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Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200
Journal Pre-proof Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsai, Zi-Yi Yang, Shao-Chun Lee, Shiang-Huei Jiang PII:
S0969-8043(19)30964-9
DOI:
https://doi.org/10.1016/j.apradiso.2020.109146
Reference:
ARI 109146
To appear in:
Applied Radiation and Isotopes
Received Date: 28 August 2019 Revised Date:
4 February 2020
Accepted Date: 23 March 2020
Please cite this article as: Tsai, W.-C., Yang, Z.-Y., Lee, S.-C., Jiang, S.-H., Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200, Applied Radiation and Isotopes (2020), doi: https://doi.org/10.1016/j.apradiso.2020.109146. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
CRediT author statement
Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsai: Writing-Original draft preparation, Methodology, Investigation. Zi-Yi Yang: Investigation. Shao-Chun Lee: Investigation. Shiang-Huei Jiang: Methodology, Investigation.
Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsaia,*, Zi-Yi Yanga, Shao-Chun Leea and Shiang-Huei Jiang.a,b, a
Institute of Nuclear Engineering and Science, National Tsing Hua University,
Hsinchu 30013 , Taiwan, R.O.C. b Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013 Taiwan, R.O.C.
*Corresponding author: Wen-Chyi Tsai Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013 Taiwan, R.O.C. Fax: +886-3-5739522 E-mail: [email protected]
Keywords: TLD-200; BNCT beam; Gamma-ray dose; Neutron activation; QA measurement
Abstract In this work TLD-200 (CaF2:Dy) chips were used to measure the gamma-ray doses in a PMMA phantom exposed to the BNCT beam at Tsing Hua Open-pool Reactor (THOR). The self-irradiation component induced by the decay of Dy-165 was corrected. The neutron dose contamination was less than 0.3%. The Dy content in the TLD-200 chip was determined by using the modified absolute calibration method of NAA. The self-irradiation TL signal was also applied for the in situ calibration.
1.
Introduction BNCT beams are mixed neutron and gamma-ray field where epithermal neutrons
predominate the radiation flux. Due to the relatively low reproducibility of the TL reading and the relatively large thermal neutron sensitivity TLD-700 (7LiF:Mg,Ti) was reported to be not suitable for accurate in-phantom BNCT dosimetry (Raaijmakers et al. 1995, 1996). CaF2 TLDs in comparison with 7LiF have very low thermal neutron sensitivity and therefore have obvious advantage for gamma-ray dose measurement in BNCT beams. There are three types of CaF2 with different activators, namely, CaF2:Dy (TLD-200), CaF2:Tm (TLD-300) and CaF2:Mn (TLD-400). When irradiated in BNCT beams all these activators became non-negligibly radioactive owing to their significant capture cross sections. As a consequence, The CaF2 TLD will receive additional internal beta and gamma-ray irradiation through the decay of the radioactive activators. Gambarini et al. (2004, 2008) successfully applied TLD-300 for the gamma-ray dose measurement in thermal and epithermal columns of a nuclear reactor. They found that the self-irradiation does not cause problems for the utilization of TLD-300 in high fluxes of thermal neutrons because its contribution is always less than 3% of the measured dose. Martsolf et al. (1995) applied TLD-400 for the gamma-ray dose measurement in the BNCT irradiation facility consisting of bismuth shielding block in the thermal column of the Oregan State TRIGA Reactor. The effect of thermal neutron induced manganese activation in the TLD chips was generally comparable in magnitude with the response of gamma-ray exposure. The manganese content in their TLD-400 chips was 2.44 ± 0.11 wt%. In our previous work (Tsai et al., 2018) TLD-400 chips with 1.6 % Mn activator were also used for the QA measurement of gamma-ray dose and neutron activation in a PMMA phantom exposed to the BNCT beam at Tsing Hua Open-pool Reactor (THOR). The self-irradiation contribution by 56Mn decay to the measured gamma-ray dose was found ranging from 3 to 8%. In parallel the measured self-irradiation response was applied for an in situ calibration of the irradiated TLD-400 chips. TLD-200 is more sensitive than TLD-400 by a factor of ~2-18 (JRT Associates Inc.), however, because Dy presents a huge thermal neutron cross section (σ=2700 barns) and a short half-life (2.35 h) radioactive product, the self-irradiation effect may cause significant contribution to the measured TL response. In this work we, therefore, intend to investigate the feasibility of the usage of TLD-200 for the measurement of gamma-ray dose and neutron activation in the BNCT beam. 2. Materials and methods 2.1 Experimental setup
