(12) Concerning the pulverization of MgF.sub.2 being a raw material, balls for ball mill were filled into a pot mill, 3 kg of the raw material was filled therein and mixed for one week so as to be pulverized. The pot mill made of alumina, having an inside diameter of 200 mm and a length of 250 mm was used. As the filled balls made of alumina, 5: 1800 g, 10: 1700 g, 20: 3000 g and 30: 2800 g were used. The particle sizes of the raw material after pulverization were measured using a laser diffraction/scattering particle size distribution analyzer LA-920 made by HORIBA. Ltd. The median diameters were approximately 1.2 m-1.3 m.

(13) As the sintering aid, two kinds, the CMC and calcium stearate, were selected.

(14) With various addition proportions of each of them, the tests for examining the effects thereof were conducted. For comparison, a test with no sintering aid was also conducted.

(15) Concerning mixing of the sintering aid, the two kinds of sintering aids were added in the proportion of 0-2% by weight, respectively. As is the case with the pulverization of the raw material, after filling the balls for ball mill into the pot mill, the sintering aid was mixed a whole day and night.

(16) This mixed raw material was filled into a mold form of a uniaxial press molding device (mold size 220 mm220 mmH150 mm) and compression molding thereof was conducted at a press pressure of 20 MPa. Then, this press molded body was put into a vinyl bag and sealed, and it was put into a molding part of a CIP device (inside size: inside diameter 350 mmheight 120 mm). The space in said molding part was filled with clean water, and cool isostatic pressing (CIP) was conducted with variations of isostatic pressures by hydraulic pressure at room temperature.

(17) Preliminary sintering was conducted on this CIP molded body in an air atmosphere with various kinds of heating conditions in the temperature range between 500 C. and 700 C. and in the time range of 3 to 18 hours. After observing the appearance of this preliminary sintered body, in a nitrogen gas atmosphere, the temperature was raised from room temperature to 550 C. at a fixed rate for 6 hours, and held there for 8 hours. Then, it was raised to 950 C. at a fixed rate for 2 hours and held there for 1 hour, and then lowered for 20 hours to 100 C. By observing the appearance of the taken-out sintered body and the compact state of the inside thereof, proper compositions, processing conditions and preliminary sintering conditions were investigated.

(18) As a result, there was no big difference between the effects of the two kinds of sintering aids, but in the case of no sintering aid, the shape keeping performance of the uniaxial press molded body was poor, so that loss of shape frequently occurred in handling to the following CIP molding step. When the mix proportion of the sintering aid was 0.03% by weight or more, the loss of shape was not noticed, while coloring which appeared to be a residual of the sintering aid was sometimes noticed on the preliminary sintered body or sintered body when the mix proportion thereof exceeded 0.6% by weight. Accordingly, the proper range of the mix proportion of the sintering aid was decided to be 0.03-0.5% by weight.

(19) When the molding pressure of the CIP device was less than 5 MPa, the bulk density of the sintered body in any of optimizing tests of the heating conditions of preliminary sintering and main sintering was lower by 2% or more than the case of the molding pressure of 5 MPa or more. For example, in the case of a molding pressure of 10 MPa, the bulk density of a sintered body sintered with the same sintering conditions was 2.95 g/cm.sup.3, while in the case of a molding pressure of 4.8 MPa, the bulk density of a sintered body was 2.86 g/cm.sup.3, 3% lower than the former. When the molding pressure was increased gradually from 5 MPa to 20 MPa, it was recognized that the bulk density of a sintered body after sintering tended to increase little by little. The tests were conducted until the molding pressure was gradually increased further to 50 MPa. The increase of the bulk density of a preliminary sintered body or a sintered body in the case of a molding pressure of 20 MPa or more was just slight, and a linear improvement like between 5 MPa and 20 MPa was not recognized. Accordingly, the proper value of the molding pressure was decided to be 5 MPa or more, preferably 20 MPa or more.

(20) Concerning the preliminary sintering conditions of a molded body in an air atmosphere, as shown in FIG. 2, at a heating temperature of less than 550 C. shrinkage was small compared with the size of the molded body, while at a heating temperature of 610 C. or more, the shrinkage was large and difficult to control. Accordingly, the proper range of the preliminary sintering temperature was decided to be between 550 C. and 600 C.

(21) Concerning the proper value of the heating time, as shown in FIG. 2, at 550 C. 8-9 hours were optimal from the evaluation of the shrinkage rate, and it could be judged that 4-10 hours were proper. At 600 C., 6-8 hours were optimal, and it could be judged that 4-10 hours were proper. From these results, the heating condition of the preliminary sintering was decided to be at 550 C.-600 C. 4-10 hours in an air atmosphere.

(22) The main sintering step important in producing a MgF.sub.2 sintered body suitable for a neutron moderator and the sintering mechanism thereof are described below.

(23) The definition of primary flocculation process and secondary flocculation process which are the terms expressing the degree of progress of the sintering step, is described below. The primary flocculation process is the first half of the stage of sintering, and at the initial stage thereof, the intervals between particles gradually become narrower and the voids among particles also become smaller. Furthermore, the particle-to-particle contact portions become thick and the voids among them become small. Here, the majority of the voids are open pores connecting to the surrounding atmosphere. Such whole stage is called primary flocculation process.

(24) After the end of the primary flocculation process, with further progress of sintering, the open pores gradually decrease and turn into closed pores. Roughly the stage of turning into closed pores and the following stage of defoaming and compacting are called secondary flocculation process.

