METHOD AND DEVICE FOR MULTI-DIMENSIONAL DIRECTION MEASUREMENT OF GAMMA RADIATION IN THE FAR FIELD

Abstract

A method for multidimensional direction measurement of gamma radiation in the far field by means of a group of several energy discriminating detectors synchronized with each other for detection of radiation can use unidirectional and bidirectional Compton scattering processes and lookup tables LUT.sup.SK, a defined functional value f(E1,E2), a list of defined detector pairs with an identification number i for defined detector pairs, and one or more frequency distributions Y for the acquisition of the measurement values. In some embodiments, the method can include setting up a detector system, acquiring measurement values, associating coincidence events with an Identification number, calculating a functional value, acquiring coincidence events in frequency distributions, and calculating one or more direction distributions from the frequency distributions.

Claims

1-15. (canceled)

16. A method for the multidimensional direction measurement of gamma radiation in the far field via a group of several energy discriminating detectors synchronized with each other for detecting radiation, wherein the method uses unidirectional and bidirectional Compton scattering processes and lookup tables LUT.sup.SK, a defined functional value f(E1, E2), a list of defined detector pairs with an identification number i for defined detector pairs, and one or more frequency distributions Y for acquiring the measurement values, and the method comprises: a) setting up the detector system for a measurement, wherein the following sequence of steps is carried out: creating a list with defined detector pairs, wherein the defined detector pairs comprise all pairs, which may be formed in combination from the quantity of the detectors, and wherein each pair comprises at least one detector made from a material having an atomic number of Z.sub.eff>30, and identifying the latter with an ID number i; interconnecting all detectors in a coincidence circuit such that coincidence events are acquired in all defined detector pairs i=1, . . . , I; identifying both detectors of each defined detector pair i with the numbers 1 and 2, respectively, wherein the detector having the lower atomic number receives the number 1, and that one having the higher atomic number receives the number 2, wherein in case both detectors consist of the same material, the identification as 1 and 2, respectively, is made arbitrarily; defining a function f(E1, E2), which is calculated from two energy values E1 and E2, wherein such functions f(E1, E2) are allowable, which are traceable back to a function f(E1) when substituting E2 by C−E1, which function over the entire interval [0,C] is clearly defined, constant, and monotonous; wherein C is a constant, which represents the radiation energy C=E1+E2; b) acquiring measurement values of coincidence events, if interactions take place simultaneously in respectively two detectors of all defined detector pairs i, wherein the measurement values originate from a radiation distribution in the far field, and the measurement values are the interaction energies E1 and E2 of the radiation measured in the detectors; c) associating coincidence events with an identification number i; d) calculating the functional value f(E1, E2) from two energy values E1, E2 per coincidence event by means of the function f(E1, E2) defined in step a); e) acquiring the coincidence events corresponding to their identification number i and their functional values f(E1, E2) in one or more frequency distributions Y, wherein a separate frequency distribution Y is available for each radio nuclide, f) calculating one or more direction distributions X from the frequency distributions Y by means of a statistic image reconstruction method of emission tomography using lookup tables LUT.sup.SK, wherein a separate direction distribution X is available for each radio nuclide.

17. The method of claim 1 wherein: the radiation sources emit a discrete and/or continuous distribution of radiation; and/or that the radiation sources emit gamma, electron, positron, proton, ion, and/or neutron radiation; and/or that the radiation originates from the radioactive decay of one or more radio nuclides; and/or that the radiation is the prompt gamma radiation, which is generated during the absorption of proton or ion radiation in target materials; and/or that the radiation is of low intensity, as, for example, in astronomy.