Measurements were carried out in a Polymethylmethacrylate (PMMA) phantom with dimensions of 21 × 21 × 21 cm3 in direct contact to the exit of the BNCT beam at THOR as shown in Fig. 1. TLD chips were put along the central axis of the PMMA phantom at depths of 1, 2, 5, and 10 cm behind the beam entrance surface. THOR is a swimming pool type research reactor with a thermal power of 2 MW. The characterization of the BNCT beam at THOR has already well been established (Liu, 2009) and the source strength of different components have been presented in the literature (Tsai et al., 2018). An on-line neutron monitoring system consisting of three miniature fission chambers was installed around the beam collimator to monitor the time variation of the intensity of the neutron beam. 2.2 Annealing and TL readout process for TLD-200 In this study TLD-200 (CaF2:Dy) chips with dimensions of 3.2 x 3.2 x 0.89 mm3 from Thermo Fisher Scientific Inc. were adopted. The UL-320 TLD reader is a direct contact planchet heating type from Rexon Components & TLD Systems, Inc.. All the annealing were carried out on a specially designed hot plate, which can be heated up to 400 °C precisely (Tsai and Jiang, 2011). Before exposure TLD-200 chips on a 2-mm thick stainless steel (SS) storage tray underwent a 400 °C annealing for 3 min and followed a fast cooling by transferring the SS storage tray directly to an aluminum quenching block stored in the frozen drawer of a refrigerator. After exposure TLD-200 chips received a 3-min 130 °C post-irradiation annealing to get rid of the low-temperature signal, which is prone to fading, so as to improve the signal reproducibility. Fig. 2 shows the glow curves of TLD-200 with and without post-irradiation annealing. Also shown in Fig. 2 is the glow curve of TLD-400 for comparison. It can be seen from Fig. 2 that the glow curve after post-irradiation annealing shows a peak at 200 °C and the integral area is significantly reduced. However, the sensitivity is still higher than that of TLD-400, for which the glow curve peaks at 260 °C, by a factor of around 2 to 4. For each readout process, firstly, the reader will read a two-second dark current counts and then a two-second light reference counts. The TLD on the planchet was heated from room temperature at a heating rate of 10 °C/s to 300 °C at 30th second and then kept at 300 °C for 10 seconds. TL signal was collected from 5th to 26th second during the heating process. All the read-out counts were expressed in terms of light reference unit (LRU) that is the net counts of the two-second light reference reading. It is emphasized that each TLD-200 chip was treated as an individual detector with a given ID number in this work to improve the reproducibility of the readout signal. 2.3 Gamma-ray dose measurement
Prior to carrying out gamma-ray dose measurement TLD-200 chips were send to National Ionizing Radiation Standard Laboratory for a conventional calibration using a reference Co-60 gamma-ray source, where they were exposed at 5 mm beneath the surface of a 10 x 10 x 1 cm3 PMMA phantom with a SAD of 100 cm for 3 Gy air kerma. The air kerma to TLD-200 dose conversion factor (DTLD / Kair) for TLD in the phantom was calculated by the MCNPX code (Pelowitz, 2008) to be 0.965; hence each of TLD chips received a 2.895-Gy absorbed dose. Prior to the readout of the TL signals by the TLD reader with high voltage (HV) set at 450 V each TLD-200 chip underwent a 3-min 130 °C post-irradiation annealing with a fast cooling. For QA measurement of gamma-ray dose TLD-200 chips were put at 1-, 2-, 5-, and 10-cm depths along the central axis of the PMMA phantom in direct contact with the exit of the BNCT beam at THOR. There were three chips at each depth. The irradiation time was around 20 to 40 minutes for THOR operated at 1.2 MW. After the irradiation each TLD-200 chips were measured for activities of the induced 165Dy by using an HPGe detector. Following the activity measurement each TLD-200 chips underwent a post-irradiation annealing with fast cooling and then the TL signals were read out with HV set at 450 V at the time denoted by t1. This measured gamma-ray dose, D(t1), contained the self-irradiation contribution originating from the beta decay of the induced 165Dy activity in the TLD chip in addition to the gamma-ray dose in the PMMA phantom. The self-irradiation component can be corrected by applying an additional readout, with HV set at 600 V due to much lower TL signal, of each irradiated TLD chip after the induced 165Dy has completely decayed, normally at one day after the first readout at t1.This secondly read-out dose was denoted by Dn(>t1). Note that after the first readout at t1 each TLD-200 chip underwent a 3-min 400 °C main annealing with fast cooling to clean out the existing signal and get ready to accumulate the self-irradiation signal, while prior to the second readout a post-irradiation annealing with fast cooling was applied to each TLD-200 chip. The gamma-ray dose in the PMMA phantom, Dγ, was then determined by subtracting Dn(>t1) from D(t1) using Eq. (2) derived by Martsolf et al. (1995). D =D
−D >
− 1 ,……………………………………(1)
where λ is the decay constant of 165Dy and ti is the irradiation time. It is worthwhile to note that the secondly read-out dose Dn(>t1) in Eq. (1) can also be acquired from the activity of 165Dy measured using the HPGe detector and the energy deposition in the TLD-200 chip per disintegration of 165Dy calculated by using MCNPX code as will be described in the following section 2.5. The second term in the right hand side of Eq. (1) is the contribution due to self-irradiation and is denoted as DSI.
