NEUTRON AND GAMMA MULTI-ELEMENT ALANINE DOSIMETER HOLDER

Abstract

An alanine criticality accident dosimeter comprising at least two alanine pellets and at least two alanine pellets enhanced with moderator, and each pair of alanine pellet and enhanced alanine pellet is covered with a neutron filer or a photon filter. An improved method to determine radiation dose of a subject after an accident by measuring alanine pellets from a criticality accident dosimeter with neutron sensitivity, and improved discrimination between photon and neutron dose contributions.

Claims

1) A criticality accident dosimeter for ascertaining radiation dosage comprising a) at least four alanine dosimeter pellets; and b) a dosimeter holder, which comprises (i) a front cover and a back cover joined by one or more fasteners; (ii) a plurality of supports on said back cover, wherein each support is adapted to hold one said alanine pellet; and (iii) one or more filter affixed on said front cover, wherein each of said filter is adapted to cover at least two of said alanine pellets when the front and back cover of said holder is closed, whereas said filter enhances sensitivity of said alanine pellet.

2) The criticality accident dosimeter claim 1, wherein said alanine pellet comprising alanine and a binder.

3) The criticality accident dosimeter of claim 2, wherein the alanine is in crystalline form.

4) The criticality accident dosimeter of claim 1, wherein the filter is a cadmium filter or .sup.6Li filter.

5) The criticality accident dosimeter of claim 4, wherein said cadmium filter is a cadmium dosimetric pellet/chip.

6) The criticality accident dosimeter of claim 4, wherein said .sup.6Li filter is a .sup.6Li dosimetric pellet/chip.

7) A method for determining the radiation dose by Electron Paramagnetic Resonance (EPR) dosimetry using alanine criticality accident dosimeter, comprising a) providing a subject with a criticality accident dosimeter holder containing at least two alanine pellets and at least two alanine pellets covered by a cadmium filter or .sup.6Li filter; b) measuring the EPR spectrum of alanine pellets contained in said holder after a radiation exposure; and c) determining radiation dose of said alanine pellet with the cadmium filter or .sup.6Li filter and radiation dose of said alanine pellet without the cadmium filter or .sup.6Li filter using measurements from step b). c) calculating radiation dose exposed to said subject.

Description

DETAILED DESCRIPTION OF THE FIGURES

[0029] FIG. 1 Diagram of the alanine dosimeter holder.

[0030] FIG. 2 Photo of the alanine dosimeter holder prototype.

[0031] FIG. 3 Enhanced Alanine pellet made with alanine pellet and .sup.6LiF chip/pellet.

[0032] FIG. 4 Two prototype dosimeter holder designs.

[0033] FIG. 5 shows Neutron irradiation setup: prototype holders are placed on the PMMA slab phantom.

[0034] FIG. 6 Comparison of EPR signals from neutron-irradiated bare alanine (black line on top), alanine pellets irradiated behind .sup.6LiF single pellet (red line) and alanine pellets irradiated behind cadmium plate (blue line).

[0035] FIG. 7 EPR signals of neutron-irradiated alanine pellets behind .sup.6LiF filters with different thicknesses.

[0036] FIG. 8 EPR signals of gamma-irradiated alanine pellets behind .sup.6LiF and cadmium filters (top to bottom: no filter, 1.sup.6LiF filter and cadmium filter).

[0037] FIG. 9 Comparison of the EPR response from alanine pellets behind different filters during irradiation with BT-2 (neutron) and .sup.60Co (gamma). The EPR responses are normalized to the same nominal dose of 10 Gy.

DETAILED DESCRIPTIONS OF THE INVENTION

[0038] The present invention aims to improve criticality accident dosimetry. The criticality accident dosimeter of present invention provides better neutron sensitivity, and is capable of discriminate photon and neutron dose contributions without requiring special Gd.sub.2O.sub.3 doped alanine pellets or additional photon dosimeter.

[0039] Instead of using two types of alanine pellets with different composition (e.g. with and without Gd.sub.2O.sub.3), this invention proposes a new design of criticality accident dosimeter based on the same dosimetric material (alanine) and the same method of readout, e.g. EPR. In order to enhance the photon-neutron differentiation of alanine, filters made of material that is capable of improving alanine's neutron sensitivity are placed in front of standard alanine pellets.