18. The method of claim 17, wherein the method in step a) also comprises at least one of: calibrating the signals of all detectors as absorbed radiation energy E; determining a suitable coordinate system; acquiring the directions of the defined detector pairs, wherein in the 2-dimensional direction measurement, the direction of the detector pair i is acquired with the azimuth angle φ.sub.i, and wherein in the 3-dimensional direction measurement, the direction of detector pair i is acquired with the azimuth angle φ.sub.i and the height angle β.sub.i; dividing the measurement range for the functional value f(E1,E2) into a number J of equidistant measurement value channels j=1, . . . , J; creating one or more 2-dimensional arrays with I.Math.J fields as data structures for storing the frequency distributions Y, in which coincidence events are registered according to their ID number i and are registered according to their measurement value channel j; wherein a separate 2-dimensional array Y is created for each radio nuclide; dividing the detector pairs i into symmetry classes SK, wherein those detector pairs i, which are mapped to other detector pairs with similar construction upon rotation or translation are aggregated to respectively one symmetry class SK(i); creating one or more lookup tables LUT.sup.SK for each symmetry class SK and for each radio nuclide, wherein these cover an angular range ϑ from 0° to 180° and are divided into equidistant angular steps; creating the lookup tables LUT.sup.SK by measurements with the detector system or by Monte Carlo simulations or by means of a theoretical model; transferring all lookup tables LUT.sup.SK for all symmetry classes and all radio nuclides to an algorithm of image reconstruction, which processes the measurement data and which calculates the direction distributions X.

19. The method of claim 18, wherein the lookup tables LUT.sup.SK are created by measurements with the detector system by a creation process comprising: creating a prescription for generating and validating the lookup tables LUT.sup.SK from reference measurements, which selects the reference sources, considers the nuclide type, as well as defines the measurement conditions, under which the measurements are to be carried out; performing reference measurements for all steps defined previously in the prescription; creating and validating the lookup tables LUT.sup.SK according to the prescription defined previously for analyzing the measurement data of the reference measurements; detecting the natural radiation background b.sup.SK(i) for all symmetry classes SK; excluding the natural radiation background b.sup.SK from the lookup tables LUT.sup.SK.

20. The method of claim 1, wherein the functional value f(E1, E2) used in steps a) and d) is energy asymmetry ƒ(E1,E2)=(E2−E1)/(E1+E2).

21. The method of claim 16, wherein: the selection condition for the energy sum E1+E2 of the energies detected in both detectors of a pair is applied; and/or that for each radio nuclide, a separate frequency distribution Y is created; and/or that for each radio nuclide, a separate direction distribution X is calculated.

22. The method of claim 16, wherein: several selection conditions are applied for the energy sum E1+E2 for radio nuclides with several gamma energies; and/or that for such radio nuclides with several gamma energies, one or more frequency distributions Y are created.

23. The method according to claim 16, characterized in that in step f), the statistic image reconstruction method partially or completely is the Maximum Likelihood Expectation Maximization (MLEM) method, the Ordered Subset Expectation Maximization (OSEM) method, the List Mode-Maximum Likelihood Expectation Maximization (LM-MLEM) method and/or the List Mode-Ordered Subset Expectation Maximization (LM-OSEM) method.

24. The method of claim 16, wherein, for each detected radio nuclide, a 2-dimensional direction distribution X.sub.k=X(ω.sub.k) is calculated according to $X_k^{\left[n+1\right]}=X_k^{\left[n\right]}.Math.\frac{K}{{.Math.}_{i,j}.Math.Y_{i.Math.j}}.Math.\underset{i,j}{.Math.}.Math.\frac{L.Math.U.Math.T_j^{S.Math.K\left(i\right)}\left[\vartheta \left({\omega }_k,{\varphi }_i\right)\right].Math.Y_{i.Math.j}}{{.Math.}_{k^{\prime }=1}^K.Math.{\mathrm{LUT}}_j^{S.Math.K\left(i\right)}\left[\vartheta \left({\omega }_{k^{\prime }},{\varphi }_i\right)\right].Math.X_{k^{\prime }}^{\left[n\right]}}$ from the initial distribution
∀k=1, . . . ,K X.sub.k.sup.[0]=1 using the lookup tables LUT.sup.SK and an angular distance function according to
ϑ(ω.sub.k,φ.sub.i)=arccos(cos(ω.sub.k−φ.sub.i)) wherein K is a number of pixels for the azimuth angle ω.sub.k defined by the user.