(25) In the present invention, due to pulverization of raw materials, particle size control, mixing of a sintering aid, uniaxial press molding, CIP molding, preliminary sintering and the like, it was recognized that the voids among particles of the preliminary sintered body were small, and that the voids almost uniformly scattered without gathering (the first half stage of the primary flocculation process).

(26) In the heating process of the next main sintering step, the heating temperature is gradually raised. Around the preliminary sintering temperature limits (550 C.-600 C.), particles start to gather, and thereafter, solid phase reaction starts in the temperature limits far lower than 980 C. at which a solid solution starts to be formed. With that, flocculation of particles makes progress, the particle-to-particle distances become short and the voids become small. Here, in the case of heating at a relatively low temperature (in the vicinity of 550 C.) like preliminary sintering for a short period of time, most of the voids remain in the open pore state (the second half stage of the primary flocculation process).

(27) It is generally said that the solid phase reaction starts in the temperature limits lower by the order of 10% or further lower than 980 C. From the observation of the sections of the sintered bodies in the preliminary test by the present inventors, it was considered that the solid phase reaction started in further lower temperature limits than generally said, at approximately 500 C. Its grounding is that at 550 C. the lowest limit of the proper preliminary sintering temperature, sintering had already made progress considerably and that the preliminary sintered body considerably shrunk compared with the molded body. In this preliminary test, the bulk volume shrunk in the order of 10-20% by volume. It was considered that the reaction made progress at a slow reaction rate in the temperature limits and that it made progress at a quite high reaction rate in the temperature limits in the vicinity of approximately 700 C. or more up to 980 C.

(28) What attention should be paid to is behavior of fine bubbles (foaming gas) generated through vaporization of part of a raw material in the temperature limits of about 8500 C. or more. In the case of heating at about 850 C. or more, it was considered that the heating time should be as short as possible, since this formation of bubbles became noticeable.

(29) Micro behavior of raw material particles is described below. Around a temperature exceeding 980 C. at which a solid solution starts to be formed, melting starts in the vicinity of a particle interface where fine particles of MgF.sub.2 are present, and a solid solution of MgF.sub.2 starts to be formed. As described by reference to FIG. 1, the present inventors presumed from the observation of the sections of the sintered bodies in the preliminary test that in the case of MgF.sub.2 simple, a solid solution would start to be formed in the temperature limits in the order of dozens of degrees lower than 980 C. as the true state. It was presumed that this solid solution filled the voids among particles and that in some parts, more fine voids were also filled in through capillary phenomenon.

(30) On the other hand, even if the heating temperature is lower than 980 C. by heating at about 700 C. or more for a long period of time as described above, the solid phase reaction makes progress, the voids gradually decrease with the elapse of time so as to be closed pores. Parallel with that, a gas component within the closed pores scatters within the bulk (parent) of the sintered body, leading to the progress of defoaming so as to make the sintered body compact with few bubbles (secondary flocculation process). Here, in order to make it compact by heating in the relatively low temperature limits of the order of 700 C., heating for a quite long period of time is required, resulting in low productivity and being uneconomical.

(31) Also here, in heating at about 850 C. or more, attention should be paid to the presence of line bubbles (foaming gas) generated through vaporization of a raw material. It is presumed that the bubbles contain fluorine gas. Fluorine, the atomic number of 9, having an atomic weight of 18.998, is heavier than the air, and a relatively heavy element among light elements. The diffusion velocity thereof within the bulk (equivalent for the parent) of the sintered body is slow (it is difficult to diffuse), and it is considered that once formed bubbles do not easily disappear. As measures for suppressing foaming, to avoid heating in the temperature limits of foaming as much as possible, and to hold heating in the temperature limits thereof to a short period of time are exemplified.

(32) The difference in appearance between such foaming gases and bubbles left after pores became closed but could not be defoamed in the sintering step (hereinafter, referred to as residual bubbles) is described below. The sizes of the foaming gases generated by general heating for a relatively short period of time are approximately several m diameter, and the shapes thereof are almost perfect spheres. On the other hand, the sizes of the residual bubbles are all mixed up, large, medium and small, and the shapes thereof are not perfect spheres but irregular. Therefore, it is possible to distinguish the both according to the difference in shape. Here, in the case of heating at a high temperature far exceeding 980 C., or heating in the temperature limits exceeding 980 C. for a long period of time, a foaming gas and a foaming gas, or a residual bubble and a foaming gas gather and grow to a large irregular bubble, resulting in difficulty in judging its origin.

(33) With the progress of the secondary flocculation process, the voids among particles become smaller, all or most of the voids are surrounded by particles or a bridge portion of the sintered body so as to be closed pores (bubbles). Depending on the conditions, gases are released through the voids (open pores), or gas components within the bubbles permeate into the bulk such as the particles or the bridge portion of the sintered body to degas, resulting in extinction of the bubbles (defoaming phenomenon). Whether the voids among particles are left as closed pores (bubbles) or by degassing, they do not remain as bubbles so as to disappear, is a significant element for deciding the degree of achievement of compactness of the sintered body, leading to the characteristics of the sintered body. Particularly in the case of sintering in an inert gas atmosphere (a light element gas such as Hie or Ne), it was considered that the lighter element easily diffused within the pores or bulk of the sintered body, leading to promoting the capillary phenomenon and defoaming phenomenon, and that bubbles were difficult to remain, leading to easy compacting. Thus, in order to make the whole compact, it is important to advance the primary flocculation process and the secondary flocculation process continuously with good balance.