[0040] It is well known that some materials, such as cadmium and .sup.6Li, can convert neutrons into other types of particles to which alanine has higher sensitivity. A number of materials exist with high neutron-capture cross sections. The capture of neutron then directly or indirectly provides particles that subsequently produce radiation-induced radicals in alanine. A partial list of these types of nuclear reactions is given below:

n+.sup.6Li.fwdarw.α+.sup.3H  (1)

n+.sup.113Cd.fwdarw..sup.114Cd*.fwdarw..sup.114Cd+γ  (2)

n+.sup.157Gd.fwdarw..sup.158Cd*.fwdarw..sup.158Cd+γ  (3)

[0041] In case of a .sup.6Li filter, the neutron irradiation will generate a particles and tritium ions as a result of the reaction (1), which will produce radiation-induced radicals in alanine pellets placed behind the filter. In case of a cadmium filter, neutrons will produce a photon component (see nuclear reactions (2) and (3)) contributing to the dose measured by the alanine pellet behind the cadimium filter. Although reactions (1-3) are well known in the field. No one has applied these reactions in NAD design. Without building a prototype, and experimentally measuring the effect of the neutron sensitivity enhancement, it was unclear if this dosimetry design is feasible or sensitive enough for NAD application.

Holder Design

[0042] A criticality accident dosimeter of the present invention, comprises at least four alanine pellets, of which at least two of the alanine dosimeter pellets are placed behind a filter; wherein said filter enhances the sensitivities of said alanine dosimeter pellets covered by the filter. The filter may be made of .sup.6Li or cadmium. The personal criticality dosimeter of the present invention upon exposure to ionizing radiation, can produces radicals that remain stable for at least a year.

[0043] In one embodiment, a personal criticality accident dosimeter of present invention for ascertaining radiation dosage comprising: at least four alanine dosimeter pellets, and a dosimeter holder, which comprises a plurality of supports, each designed to hold one said alanine dosimeter pellet and a .sup.6Li filter or a cadmium filter, wherein said .sup.6Li filter or a cadmium filter covers at least two of said alanine pellets.

[0044] Referring to FIG. 1 and FIG. 2, a dosimeter holder 20 of the present invention, comprises a front cover 21 and a back cover 22, which are joined by hinges 23 or other fasteners so that the dosimeter holder 20 may be opened or closed as desired. A fastener, such as a clip, a clasp or a hasp 28, for securing the dosimeter to a person's clothing, is also provided on the outer rim of the holder 20. The body of the holder 20 can be made from Plexiglas to support tissue equivalency of alanine. The inside surface of said back cover 22 comprises at least four of mounting supports 29, each adapted to hold and affix one alanine pellet 24. One or more filters 25 or 26 are affixed to the inside surface of said front cover 21, wherein said filter 25/26 is adapted to be placed in front of at least two of the alanine pellets when the front cover and back cover of the holder is closed. The addition of .sup.6LiF (TLD-600H) 26 filter or a cadmium filter 25 to the alanine dosimeter pellets 24 enhances the sensitivities of said alanine dosimeter pellet. The other filters with similar properties to discriminate photon exposure against neutron exposure can also be used.

[0045] In another embodiment, a personal criticality accident dosimeter of present invention for ascertaining radiation dosage comprises: at least two standard alanine dosimeter pellets 24, and at least two enhanced alanine dosimeter pellets 28, wherein said enhanced dosimeter pellets is made by adding one or more filter chip/pellets 27 to an alanine dosimeter pellet as shown in FIG. 3 or FIG. 4. As shown in FIG. 3, an enhanced alanine pellet 28 is made by sandwich an alanine dosimeter pellet 24 between two .sup.6Li filter chips or gadolinium filter chips 27. Dosimetric chip/pellet 27 made of .sup.6LiF: Mg,Cu,P (TLD-600H™, THERMO SCIENTIFIC™, Waltham, Mass.) or cadmium can be used for this purpose. These dosimetric chips/pellets are commercially available, and typically have a diameter of 4 mm, which is exactly equal to the diameter of a standard alanine pellet.

Example 1: Prototype Dosimeter Holders

[0046] Two types of dosimeter holder have been designed and built (as shown in FIG. 4).

[0047] Type 1 dosimeter holder is designed to hold four standard alanine pellets (size 4×2.4 mm, AERIAL®, Illkirch—France) without any filtration and another four standard alanine pellets placed behind a .sup.6LiF filter when the holder is closed. Standard .sup.6LiF pellets (size 3.6×0.015 mm) were used as .sup.6LiF filter. These .sup.6LiF pellets were purchased from Thermo Eberline LLC (Franklin, Mass.). In order to check the effect of .sup.6LiF filter thickness on sensitivities of dosimeter, .sup.6LiF filter thickness (e.g. three, two and one .sup.6LiF chips) are ted.

[0048] Type 2 dosimeter holder is designed to hold four alanine pellets without any filtration and another four alanine pellets behind a cadmium filter (44×15×0.5 mm).