25. The method of claim 16, wherein, for each detected radio nuclide, a 3-dimensional direction distribution X.sub.kl=X(ω.sub.k,h.sub.1) is calculated according to $X_{k.Math.l}^{\left[n+1\right]}=X_{k.Math.l}^{\left[n\right]}.Math.\frac{K.Math.L}{{.Math.}_{i,j}.Math.Y_{i.Math.j}}.Math.\underset{i,j}{.Math.}.Math.\frac{L.Math.U.Math.T_j^{S.Math.K\left(i\right)}\left[\vartheta \left({\omega }_k,h_l,{\phi }_i,{\beta }_i\right)\right].Math.Y_{i.Math.j}}{{.Math.}_{k^{\prime }=1}^K.Math.{.Math.}_{l^{\prime }=1}^L.Math.{\mathrm{LUT}}_j^{\mathrm{SK}\left(i\right)}\left[\vartheta \left({\omega }_{k^{\prime }},h_{l^{\prime }},{\varphi }_i,{\beta }_i\right)\right].Math.X_{k^{\prime }.Math.l^{\prime }}^{\left[n\right]}}$ from the initial distribution
∀k=1, . . . ,K, l=1, . . . ,L X.sub.kl.sup.[0]=1 using the lookup tables LUT.sup.SK and an angular distance function according to
ϑ(ω.sub.k,h.sub.l,φ.sub.i,β.sub.i)=arccos(cos β.sub.i cos h.sub.l cos(ω.sub.k−φ.sub.i)+sin β.sub.i sin h.sub.l) wherein K and L are numbers of pixels defined by the user for the azimuth angle ω.sub.k and the height angle h.sub.l.

26. A device for carrying out a method for multidimensional direction measurement of gamma radiation in a far field, comprising: a group of several synchronized detectors for detection of radiation, wherein at least one detector material has an atomic number Z.sub.eff>30 and all detectors measure the energies E, which occur in interactions of the radiation with the detector materials; a system electronics, which registers coincidence events, if interactions take place simultaneously in respectively two detectors from a list of defined detector pairs i, wherein the list of defined detector pairs comprises all pairs, which can be formed in combination from the quantity of all detectors, and wherein the defined detector pairs comprise at least one detector made from a material having an atomic number of Z.sub.eff>30; a data acquisition system, which determines a ranking for both detectors involved in a coincidence event, which defines a first and a second detector, and which sorts the energies (E1, E2) measured in the coincidence events corresponding to their identification 1, 2 and stores them in a chronological list with the attributes {i, E1, E2} and the detection time t; wherein in each detector pair i the detector having the lower atomic number receives the number 1, and that one having the higher atomic number receives the number 2, wherein if both detectors of a pair should have the same atomic number, the identification of 1 and 2, respectively, is made arbitrarily; and an analysis unit, which creates one or more frequency distributions Y from the data stored in the data acquisition system, wherein a function f(E1, E2) is applied, which is dependent on two energy values (E1, E2), and wherein this function f(E1, E2) is traceable back to a function f(E1) when substituting E2 by C−E1, which function over the entire interval [0,C] is clearly defined, constant, and monotonous, wherein C is a constant, which represents the radiation energy C=E1+E2, and an analysis unit, which reconstructs one or more direction distributions X of the radiation field.

27. The device according to claim 26, wherein the device is configured partially or completely as a Compton camera, a Compton telescope, a single plane Compton camera, a neutron camera, and/or a dual gamma/neutron camera; and/or wherein the entirety of all detectors is arranged in a ring, and/or wherein one or more central detectors may be present in the interior of the ring; and/or wherein the ring preferably comprises four of five, and particularly preferred six plastic—scintillation detectors, and/or one or more scintillation detectors made from NaI, CsI, CeBr.sub.3 and/or LaBr.sub.3 are provided in the interior of the ring; and/or the scintillation detectors preferably are formed with a dimension of 1″×1″, 1.5″×1.5″, 2″×2″ and/or 3″×3″.