(34) In the present invention, the preliminary sintering step chiefly corresponding to the first half stage of the primary flocculation process and the main sintering step chiefly corresponding to the second half stage of the primary flocculation process and the secondary flocculation process are separately conducted, so as to make the two flocculation processes easy to make progress uniformly throughout the sintered body. However, it is meaningless to divide the sintering step into two steps of preliminary sintering and main sintering like this, if the heating conditions are not proper. For example, in the case of heating at a high temperature exceeding the proper limits in the preliminary sintering step, in the case of rapidly heating at the temperature raising stage of the main sintering step, or in cases where the holding temperature in the main sintering step is a high temperature exceeding the proper limits, a remarkable difference in the degree of compactness is caused between the periphery portion of the sintered body and the inside thereof. When such situation is caused, degassing becomes difficult in the process of compacting the inside of the sintered body, resulting in insufficient compactness of the inside thereof. Therefore, it is important to make the heating temperature pattern in the sintering step according to the size proper.

(35) As described above, the proper conditions till just before the main sintering step are disclosed. The preliminary sintered body provided to this main sintering step is in a state that the whole body has already advanced to the first half stage of the primary flocculation. What is important here is the whole of the preliminary sintered body has already advanced uniformly to the middle of the primary flocculation.

(36) In order to find a method for producing a fluoride sintered body suitable for a neutron moderator, various kinds of main sintering steps were conducted.

(37) Preliminary sintered bodies obtained by conducting uniaxial press molding and CIP molding on a compound material made of MgF.sub.2 being a pulverized raw material with CMC of 0.2% by weight as a sintering aid added thereto and conducting preliminary sintering thereon at 550 C. for 6 hours were used. In any case, the heating time was set to be 6 hours. In each case of a sintering temperature varying between 600 C. and 1200 C., at an interval of every 50 C., the bulk density of the sintered body was measured. In the case of a range approximately between 900 C. and 1100 C., the bulk densities exceeded 2.90 g/cm.sup.3, which were high, but in either case of a sintering temperature of 850 C. or less, and that of 1150 C. or more, the bulk densities were lower than 2.90 g/cm.sup.3. When observing the sections of those sintered bodies, in the case of those sintered at 800 C. or less, the bridge widths of the sintering portions were narrow, so that it could be judged as absolutely insufficient progress of sintering. In the case of a body sintered at 850 C. a few open pores were noticed. In the case of a body sintered at 1100 C., some irregular bubbles were found inside, and in the case of those sintered at 1150 C. or more, those had a porous pumiceous structure as if irregular bubbles were innumerably formed inside. Fine bubbles which were almost perfect spheres of several to dozen m in diameter were observed all over the sintered body and innumerable irregular bubbles of 10 m or more in diameter were found all over the sections observed. It could be judged that these perfect sphere bubbles were foaming gases from their shapes, and that these irregular bubbles were bubbles in clusters similarly from their shapes.

(38) On the other hand, from the examination by the present inventors, it was found out that in the process of heating the MgF.sub.2 raw material obtained by pulverizing those as measured by a differential thermal analyzer, the weight clearly started to decrease at about 800 C., and that the weight started to drastically decrease at about 850 C. This means that a sublimation phenomenon in which MgF.sub.2 starts to dissolve/vaporize to generate fluorine gas starts at about 800 C., and that this phenomenon becomes brisk at about 850 C. (what is called presenting a foaming phenomenon).

(39) Through this sublimation, as described above, fine bubbles are formed all over the sintered body. The behavior of the formed fine bubbles (foaming gases) such as defoaming or remaining as bubbles is decided according to the degree of progress of the sintering step, in which portion of the sintered body they were formed and the like.

(40) In the primary flocculation process, for example, since the whole sintered body contains mainly open pores, the majority of bubbles are defoamed through the open pores, leading to few bubbles left. In the secondary flocculation process, since the sintered body contains mainly closed pores, a lot of foaming gases cannot be defoamed, leading to bubbles left. Basically it can be said that to swiftly complete the sintering in the secondary flocculation process is a course to be taken to reduce residual bubbles.

(41) Thus, it is preferable that the transition from the primary flocculation process to the secondary flocculation process should be advanced in the whole sintered body with as small a time lag as possible. However, it is not easy to undergo the transition from the primary flocculation process to the secondary flocculation process in the whole sintered body without time lag.

(42) Then, the present inventors decided to complete the primary flocculation process and the first half of the secondary flocculation process by heating at a rather low temperature in the temperature limits just below the starting temperature of foaming (about 850 C.) for a relatively long period of time, and then, to complete the second half of the secondary flocculation process by heating at a temperature in the vicinity of the temperature at which a solid solution starts to be formed (980 C.) for a relatively short period of time. They found out that this was an excellent sintering method by which the degree of progress of sintering in the whole sintered body could be made uniform with formation of few foaming gases.

(43) The proper range of the sintering conditions is described below. As preliminary sintering, a molded body was held at 600 C. for (hours in an air atmosphere. The preliminary sintered body was about 212 mm212 mmt72 mm in size and a cuboid shape with two square surfaces on the top and bottom.

(44) The heating atmosphere was set to be a nitrogen gas atmosphere. Preliminary tests concerning each of heating and cooling conditions in the heating pattern were conducted in three cases of the required time of 3, 6 and 9 hours. As a result, in the case of 3 hours, small cracks occurred in the sintered body, while in the other cases, the results were good. Therefore, the required time was set to be 6 hours.