Example 2: Irradiation Testing

Irradiation Set up:

[0049] Neutron Irradiation: 20 MW research reactor of the National Institute of Standards and Technology (NIST) was utilized for the testing of the prototype holders. The reactor is D.sub.2O cooled and moderated. The core is comprised of thirty enriched-uranium fuel elements of a unique, split-core design. It utilizes low-energy neutrons, which are often described as thermal and cold. Reactor neutron imaging station BT2 was used for the holders' irradiation. The irradiation was done on the standard ANSI PMMA phantom (as shown in FIG. 5). Four holders with alanine dosimeters were irradiated simultaneously. BT-2 neutron flux and energy spectrum can be changed using different combinations of apertures and beams (see Appendix A). Flux 1.38E+07 n/(cm2 sec) was used for irradiation. Neutron dose rates were calculated by Monte Carlo N-Particle Transport Code (MCNP) separately for fast and slow neutrons using 1 eV as the cutoff. According to MCNP 1.38E+07 n/(cm2 sec) flux has about 20% of thermal neutrons (<1 eV). Calculated neutron dose rates (Table 2) were used for comparison of alanine response in the designed holders to the neutron irradiation response of bare alanine to .sup.60Co. Dosimeter prototypes with various .sup.6LiF moderator thickness (three, two and one .sup.6LiF discs) are irradiated at high dose. Compare the intensity and shapes of the resulting signals. Determine the best moderator to enhance its neutron sensitivity.

TABLE-US-00002 TABLE 2 MCNP calculated dose rates used for testing. Type of exposure Dose rate, mrem/s Fast neutron 62.639 Thermal neutron 14.082 Gamma 0.5694

[0050] Gamma Irradiation: an AFRRI Co60 source was utilized for the testing of the prototype holders with similar set up.

EPR Measurements

[0051] A Bruker ELEXSYS 500 (Bruker BioSpin, Billerica, Mass.) spectrometer equipped with a super-high-Q resonator ER 4123 SHQE in the X-band (9-10 GHz) was used for the measurements. All four pellets irradiated under the same conditions were put together into a sample tube and measured. Table 3 shows the recorded settings. The EPR measurement (dose evaluation) takes only 3.5 minutes (Table 3). For statistical purposes, the measurement of each sample was performed for five times.

TABLE-US-00003 TABLE 3 EPR recording conditions - Spectrometer settings Parameter Value HF modulation 100 kHz Amplitude of HF modulation 3 G Microwave power 2 mW Receiver gain 20000 Time constant 1.28 ms Converse time 5.12 ms Number of points 1024 Sweep time 5.243 s Number of scans 40 Total recording time 3.5 min Central field 3480 G Sweep field 50 G

Dose Algorithm

[0052] According to Trompier et al., 2008 the total EPR radiation response of alanine at mixed neutron-photon irradiation in terms of its sensitivity to .sup.60Co gamma rays, R can be generally described by the following formula:

R=kD.sub.n+hD.sub.γ  (1)

[0053] Where k is the sensitivity of alanine to neutrons relative to .sup.60Co gamma rays, h is alanine sensitivity to the photons relative to .sup.60Co gamma rays. Both sensitivities are measured in terms of the EPR radiation response per dose unit. D.sub.n and D.sub.γ are the neutron and photon absorbed doses in tissue. In case of alanine use without different filtration, it is impossible to determine photon and neutron doses separately. Therefore 2008 Trompier et al. 2008 additionally used thermoluminescent dosimetry (Al.sub.2O.sub.3) to measure photon dose alone. If there is a holder with alanine pellets behind different filters then Eq. (1) can be written for each filtration separately and neutron and photon doses can be determined without the use of additional dosimeters, e.g. TLD. In case of holder prototype using a cadmium filter the system of two equations can be written in the following way:

[00001]$\begin{array}{cc}{R}_{\mathrm{Cd}}=k_{\mathrm{Cd}}{D}_n+h_{\mathrm{Cd}}{D}_{\gamma }& \left(3\right)\\ R_{\mathrm{bare}}=k_{\mathrm{bare}}{D}_n+h_{\mathrm{bare}}{D}_{\gamma }\mathrm{or}& \left(4\right)\\ D_n=\frac{R_{\mathrm{Cd}}-h_{\mathrm{Cd}}{D}_{\gamma }}{k_{\mathrm{Cd}}}& \left(5\right)\\ D_{\gamma }=\frac{R_{\mathrm{bare}}-k_{\mathrm{bare}}{D}_n}{h_{\mathrm{bare}}}& \left(6\right)\end{array}$