28. The device of claim 27, wherein a scintillation detector is used as the detector, and/or the scintillator is formed as monolithic block or as pixelated scintillator module, and/or which is made from pure or doped materials from the group of PVT, anthracene, stilbene, p-terphenyl, CaF.sub.2, BaF.sub.2, NaI, CeBr.sub.3, LaBr.sub.3, LaCl.sub.3, La(Br.sub.xCl.sub.1-x).sub.3, CsI, SrI.sub.2, CLYC, CLBC, CLCB, CLLB, BGO, LSO, LYSO, GAGG, YAP and/or YAG; and/or that a semiconductor detector is used as detector, and/or that the semiconductor is formed as segmented or non-segmented semiconductor, and/or which has a planar or coaxial geometry, and/or which is made from materials of the group of Ge, GaAs, CdTe and/or CdZnTe.

29. The device of claim 28, wherein at least two detectors are used; and/or that all detectors used are substantially similar in construction, and that at least two of the detectors used are different from each other.

30. The device of claim 26, wherein a detector and system electronics is used, which uses analog and/or digital electronic components; and/or that the analog electronic components comprise a combination of various modules, which comprise a high-voltage supply, a preamplifier, an amplifier, a pulse shaper, the charge integrator, a pulse height analyzer, a multichannel analyzer (MCA) and/or a coincidence circuit; and/or that the digital electronic components comprise a combination of different hardware and software components, which comprise a high-voltage supply, an A/D converter per detector, a Field Programmable Gate Array (FPGA), a storage medium, a digital signal processor, and/or an analysis software.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0169] The invention and embodiments thereof will be described below in further detail in connection with the drawings. It should be understood that like reference characters used in the drawings may identify like components. In the drawings:

[0170] FIG. 1 shows a schematic illustration that is not to scale of a unidirectional detector pair in a 2-dimensional measurement situation according to an embodiment of the invention;

[0171] FIG. 2 shows a schematic illustration that is not scale of a bidirectional detector pair in a 2-dimensional measurement situation according to a further embodiment of the invention;

[0172] FIG. 3 shows a schematic illustration that is not to scale of the radiation from a source incident on a detector pair 1, 2 in a 2-dimensional measurement situation according to a further embodiment of the invention;

[0173] FIG. 4a shows a schematic illustration that is not to scale of a device model with a central cerbromide detector which is surrounded by six plastic detectors that are arranged circularly according to a further embodiment of the invention;

[0174] FIG. 4b shows a schematic illustration that is not to scale of a device module according to FIG. 4a in which a further cerbromide detector is included according to a further embodiment of the invention;

[0175] FIG. 5 shows a schematic illustration of a frequency distribution Y of the device model of FIG. 4a in a 2-dimensional direction measurement according to a further embodiment of the invention;

[0176] FIG. 6a shows a schematic illustration that is not to scale of a first symmetry class of four cube-shaped detectors in total, arranged in a square, wherein two horizontal and two vertical detector pairs are present, according to a further embodiment of the invention;

[0177] FIG. 6b shows a schematic illustration that is not to scale of a further symmetry class of four cube-shaped detectors in total arranged in a square of FIG. 6a, according to a further embodiment of the invention;

[0178] FIG. 7 shows a schematic illustration of a frequency distribution Y of the device model of FIG. 4b in a 3-dimensional direction measurement according to a further embodiment of the invention;

[0179] FIG. 8a shows a schematic illustration that is not to scale of a direction distribution for a radiation far field, the emission points of which are on lying within one plane. The direction distribution is modelled along the horizontal circle as a function X(ω) that is dependent on the azimuth angle ω according to a further embodiment of the invention;

[0180] FIG. 8b is a schematic illustration that is not to scale of a direction distribution for a radiation far field, the emission points are distributed in space arbitrarily. The direction distribution is modelled on the celestial sphere as a function X(ω,h) that is dependent on the azimuth angle ω and the height angle h according to a further embodiment of the invention;

[0181] FIG. 9 is a schematic illustration of a lookup table according to a further embodiment of the invention;

[0182] FIG. 10 is a diagram comprising measured functions of representation from a lookup table of the device module of FIG. 4a for CeBr.sub.3—plastic detector pairs at selected asymmetry values A; according to a further embodiment of the invention;