(45) The heating atmosphere was continuously set to be the nitrogen gas atmosphere. The heating temperature was varied in the range of 700 C. to 1250 C., and in eleven cases of the holding time of 2, 3, 4, 5, 6, 8, 10, 12, 14, 16 and 18 hours, the tests were conducted. As shown in FIG. 3, in the case of 750 C. or less, the compactness was insufficient, regardless of the holding time. In the case of a holding time of 4 hours or less, the compactness was insufficient in any case other than 1100 C. On the other hand, in the case of a heating temperature exceeding 1150 C., a large number of bubbles were generated due to too fast sintering speed, regardless of the holding time, while in the case of a holding time of 16 hours or more, foaming occurred in part of the periphery of the sintered body, leading to getting out of shape in appearance.

(46) Reviewing the results in FIG. 3 in detail, in the case of heating at 850 C., the sintering state was good with a holding time of 8 hours or more, while slightly insufficient with 6 hours or less.

(47) In the case of 900 C., the sintering state was good with a holding time of 5 hours or more, while slightly insufficient with 4 hours or less, and beyond decision of quality with 16 hours or more.

(48) In the case of 950 C., the sintering state was good with a holding time of 5 to 14 hours, while slightly insufficient with 4 hours or less, and beyond decision of quality with 15 hours or more.

(49) In the case of 1000 C. the sintering state was good with a holding time of 5 to 12 hours, while slightly insufficient with 4 hours or less, and much foaming with 14 hours or more.

(50) In the case of 1100 C., the sintering state was good with a holding time of 3 to 8 hours, while much foaming with 10 hours or more.

(51) In the case of 1150 C., much foaming was seen with any holding time.

(52) In the case of 1200 C., the sintering was insufficient with a holding time of 3 hours or less, while beyond decision of quality or poor because of too much melting with 4 hours or more.

(53) When the heating temperature was a comparatively low temperature of 800 C. to 850 C., the sintering was slightly insufficient when the holding time was between 4 and 8 hours. However, since the main sintering step was divided into two, the first main sintering step and the following second main sintering step in the present invention, that was regarded as being good as the evaluation in the first main sintering step.

(54) In order to examine the relationship among the heating temperature, the bulk density of the sintered body and the mass decrease TG thereof corresponding to a yield, using the same preliminary sintered body as the above, the holding time was set to be 6 hours, and the heating temperature was varied within the range of 600 C. to 1300 C.

(55) As a result, in the case of a heating temperature of 900 C., the bulk density was approximately 2.90 g/cm.sup.3. Like the results shown in FIG. 3, the sintered body having a bulk density of that or more could be judged to have sufficient compactness without troubles such as losing its shape in the treatment of the second step.

(56) On the other hand, in the case of a heating temperature of 1150 C. or more, the mass decrease TG was 0.8% or more, and the decrease of the yield was remarkable. When the heating temperature was more than that, foaming occurred in part of the periphery of the sintered body, resulting in a trouble such as getting out of shape in appearance.

(57) From the results shown in FIG. 3, it could be judged that, if the sintering step was one of the heating steps, the heating temperature of 850 C. to 1100 C. and the holding time of 3 to 14 hours (high-temperature short-time heating, or low-temperature relatively-long-time heating within these ranges) were proper conditions.

(58) What was clarified here is, when relatively long time heating, such as at 900 C. for 16 hours or more, at 1000 C. for 14 hours or more, or at 1100 C. for 10 hours or more, was conducted, the quantity of bubbles was large and part of those gathered and was growing to a large bubble. It was confirmed that such sintered body involved defects which would cause cracks to occur from a large bubble portion or cause splitting in the next mechanical processing step.

(59) From these situations, the present inventors decided as a fundamental plan of the main sintering step that foaming should be suppressed as much as possible, as well as the sintering reaction should be allowed to sufficiently make progress, leading to producing a sintered body having a good processability in the subsequent mechanical processing step.

(60) The fundamental plan at the beginning of the main sintering step was that forming should be tried not to occur as much as possible, that the sintering should be allowed to make slow progress, and that the difference between the degree of progress of the inner portion of the sintered body and that of the periphery portion thereof should be kept as small as possible. The heating temperature limits were decided to be within the range of 800 C. to 1100 C. as described above. Since the temperature at which foaming became noticeable was about 850 C., the heating temperature at the beginning of the main sintering step was set to be below 850 C., 840 C. or less, that is, from 750 C. to 840 C., and the holding time was set to be 5 to 12 hours.

(61) Heating at the next stage of enhancing the sintering reaction of the sintered body was decided to be conducted in the temperature limits in the vicinity of 980 C. at which a solid solution started to be formed, that is, 900 C. to 1100 C. within the above proper conditions. The holding time was decided to be made as short as possible in order to enhance the sintering reaction and suppress foaming. Judging from the results in FIG. 3 and the below-described examples and comparative examples, the holding time was decided to be 0.5 to 3 hours, since the enhancement of the sintering reaction was poor in the case of less than 0.5 hour, and too many bubbles were formed in the case of 4 hours or more.

(62) When the atmospheric gas was changed to helium, the results were not different from those in the case of nitrogen gas. At less than 800 C., the compactness was not sufficient regardless of the holding time, and in the case of a holding time of 4 hours or less, the compactness was insufficient. In the case of 1110 C. or more, the sintering speed was too fast regardless of the holding time as is the case with the nitrogen gas, resulting in occurrence of many bubbles, and in the case of a holding time of 4 hours or more, because of foaming, the appearance got out of shape in some cases.