D.sub.γ in the first equation can be substituted by the second equitation:

[00002]$\begin{array}{cc}{D}_n=\frac{R_{\mathrm{Cd}}}{k_{\mathrm{Cd}}}-\frac{h_{\mathrm{Cd}}}{k_{\mathrm{Cd}}}\left(\frac{R_{\mathrm{bare}}-k_{\mathrm{bare}}{D}_n}{h_{\mathrm{bare}}}\right)& \left(7\right)\end{array}$

This equation can be solved relatively to D.sub.n:

[00003]$\begin{array}{cc}{D}_n=\frac{R_{\mathrm{Cd}}{h}_{\mathrm{bare}}-h_{\mathrm{Cd}}{R}_{\mathrm{bare}}}{k_{\mathrm{Cd}}{h}_{\mathrm{bare}}-h_{\mathrm{Cd}}{k}_{\mathrm{bare}}}& \left(8\right)\\ D_{\gamma }=\frac{R_{\mathrm{bare}}{k}_{\mathrm{Cd}}-h_{\mathrm{bare}}{R}_{\mathrm{Cd}}}{k_{\mathrm{Cd}}{h}_{\mathrm{bare}}-h_{\mathrm{Cd}}{h}_{\mathrm{bare}}}& \left(9\right)\end{array}$

[0054] Eqs. (8) and (9) allow to calculate neutron and gamma absorbed doses if the proposed prototype with a cadmium filter was used. Data (k, h, and R) from Tables 3 and 4 allow to verify the correctness of the proposed equations i.e. that the calculated neutron and photon doses are equal to the actually delivered neutron and photon doses.

[0055] In the case of the prototype with a .sup.6LiF filter similar equations (with the change of Cd to Li) can be proposed for neutron and gamma dose calculations, as follows:

[00004]$\begin{array}{cc}{D}_n=\frac{R_{\mathrm{Li}}{h}_{\mathrm{bare}}-h_{\mathrm{Li}}{R}_{\mathrm{bare}}}{k_{\mathrm{Li}}{h}_{\mathrm{bare}}-h_{\mathrm{Li}}{k}_{\mathrm{bare}}}& \left(10\right)\\ D_{\gamma }=\frac{R_{\mathrm{bare}}{k}_{\mathrm{Li}}-h_{\mathrm{bare}}{R}_{\mathrm{Li}}}{k_{\mathrm{Li}}{h}_{\mathrm{bare}}-h_{\mathrm{Li}}{h}_{\mathrm{bare}}}& \left(11\right)\end{array}$

Results of Porotype Dosimeter Holders' Testing

[0056] FIG. 6 shows EPR signals of the simultaneously irradiated (with the same dose) bare alanine (black line on top), alanine pellets irradiated behind a .sup.6LiF single pellet (red line) and alanine pellets irradiated behind a cadmium plate (blue line). It is obvious that both proposed filters (.sup.6LiF and Cd) result in a significant increase of alanine sensitivity to neutron exposure. FIG. 7 shows the effect of the thickness of the .sup.6LiF filter on the EPR spectrum of alanine irradiated with neutrons. One can see that the effect of the sensitivity enhancement is reduced with .sup.6LiF thickness increases. This is probably due to the higher absorption of the generated α-particles inside thicker filters. Filter with one .sup.6LiF chip offers the best sensitivity enhancement.

[0057] The simulated neutron flux spectrum, neutron dose rate spectrum, core gamma flux spectrum, and core gamma dose rate spectrum (with the cooled 10 cm bismuth filter in the beam) are shown in FIG. 7, FIG. 8, and FIG. 9 respectively. The lower figures show the corresponding cumulative distribution functions for which the high energy limits give the energy-integrated quantities. The neutron and core gamma dose rate curves in FIG. 9 were obtained from modified flux tallies in which NCRP dose rate factors extrapolated to each energy bin center were applied. Table 4 provides absolute numbers of the neutron sensitivity increase for different filters tested. One can see from Table 4 that the cadmium filter provides almost a factor of five neutron sensitivity increase, and the .sup.6LiF effect is only slightly less. It is important to note that this effect for both filters tested is more significant than from doping alanine with Gd.sub.2O.sub.3, proposed by Marrale et al., 2015. Table 5 shows the result of the .sup.60Co irradiation