[0183] FIGS. 11a, 11b, 11c, and 11d illustrate four different diagrams of calculated asymmetry curves in the lookup tables of unidirectional and bidirectional detector pairs for the radio nuclides Co-60 and Cs-137, according to a further embodiment of the invention;

[0184] FIG. 12 shows a schematic illustration that is not to scale of a measurement plan with angular positions of a reference source for creating a lookup table for the device model of FIG. 4a according to a further embodiment of the invention;

[0185] FIG. 13 shows a schematic illustration of a calculation scheme for an MLEM method for reconstruction of the direction distribution of a radiation field according to a further embodiment of the invention;

[0186] FIG. 14 shows nine diagrams of measurement direction distributions for radiation fields with one, two, and three point radiation sources. The illustrations are examples for 2-dimensional direction measurement by means of the device model of FIG. 4a, according to a further embodiment of the invention.

DETAILED DESCRIPTION

[0187] In FIG. 1, a schematic illustration that is not to scale of a unidirectional detector pair is illustrated in a 2-dimensional measurement situation. The detector having an atomic number of Z.sub.eff≤30 is indicated by the number 1, wherein the detector having a higher atomic number of Z.sub.eff>30 is indicated by the number 2. The reconstruction of the direction of incidence remains ambiguous. The angle of the incident radiation with respect to the connection line of both detectors can be measured, but not the side, on which the source is located.

[0188] FIG. 2 shows a schematic illustration that is not to scale of a bidirectional detector pair in 2-dimensional measurement situation. Both detectors consist of materials having the same or similar atomic number of Z.sub.eff>30. The indication of the two detectors of a bidirectional pair as detector 1 and detector 2 is set once, and then has to be used consistently in all calculations. As with respect to the unidirectional detector pair in FIG. 1, the reconstruction of the direction of incidence remains ambiguous. The angle of the incident radiation with respect to the connection line of both detectors can be measured, however, not the side on which the source is located.

[0189] FIG. 3 shows the radiation of a source incident on a detector pair 1, 2 in a 2-dimensional measurement situation.

[0190] FIG. 4 shows two device models according to the invention. In the model of FIG. 4a, a central cerbromide detector is surrounded by six plastic detectors circularly. This model is suitable for the 2-dimensional direction measurement. If a further cerbromide detector is arranged below the central cerbromide detector, the model of FIG. 4b is created, which allows for 3-dimensional direction measurements.

[0191] FIG. 5 shows an example for a frequency distribution Y (the measurement data) for the device model of FIG. 4a. The system electronics determines, which detectors have been coincident, and calculates the asymmetry value A=(E2−E1)/(E1+E2) from the coincident energy inputs E1 and E2. The coincidence events are registered according to their detector pair identification number and their asymmetry value A in the frequency distribution Y.

[0192] FIG. 6 illustrates how detector pairs are grouped into the symmetry classes. Four cube-shaped detector are arranged in a square. This detector arrangement has two symmetry classes. The first symmetry class is shown in FIG. 6a and comprises two horizontal and two vertical detector pairs. The two diagonal detector pairs, which can be seen in FIG. 6b, form a further symmetry class.

[0193] FIG. 7 shows an example for a frequency distribution Y (the measurement data) for the device model of FIG. 4b. The measurement data Y has been grouped corresponding to three symmetry classes of the device model of FIG. 4b into three groups. The detector pairs I to VI belong to the symmetry class 1, the detector pairs VII to XII to the symmetry class 2. The 13.sub.th detector pair forms an own symmetry class 3.

[0194] FIG. 8 shows two schematic illustrations of direction distributions for radiation far fields. A radiation far field is characterized completely by its direction dependency. Depending on the measurement situation, there are two types of distributions. If all radiation sources and the measurement device are lying in one plane, the distribution X(ω) along the horizontal circle dependent on the azimuth angle ω is used, as shown in FIG. 8a. The universal direction distribution is the distribution X(ω,h) on the celestial sphere shown in FIG. 8a, which is dependent on the azimuth angle ω and the height angle h. The value X represents the radiation intensity. It may be standardized either to the largest value that has occurred, or may be specified by means of a measuring unit.