(63) In order to examine the relationship among the heating temperature, the bulk density of the sintered body and the mass decrease TG thereof corresponding to a yield, using the same preliminary sintered body as the above, the holding time was set to be 6 hours, and the heating temperature was varied within the range of 600 C. to 1300 C. As a result, as is the case with the nitrogen gas, the bulk density was approximately 2.90 g/cm.sup.3 at a heating temperature of 900 C. It was judged that the sintered body having a bulk density of that or more would not lose its shape in the treatment of the subsequent step, as is the case with the nitrogen gas, resulting in having sufficient compactness. On the other hand, at a heating temperature of 1110 C. or more, the mass decrease TG was 0.8% or more and the yield decrease was remarkable. And foaming occurred in part of the periphery of the sintered body, resulting in a trouble such as getting out of shape in appearance.

(64) Therefore, it was judged that the heating temperature of 900 C. to 1100 C. and the holding time of 0.5 to 2.5 hours were proper conditions. Furthermore, since in the case of a heating temperature of 950 C. to 1050 C. and a holding time of 0.5 to 3 hours, defects such as cracks were difficult to occur when providing the sintered body to the mechanical processing, resulting in good mechanical processability, it was judged that the heating temperature of 950 C. to 1050 C. and the holding time of 0.5 to 3 hours were desirable. Therefore, as proper heating conditions of the main sintering step in a helium gas atmosphere, as is the case with the above nitrogen gas atmosphere, the proper condition of the first heating of the main sintering step was at 750 C. to 840 C. for a holding time of 5 to 12 hours, while that of the second heating thereof was at 900 C. to 1100 C. for a holding time of 0.5 to 3 hours.

(65) The inert gas is not limited to nitrogen and helium. In the case of argon or neon, the same effects can be obtained. Since neon is expected to have a high resolution degree or a scattering characteristic in the parent of this sintered body, like helium, the defoaming phenomenon can be further promoted and an improvement thereby equal to or more than that by helium can be expected.

(66) When the heating conditions of the main sintering step were within the proper range, the state of the completed sintered body was wholly compact in any case, and no clearly defective portion such as a locally large void or a crack seen in a general ceramic sintered body could be found.

EXAMPLES

(67) The present invention is more specifically described below by reference to Examples, but the present invention is not limited to these Examples.

(68) First, a typical characteristic evaluation test conducted on sintered bodies in the Examples is described.

(69) In order to evaluate the neutron moderation performance, a beam emitted from an accelerator is allowed to collide with Be being a target, and by nuclear reaction, high-energy neutrons (fast neutrons) are mainly generated. Using Pb and Fe each having a large inelastic scattering cross section as a moderator in the first half of moderation, the neutrons are moderated to some extent (approximately up to 1 MeV) while suppressing the attenuation of the number of neutrons. These are irradiated to a moderator to be evaluated (a moderator in the second half of moderation), and by examining the neutrons after moderation, the moderator is evaluated. The measurement of the contents of the neutrons (hereinafter, referred to as a neutron flux) was conducted according to the method devised by the present inventors (the above Non-Patent Document 3). The total thickness of each moderator in the second half to be evaluated was set to be 320 mm, and two kinds of moderators, MgF.sub.2 and CaF.sub.2 were selected.

(70) Furthermore, the case wherein MgF.sub.1 and CaF.sub.2 were superposed on each other (the total thickness was set to be 320 mm) was also evaluated.

(71) What was evaluated here is how many fast neutrons having high possibilities of adversely influencing a patient remained in the neutrons moderated by the moderator. The results are shown in FIG. 4. Here, as MgF.sub.2 and CaF.sub.2, compact sintered bodies thereof each having a relative density (100(actual density)/(true density), unit %) of 952% were used.

(72) From FIG. 4, as the layer thickness of MgF.sub.2 increases as a moderator, (goes in a right direction on the axis of abscissas), the number of fast neutrons having possibilities of adversely influencing a patient decreases. In the case of MgF.sub.2 only, compared with the case of CaF.sub.2 only, the number thereof could be reduced to about to . It can be known that MgF.sub.2 is superior as a moderator.

(73) Using the above evaluation device, in the same manner, how the relative density (i.e. the compactness) of MgF.sub.2 influences the moderation performance was examined. As a moderator, only MgF.sub.2 sintered bodies, having a relative density of 90% to 97% were used.

(74) The results are shown in Table 1. The higher relative density the sintered body had, the less the quantity of mixed fast neutrons was, leading to obtaining excellent performance as a moderator. The sintered bodies having a relative density of less than 92% were varied in moderation performance. Some unstable cases, wherein the quantity of mixed fast neutrons suddenly increased, or the epithermal neutron dose drastically increased, were noticed. This appears to be because insufficient compactness resulted in insufficient moderation performance, or with the formation of open pores, impurities were mixed in the sintered body in molding thereof, resulting in irregular influence on the moderation performance. In order to allow the sintered body to present stable moderation performance, it was found that the relative density of 92% or more, that is, the bulk density of 2.90 g/cm.sup.3 or more was required.

(75) As evaluation indexes of mechanical strength, bending strength and Vickers hardness were adopted. The samples for bending strength, having a size of 4 mm46 mmt3 mm with the upper and lower surfaces optically polished were prepared according to JIS C2141, and tested according to the three-point bending test JIS R1601. To obtain the Vickers hardness, using Micro Hardness Tester made by Shimadzu Corporation, an indenter having a load of 100 g was pressed for 5 seconds of loading time so as to measure the diagonal length of the impression, which was converted into hardness.
Hardness=0.18909P/(d).sup.2
Here, P: load (N) and d: diagonal length of impression (mm)

Example 1

(76) A high-purity MgF.sub.2 raw material (mean particle diameter of 20 m and purity of 99.9% by weight or more) was pulverized using the pot mill and alumina balls described in the Mode for Carrying Out the Invention, to a high-purity MgF.sub.2 powder (mean particle diameter of 1.2 m and purity of 99.9% by weight or more). To the powder, a carboxymethyl cellulose (CMC) solution was added as a sintering aid in the proportion of 0.2% by weight to 100 of the MgF.sub.2 powder, and mixed in the pot mill for 12 hours so as to be a starting raw material.