TABLE-US-00004 TABLE 4 Summary of the BT-2 irradiation. The contribution of the photon irradiation to alanine response was considered equal to 0 at k calculations. In reality, its contribution to the total (neutron + photon) dose was 1.6%. Slow Fast Total Photon EPR neutron neutron neutron Dose, Response, Filtration dose, Gy dose, Gy dose, Gy Gy a.u. k Double .sup.6LiF 1.487 11.113 12.600 0.205 3.96 ± 0.03 0.31 k.sub.2Li No 1.487 11.113 12.600 0.205 1.04 ± 0.01 0.08 k.sub.bare Single .sup.6LiF 1.487 11.113 12.600 0.205 4.48 ± 0.03 0.36 k.sub.Li No 1.487 11.113 12.600 0.205 1.05 ± 0.03 0.08 k.sub.bare Triple .sup.6LiF 1.487 11.113 12.600 0.205 3.28 ± 0.05 0.26 k.sub.2Li Cadmium 1.487 11.113 12.600 0.205 4.93 ± 0.02 0.39 k.sub.Cd Cadmium 1.487 11.113 12.600 0.205 5.18 ± 0.02 0.41 k.sub.Cd No 1.487 11.113 12.600 0.205 1.07 ± 0.01 0.09 k.sub.bare

TABLE-US-00005 TABLE 5 Summary of the .sup.60Co irradiation Photon Filtration Dose, Gy EPR Response, a.u. h, a.u./Gy Double .sup.6LiF 20 7.16 ± 0.08 0.36 h.sub.2Li No 20 7.36 ± 0.05 0.37 h.sub.bare Single .sup.6LiF 20 7.11 ± 0.04 0.36 h.sub.Li No 20 7.16 ± 0.02 0.36 h.sub.bare No 20 7.28 ± 0.06 0.35 h.sub.bare Cadmium 20 6.18 ± 0.03 0.31 h.sub.Cd Cadmium 20 6.22 ± 0.03 0.31 h.sub.Cd No 20 7.26 ± 0.03 0.36 h.sub.bare

[0058] Thus neutron and photon doses for any photon-neutron mixture can be calculated by measuring the EPR response of alanine pellets irradiated in the proposed holders and using the determined parameters h and k.

Conclusions

[0059] The experiments on prototype dosimeter holders demonstrated that the proposed cadmium and .sup.6LiF filters (convertors) increase the sensitivity of alanine to neutron irradiation by almost a factor of five. The latter makes alanine even more sensitive to neutrons than to photons. Normally its sensitivity to neutrons is significantly lower than to photons.

Example 3: Method of Using Inventive NAD to Determine Radiation Dose

[0060] Proposed and tested holder designs in combination with the developed dose algorithm allow to measure neutron and photon doses separately using only EPR measurements in alanine. This makes the entire dose measurement process fast (<10 min) and requiring only one type of dose readout equipment (EPR).

[0061] A method for determining the radiation dose by Electron Paramagnetic Resonance (EPR) dosimetry using alanine criticality accident dosimeter, comprises of 1) providing a subject with a criticality accident dosimeter holder containing at least two alanine pellets and at least two alanine pellets covered by a cadmium filter or a .sup.6Li filter; 2) measuring the EPR spectrum of alanine pellets contained in said holder after a radiation exposure; and c) determining neutron and photon radiation doses using EPR measurements from step 2; c) calculating radiation dose exposed to said subject.

REFERENCES

[0062] American National Standard ANSl/HPS N13.3-2013. Dosimetry for Criticality Accidents. American National Standards Institute, Inc. Health Physics Society, Mclean, Va. [0063] Baffa, O., Kinoshita, A., 2014. Clinical applications of alanine/electron spin resonance dosimetry. Radiat. Environ. Biophys., 1-8. [0064] Hayes R. B., Haskell E. H, Wieser A., Romanyukha A. A., Hardy B. L. and Barrus J. K. 2000. Assessment of an alanine EPR dosimetry technique with enhanced precision and accuracy. Nuclear Instruments & Methods in Physics Research A, 440, 453-461. [0065] Regulla, D. F., 2005. ESR spectrometry: a future-oriented tool for dosimetry and dating. Appl. Radiat. lsot. 62, 117-127. [0066] ISO/ASTM 51607-2013, Standard Practice for Use of the Alanine-EPR Dosimetry System, ASTM International, West Conshohocken, Pa., 2012, www.astm.org [0067] Marrale M., Schmitz T., Gallo S., Hampel G., Longo A., Panzeca S., Tranchina L. 2015. Comparison of EPR response of alanine and Gd.sub.2O.sub.3-alanine dosimeters exposed to TRIGA Mainz reactor. Appl. Radiat. Isotop. 106, 116-120 [0068] Trompier, F., Huet, C., Medioni, R., Robbes, I., Asselineau, B., 2008. Dosimetry of the mixed field irradiation facility CALIBAN. Radiat. Meas. 43 (2), 1077-1080.