[0195] FIG. 9 shows a lookup table schematically. A lookup table is a 2-dimensional frequency distribution, which determines the coincidence counters as a function of the angular distance ϑ of a radiation source to the detector pair axis and a measurement value. Here, the measurement value is the energy asymmetry A=(E2−E1)/(E1+E2) of both energy inputs in a coincidence event.

[0196] FIG. 10 shows various measured functions of representation from a lookup table for CeBr.sub.3—plastic detector pairs at selected asymmetry values A. the functions of representation represent the frequencies according to which a radiation incident at an angle ϑ is registered in certain measurement value intervals A. A lookup table may be considered as a group of functions of representation. The example shows three of the functions recorded by means of a 10 μCi Co-60 source. Coincidence events of both Co-60 emission lines at 1173 keV and 1332 keV are stored together.

[0197] FIG. 11 shows asymmetry curves for lookup tables, which have been calculated for the radio nuclides Co-60 and Cs-137 according to an embodiment of the invention. The asymmetry curves show the areas in a lookup table with the highest frequencies. If there would be no measurement errors, all frequencies other than zero would be lying on the curves shown. Here, it is distinguished between lookup tables of unidirectional and bidirectional detector pairs. Lookup tables of unidirectional detector pairs (FIGS. 11a and 11c) are characterized by having only one prominent asymmetry curve. On the other hand, lookup tables of bidirectional detector pairs (FIG. 11b and FIG. 11d) have two prominent asymmetry curves. For Co-60, the asymmetry curves for both emission lines at 1173 keV and 1332 keV are illustrated; Cs-137 only has one emission line at 662 keV.

[0198] FIG. 12 shows exemplarily the angular positions co of a measurement plan for creating the lookup table for the device model according to an embodiment of the invention shown in FIG. 4a. According to this plan, measurements are carried out at seven angular positions. The source, thereby, always has the same distance with respect to the center of the device. From these seven measurements, a lookup table may be created for the device model. In this example, the lookup table has a step size of 5°. For each angle ϑ in the value range from 0° to 180°, there is at least one detector pair, which has the correct angular distance with respect to the radiation source, and the data set of which can be copied into the respective location within the lookup tables. The measurement plan of FIG. 12 defines the assignments between the angular positions co of the reference source, the detector pairs, and the angular values ϑ of the lookup table.

[0199] FIG. 13 shows a calculation scheme for an MLEM method for the reconstruction of the direction distribution of a radiation field according to an embodiment of the invention. With each iteration step n all calculation steps shown in FIG. 13 are carried out. With a progressing number n of iterations, the MLEM method leads to the direction distribution X, which has the highest probability to have the observed frequency distribution Y. The equations (13) and (16) comprise all calculation steps shown in FIG. 13 in compact form, including forward and backward projection.

[0200] FIG. 14 shows various areas for 2-dimensional direction measurement by means of the device model of FIG. 4a. All measurements have been carried out respectively by 10 μCi Co-60 sources respectively in a distance of 105 cm. The radial measurement value scale is identical in all illustrations, and extends from 0 to 300 nSv rad′ FIGS. 14a, 14b and 14c show measurement results for a Co-60 source at an angle of incidence of 125°. Subsequently, a second Co-60 source has been added at 50° and successively a third Co-60 source has been added at 90°. FIGS. 14d, 14e and 14f show the measurement direction distributions for the 2-sources-radiation field, FIGS. 14g, 14h and 14i for the 3-sources-radiation-field.

[0201] The method according to the invention as well as the device use unidirectional and bidirectional detection processes for the multidimensional direction measurement of gamma radiation. The direction measurement, which partially or completely uses bidirectional detection processes, has been described in the above mentioned embodiments in detail. The bidirectional direction measurement is based on the functional principle shown in FIG. 11, which will be explained briefly.