(77) This starting raw material was filled into a mold form (mold size of 220 mm220 mmH150 mm) using a uniaxial press device and compressed at a uniaxial press pressure of 10 MPa to be molded.

(78) This press molded body (size of about 220 mm220 mmt85 mm), which was put into a thick vinyl bag and sealed after deairing, was put into a molding part (inside size: inside diameter 350 mmH120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with this press molded body therein and the molding part of the CIP device, and isostatic pressing was conducted at a molding pressure of 20 MPa, resulting in a CIP molded body (size of about 215 mm215 mmt75 mm).

(79) Preliminary sintering at 600 C. for 5 hours in an air atmosphere was conducted on this molded body, resulting in a preliminary sintered body having a size of about 208 mm208 mm t72 mm.

(80) This preliminary sintered body was heated from room temperature to 830 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1000 C. at a fixed rate for 2 hours and held there for 1 hour. Heating was then stopped, and the temperature was lowered by self-cooling (furnace cooling) for about 20 hours to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out.

(81) The bulk density of the sintered body was calculated at 3.05 g/cm.sup.3 (relative density of 96.8%) from the rough size (193 mm193 mmt62 mm) and the weight thereof. The sintering state was good. Since the appearance of the sintered body was a square form in a plan view, the bulk density here was obtained by a method wherein the bulk volume was calculated from the measured two sides of the square and thickness, and the weight separately measured was divided by the bulk volume. This also applied to the following.

(82) Using samples taken from this sintered body, by the method shown in the Non-Patent Document 3, evaluation tests of neutron moderation performance and characteristics of every kind were conducted. The results are shown in Table 1. This also applied to the following Examples and Comparative Examples.

(83) Concerning the neutron moderation performance, the decrease of epithermal neutron dose was slightly small compared with CaF.sub.2 as a comparative material, but the dose of fast neutrons having high possibilities of adversely influencing a patient was reduced to about , so that it was found that MgF.sub.2 had an excellent moderation performance.

(84) As shown in Table 2, the other mechanical strengths were also good without problems.

Example 2

(85) Using the same starting raw material as in the above Example 1, preliminary sintering at 550 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 208 mm208 mmt73 mm. This preliminary sintered body was heated from room temperature to 750 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 9 hours. It was then raised to 920 C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 195 mm195 mmt64 mm, the bulk density thereof was 2.90 g/cm.sup.3 (relative density of 92.1%), and the sintering state was good.

(86) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 3

(87) Using the same starting raw material as in the above Example 1, preliminary sintering at 600 C. for 8 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 206.5 mm207 mmt71 mm. This preliminary sintered body was heated from room temperature to 840 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 12 hours. It was then raised to 1080 C. at a fixed rate for 2 hours and held there for 1 hour. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 192 mm192 mmt61 mm, the bulk density thereof was 3.00 g/cm.sup.3 (relative density of 95.2%), and the sintering state was good.

(88) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 4

(89) Using the same starting raw material as in the above Example 1, this raw material was filled into the mold form of uniaxial press molding and compressed at a uniaxial press pressure of 70 MPa to be molded. Then, molding was conducted using the cold isostatic pressing (CIP) device at a molding pressure of 40 MPa, so as to obtain a molded body (size of about 213 mm214 mmt74 mm).

(90) Preliminary sintering at 600 for 10 hours in an air atmosphere was conducted on this molded body to obtain a preliminary sintered body of 204.5 mm205 mmt70 mm. This preliminary sintered body was heated from room temperature to 830 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 12 hours. It was then raised to 1080 C. at a fixed rate for 2 hours and held there for 1 hour. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 190.5 mm191 mmt60 mm, the bulk density thereof was 3.07 g/cm.sup.3 (relative density of 97.5%), and the sintering state was good.

(91) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 5

(92) Using the same starting raw material as in the above Example 1, preliminary sintering at 580 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 206 mm206 mmt70.5 mm. This preliminary sintered body was heated from room temperature to 800 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 12 hours. It was then raised to 920 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 191.0 mm191.5 mmt62 mm, the bulk density thereof was 3.02 g/cm.sup.3 (relative density of 95.9%), and the sintering state was good.

(93) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 6

(94) Using the same starting raw material as in the above Example 1, preliminary sintering at 580 C. for 7 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 207 mm207 mmt71.5 mm. This preliminary sintered body was heated from room temperature to 830 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 12 hours. It was then raised to 1000 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 192.5 mm192.5 mmt63 mm, the bulk density thereof was 2.99 g/cm.sup.3 (relative density of 94.9%), and the sintering state was good.

(95) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 7

(96) Using the same starting raw material as in the above Example 1, preliminary sintering at 580 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 206 mm206 mmt70.5 mm. This preliminary sintered body was heated from room temperature to 840 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 980 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 193 mm 193.5 mmt62.5 mm, the bulk density thereof was 2.96 g/cm.sup.3 (relative density of 94.0%), and the sintering state was good.

(97) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 8

(98) Using the same starting raw material as in the above Example 1, preliminary sintering at 560 C. for 8 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 207 mm206 mmt70.5 mm. This preliminary sintered body was heated from room temperature to 8409 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 920 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 194.5 mm194.5 mmt64 mm, the bulk density thereof was 2.91 g/cm.sup.3 (relative density of 92.4%), and the sintering state was good.

(99) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 9

(100) Using the same starting raw material as in the above Example 1, preliminary sintering at 580 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 205 mm205 mm170.5 mm. This preliminary sintered body was heated from room temperature to 840 t at a fixed rate for 6 hours in a helium gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 980 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 192.5 mm192.5 mmt62 mm, the bulk density thereof was 3.00 g/cm.sup.3 (relative density of 95.2%), and the sintering state was good.

(101) Any of the evaluation results of the neutron moderation performance and characteristics oft every kind were good as shown in Table 2.

Example 10

(102) Using the same starting raw material as in the above Example 1, preliminary sintering at 560 C. for 6 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 207 mm207 mmt70.5 mm. This preliminary sintered body was heated from room temperature to 770 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 10 hours. It was then raised to 900 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 194.5 mm194.5 mmt64 mm, the bulk density thereof was 2.90 g/cm.sup.3 (relative density of 92.1%), and the sintering state was good.

(103) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Example 11

(104) Using the same starting raw material as in the above Example 1, preliminary sintering at 550 C. for 8 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 207 mm207 mmt70 mm. This preliminary sintered body was heated from room temperature to 790 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 940 C. at a fixed rate for 2 hours and held there for 1.5 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 194.5 mm194.5 mmt64 mm, the bulk density thereof was 2.91 g/cm (relative density of 92.4%), and the sintering state was good.

(105) Any of the evaluation results of the neutron moderation performance and characteristics of every kind were good as shown in Table 2.

Comparative Example 1

(106) Using the same starting raw material as in the above Example 1, preliminary sintering at 550 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 208 mm208 mmt73 mm. This preliminary sintered body was heated from room temperature to 750 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 9 hours. It was then raised to 920 C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 195 mm195 mmt64 mm, the bulk density thereof was 2.90 g/cm.sup.3 (relative density of 92.1%), and the sintering state was good.

(107) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of low mechanical strength.

Comparative Example 2

(108) Using the same starting raw material as in the above Example 1, preliminary sintering at 530 C. for 5 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 209 mm209 mmt76 mm. This preliminary sintered body was heated from room temperature to 740 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 4 hours. It was then raised to 890 C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 198 mm198 mmt68 mm, the bulk density thereof was 2.80 g/cm.sup.3 (relative density of 88.9%), and the sintering state was obviously porous and inconvenient, leading to a problem in handling.

(109) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of unmeasurably low mechanical strength.

Comparative Example 3

(110) Using the same starting raw material as in the above Example 1, preliminary sintering at 550 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 208 mm208 mmt73 mm. This preliminary sintered body was heated from room temperature to 750 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 9 hours. It was then raised to 880 C. at a fixed rate for 2 hours and held there for 1.5 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 197 mm196 mmt67 mm and the bulk density thereof was 2.88 g/cm.sup.3 (relative density of 91.4%). The sintering state was good in appearance, but at the stage of grinding wherein the sintered body was finished using a grinder, a phenomenon that a grinding fluid was absorbed into the sintered body was recognized. Therefore, the microstructure of the inside of the sintered body was examined in detail. As a result, it was clarified that a large number of open pores were formed, resulting in insufficient sintering.

(111) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of low mechanical strength.

Comparative Example 4

(112) Using the same starting raw material as in the above Example 1, preliminary sintering at 600 C. for 10 hours in an air atmosphere was conducted on a molded body to which uniaxial press molding and cold isostatic pressing (CIP) were applied in the same manner, so as to obtain a preliminary sintered body of 208 mm208 mmt73 mm. This preliminary sintered body was heated from room temperature to 840 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 1150 C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out. The rough size of the sintered body was 196.5 mm197 mmt68 mm and the bulk density thereof was 2.87 g/cm.sup.3 (relative density of 91.1%). The sintering state was porous. When examining the microstructure of the inside of the sintered body, the structure was not compact and a trace of violent foaming resulting in porosity was observed.

(113) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of low mechanical strength.

Comparative Example 5

(114) Using the same starting raw material as in the above Example 1, this raw material was filled into a mold form (mold size of 220 mm220 mmH150 mm) using a uniaxial press device, and compressed at a uniaxial press pressure of 4 MPa to be molded.

(115) This press molded body (size of about 220 mm220 mmt85 mm) was put into a thick vinyl bag, and sealed after deairing. That was put into a molding part (inside size: inside diameter 350 mmH120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with this press molded body therein and the molding part of the CIP device and isostatic pressing was conducted at a molding pressure of 4 MPa, resulting in a CIP molded body (size of about 218 mm218 mmt75 mm).

(116) Preliminary sintering at 550 C. for 5 hours in an air atmosphere was conducted on this molded body, so as to obtain a preliminary sintered body of about 211 mm211 mmt73 mm. This preliminary sintered body was heated from room temperature to 740 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 900 C. at a fixed rate for 2 hours and held there for 1 hour. Heating was then stopped, and the temperature was lowered by self-cooling (furnace cooling) for about 20 hours to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out.

(117) The bulk density of the sintered body calculated from the rough size (199 mm199 mmt68 mm) and the weight thereof was 2.86 g/cm.sup.3 (relative density of 90.8%). The sintering state was slightly porous.

(118) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of low mechanical strength.