[0202] In contrast to unidirectional detection processes, which use a function of the connection between a measurement value (here, the asymmetry A) and the scattering angle ϑ for the direction measurement, bidirectional detection processes do not have such a calibration curve A(ϑ). The lookup tables of bidirectional detector pairs show two prominent structures along the asymmetry curves shown in FIGS. 11b and 11d. There are respectively two measurement values A1 and A2 belonging to a certain scattering angle ϑ which are observed at certain frequencies. Nevertheless, a certain scattering angle ϑ can be assigned uniquely to the value pair (A1, A2). Over the entire angular range from 0° to 180°, each value pair (A1, A2) exists exactly one time. It cannot be mistaken with any other value pair (A1, A2) at another angle D. Therefore, the scattering angle ϑ can also be measured by means of a bidirectional detector pair. Statistical reconstruction methods already comprise the functionality required for the direction measurement with bidirectional detector pairs. Methods as MLEM, therefore, are suitable for the processing of the measurement data from the device models according to the invention.

[0203] Here, measurements using the device model of FIG. 4a are presented, which are composed of six 3″×3″ plastic-scintillation detectors and a 3″×3″ cerbromide-scintillation detector. The six plastic detectors form a ring around the central cerbromide detector. The diameter of the detector ring is 25 cm.

[0204] Each one of the six plastic detectors may be combined with the central cerbromide detector into a unidirectional detector pair. In such a detector pair, the radiation is predominantly scattered in the plastic detector, and is absorbed in the cerbromide detector.

[0205] FIG. 14 shows measurement direction distributions for various arrangements of several point radiation sources. All measurements have been carried out by means of respectively 10 μCi Co-60 sources in a distance of respectively 105 cm. At first, a first Co-60 source has been placed at an angle of 125°. The dose rate generated by a 10 μCi Co-60 source in a distance of 105 cm is approximately 100 nSv h.sup.−1. FIG. 14 a), b) and c) show the results of the direction measurements after respectively 10 s, 20 s and 60 s of measurement time. The number of the MLEM iterations has been dynamically coupled to the measurement time. With progressive measurement duration, the direction distribution of the point source becomes steeper and higher. The intensity averaged over all directions constantly amounts to 16 nSv rad.sup.−1h.sup.−1. Integrated over the full angle of 2π rad a dose rate of 2π rad.Math.16 nSv rad.sup.−1h.sup.−1=100 nSv h.sup.−1 is obtained. Subsequently, a second Co-60 source has been added at 50°. The designs are shown in FIG. 14 d), e) and f). The presence of two sources can already be seen at a measurement time of 10 s. The intensity averaged over all directions now amounts to 32 nSv rad.sup.−1h.sup.−1. Finally, a third Co-60 source has been added to the arrangement at 90°. The results in FIG. 14 g), h) and i show that from a measurement time of 20 s on, also three point sources can be resolved separately.

[0206] A methodology has been developed, by means of which the quality of the direction measurement can be formulated as a time specification, from which on a reliable direction measurement is available. The aim of this procedure was to quantitatively evaluate the quality of the direction measurement of a point source depending on the type of the nuclide, the dose rate, and the angular position. As sources, a Co-60, a Cs-137 and a Co-57 of an activity of respectively 10 μCi have been available, respectively.

[0207] Two quality classes have been defined: ±5° and ±10° angular accuracy. At each point of time t during a measurement, the MLEM method provides an estimated value for the direction of the radiation source. The measurements respectively have been run for a certain measurement time and have been repeated 20 times. Now, at each point of time t it can be determined, how many of the measurements fall into the ±5° or ±10° quality class. A confidence level of 90% has been selected. t.sub.90 is defined as that time, from which on at least 18 of the 20 measurements are correct, that means are lying within a ±5° or ±10° interval around the true value. In 90% of all measurements, the specified accuracy has been reached or exceeded after the minimum measurement time.

[0208] The device model of FIG. 4a has a characteristic asymmetry. At first, it has been examined, which influence the device specific detector arrangement has on the minimum measurement times, which are measured at different angles of incidence. The sensitivity of the direction measurement depends on the position of the source with respect to the device. If the source is located towards one of the six ring detectors, longer measurement times are required than in a case in which the source is lying in the center between two ring detectors. In this study, all sources had a constant distance of 1 m with respect to the center of the device. The minimum measurement time t.sub.90 for ±10° accuracy of the 10 μCi Co-60 source was 2.7 s towards the ring detectors, and has shortened to 0.5 s in the middle between two ring detectors. A similar behavior has been observed for Cs-137. Here, the minimum measurement time t.sub.90 for ±10° accuracy was 5.8 s towards the ring detectors, and has shortened to 0.7 s in the middle between two ring detectors.

[0209] Next, it has been examined, how the t.sub.90 minimum measurement time depend on the dose rate. These measurements have been carried out respectively in the direction of the ring detectors. They represent the lower limit of the performance of the device. The distance of the sources has been varied from 1 to 5 meters. The dose rate has been expressed as the local dose rate {dot over (H)}*(10) of the source at the measurement point. Thereby, it has been determined that above 0.05 μSv/h, the product of local dose rate {dot over (H)}*(10) and t.sub.90minimum measurement time is constant.

[0210] With this condition, the t.sub.90 minimum measurement times may be parameterized by the simple analytic expression

[00016]$\begin{array}{cc}{t}_{9.Math.0}=T\left(\mathrm{Nuklid},\sigma \right).Math.{\left(\frac{{\stackrel{.}{H}}^{*}\left(1.Math.0\right)}{\mu .Math.S.Math.v/h}\right)}^{-1}& \left(44\right)\end{array}$

[0211] The parameters T(Nuklid, σ) are device specific parameters, which are defined depending on the type of nuclide and the quality class σ. For the device model with six 3″×3″ plastic detectors and one 3″×3″ cerbromide detector, the parameters have been determined as follows: T(.sup.60Co,±10°)=0.32 s, T(.sup.60Co,±5°)=0.48 s, T(.sup.137Cs,±10°)=0.16 s and T(.sup.137Cs,±5°)=0.24 s.

[0212] It also has been examined, how the direction measurement behaves at very low local dose rates. Also below 0.05 μSv/h, direction measurements of high-quality are possible, the required measurement times are getting longer then, and possibly lie above the value calculated by means of

[00017]$t_{9.Math.0}=T\left(\mathrm{Nuklid},\sigma \right).Math.{\left(\frac{{\stackrel{.}{H}}^{*}\left(10\right)}{\mu .Math.S.Math.v/h}\right)}^{-1}.$

By means of the available device model, it was possible without any difficulties to also measure the direction of the source reliably with an accuracy of ±5° with only few nSv/h. Then, the measurement times are lying in the range of some minutes. For example, the direction measurement for a Co-60 source with 3 nSv/h dose rate after 140 s has an accuracy of ±10° and after 220 s an accuracy of ±5°. A Cs-137 source with 4 nSv/h dose rate at the measurement location could be detected after 55 s with ±10° and after 90 s with ±5°. The direction measurement of a Co-57 source with 5 nSv/h dose rate, after 22 s had an accuracy of ±10° and after 35 s had an accuracy of ±5°.

[0213] If almost static measurement conditions are prevailing, the direction measurement may be carried out in a clocked manner. Then, it is possible, for example, to track the movement of the radiation sources. The dose rate {dot over (H)}*(10), in turn, is a suitable value for controlling the clock frequency. If, for example, the movement of a point source is to be tracked, the clock frequency may be set to the time t.sub.90.

[0214] For an experiment with almost static measurement conditions, the device model of FIG. 4a has been mounted on a rotary plate. The clock frequency of the measurement program has been set to the time t.sub.90. At a rotation speed of 0.2 rounds per minute, the current position of a 10 μCi Co-60 source could be tracked in a distance of 1 m.

[0215] The present invention has been described by means of some specific embodiments of the devices and methods. Persons skilled in the art in the field of radiation detection, of course, are able to modify these embodiments without departing from the principles and the spirit of the invention, the scope of protection of which is defined in the claims and its equivalents.