Comparative Example 6

(119) Using the same starting raw material as in the above Example 1, this raw material was filled into a mold form (mold size of 220 mm220 mmH150 mm) using a uniaxial press device, and compressed at a uniaxial press pressure of 10 MPa to be molded.

(120) This press molded body (size of about 220 mm220 mmt851 nm) was put into a thick vinyl bag, and sealed after deairing. That was put into a molding part (inside size: inside diameter 350 mmH120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with this press molded body therein and the molding part of the CIP device and isostatic pressing was conducted at a molding pressure of 20 MPa, resulting in a CIP molded body (size of about 215 mm215 mmt75 mm).

(121) Preliminary sintering at 500 C. for 4 hours in an air atmosphere was conducted on this molded body so as to obtain a preliminary sintered body of about 211 mm211 mmt72 mm. This preliminary sintered body was heated from room temperature to 730 C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 900 C. at a fixed rate for 2 hours and held there for 1 hour. Heating was then stopped, and the temperature was lowered by self-cooling (furnace cooling) for about 20 hours to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out.

(122) The bulk density of the sintered body calculated from the rough size (198 mm198 mmt68 mm) and the weight thereof was 2.85 g/cm.sup.3 (relative density of 90.5%). The sintering state was insufficient and slightly porous.

(123) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of low mechanical strength.

Comparative Example 7

(124) Using the same starting raw material as in the above Example 1, this raw material was filled into a mold form (mold size of 220 mm220 mmH150 mm) using a uniaxial press device, and compressed at a uniaxial press pressure of 4 MPa to be molded.

(125) This press molded body (size of about 220 mm220 mmt85 mm) was put into a thick vinyl bag, and sealed after deairing. That was put into a molding part (inside size: inside diameter 350 mmH120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with this press molded body therein and the molding part of the CIP device and isostatic pressing was conducted at a molding pressure of 4 MPa, resulting in a CIP molded body (size of about 218 mm218 mmt75 mm).

(126) Preliminary sintering at 550 C. for 5 hours in an air atmosphere was conducted on this molded body, so as to obtain a preliminary sintered body of 211 mm211 mmt72.5 mm. This preliminary sintered body was heated from room temperature to 740 C. at a fixed rate for 6 hours in a helium gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 900 C. at a fixed rate for 2 hours and held there for 1 hour. Heating was then stopped, and the temperature was lowered by self-cooling (furnace cooling) for about 20 hours to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out.

(127) The bulk density of the sintered body calculated from the rough size (198 mm198.5 mmt67.5 mm) and the weight thereof was 2.89 g/cm.sup.3 (relative density of 91.7%). The sintering state was slightly porous.

(128) As the evaluation results of the neutron moderation performance and characteristics of every kind, as shown in Table 2, a large quantity of fast neutrons having possibilities of adversely influencing the body remained in the neutron flux after moderation. There was a problem left that the moderation effect could not be sufficiently obtained. In addition, there was a problem of low mechanical strength.

(129) [Comparative Material: CaF.sub.2]

(130) A high-purity CaF.sub.2 raw material (mean particle diameter of 20 m and purity of 99.9% by weight or more) was pulverized using the pot mill and alumina balls to a high-purity CaF.sub.2 powder (mean particle diameter of 1.4 m and purity of 99.9% by weight or more). To the powder, a carboxymethyl cellulose (CMC) solution was added as a sintering aid in the proportion of 0.2% by weight to 100 of the CaF.sub.2 powder, and mixed in the pot mill for 12 hours to be a starting raw material.

(131) This raw material was filled into a mold form (mold size of 220 mm220 mmH150 mm) using a uniaxial press device and compressed at a uniaxial press pressure of 10 MPa to be molded.

(132) This press molded body (size of about 220 mm220 mmt85 mm), which was put into a thick vinyl bag and sealed after deairing, was put into a molding part (inside size: inside diameter 350 mmH120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with this press molded body therein and the molding part of the CIP device, and isostatic pressing was conducted at a molding pressure of 20 MPa, leading to a CIP molded body (size of about 215 mm215 mmt75 mm).

(133) Preliminary sintering at 600 C. for 6 hours in an air atmosphere was conducted on the molded body, so as to obtain a preliminary sintered body having a size of about 208 mm28 mmt72 mm.

(134) This preliminary sintered body was heated from room temperature to 870 C. at a fixed rate for 6 hours in a nitrogen atmosphere, and the temperature was held there for 6 hours. It was then raised to 1100 C. at a fixed rate for 2 hours and held there for 1 hour. Heating was then stopped, and the temperature was lowered by self-cooling (furnace cooling) for about 20 hours to 100 C. at which time it was previously set to take out the sintered body, after which it was taken out.

(135) The bulk density of the CaF.sub.2 sintered body was calculated at 3.05 g/cm.sup.3 (relative density of 95.9%, and the true density of CaF.sub.2 is 3.18 g/cm.sup.3) from the rough size (193 mm193 mmt62 mm) and the weight thereof. The sintering state was good.

(136) As the evaluation results, a sintered body in a compact sintering state could be obtained and the mechanical strength was sufficient as shown in Table 2. However, since a large quantity of fast neutrons remained, there was a big problem of the moderation performance to neutrons left. This result indicated that a CaF.sub.2 sintered body was inferior to a MgF.sub.2 sintered body in characteristics as a moderator even if the CaF.sub.2 sintered body was sufficiently compact.

INDUSTRIAL APPLICABILITY

(137) It is possible to be used as a moderator to restrict the radiation velocity of radioactive rays of every kind such as neutrons.