By applying the counting rate profile recorded by the on-line neutron monitoring system the measured gamma-ray doses were expressed in terms of gamma-ray dose rate and normalized to 1.2 MW reactor power. 2.4 Determination of Dy concentration in TLD-200 The Dy concentration which is the ratio of dysprosium (Dy) mass doped in the TLD-200 chip to the mass of TLD-200 chip and is expressed in terms of percentage was determined by using the modified absolute calibration method of neutron activation analysis (Huang and Jiang, 2017). According to this method the mass of Dy can be determined by the following equation: =
,
(
*
[ !" #
$
%
#
"∆ % ]
∙ +, ………………………………(2) -.
where, mx: mass of the irradiated element, g A: Activity of 165Dy at end of irradiation, Bq λ: decay constant, s-1 ti: irradiation time, s Ra,s: reaction rate per atom per source strength F: fission chamber calibration factor in terms of source strength per counting rate FCj: counting rate of fission chamber at jth time interval ∆t: =ti/J, time interval where the neutron fluence rate is assumed to be constant Ma: atomic mass, g mol-1 θ: isotopic abundance of the target isotope NAV: Avogadro’s number, mol-1 One of the key point of this method is that Ra,s (reaction rate per atom per source strength at the measurement depth in the PMMA phantom ) in equation (2) was acquired from the MCNPX (Pelowitz, 2008) calculation applying the source term of the BNCT beam. The activity A of 165Dy was obtained by measuring the gamma rays emitted from 165Dy decay using an HPGe (high purity Germanium) detector. After the mass of Dy in the TLD-200 has been determined, the TLD chip can be applied to verify the fission chamber calibration factor, F, in the QA measurement of neutron activation for the BNCT beam. 2.5 In situ calibration of TLD-200
After TLD-200 chips in the PMMA phantom being exposed to the BNCT beam for a time period, normally 20 to 40 min, the TL signal of the TLD chip was read out for gamma-ray dose estimation and the activity of each TLD chip was measured using an HPGe detector. After complete decay of 165Dy, normally one day after the activity measurement, the TL signal due to 165Dy decay was read out. This self-irradiation signal can be used not only for the correction of the measured gamma-ray dose during beam irradiation but also for the calibration of the TLD chip in situ. Fig. 3 shows the beta-particle and gamma-ray energy distributions per disintegration of 165Dy taken from RADAR On-Line Decay data (RADAR Home Page, 2003) and IAEA Isotope Browser App (IAEA, 2017), respectively. MCNPX was applied to calculate the energy deposition in TLD-200 chips per 165Dy disintegration for TLD chip on the SS-304 storage stay. The result is listed in Table 1. It is clear to note that the energy deposition is essentially caused by beta particles while the contribution of gamma-rays is less than 0.1%. From the read-out TL signal due to 165Dy decay, the measured 165Dy activity and the energy deposition per disintegration of 165Dy, the TL calibration factor of theTLD-200 chip can be determined in situ. 3.
Results Table 2 lists the measured data of each TLD-200 chips. It is worthwhile to
mention that the self-irradiation percentage contributions ranged from 5.4 to 12.3%. For TLD chips at 1- and 2-cm depths they were about the same. This percentage contributions decreased by ~20% for TLD chips at 5-cm depth and ~50% at 10-cm depth. In order to verify the measured data we have carried out a detailed radiation transport calculation of different dose rate components for the experimental setup by using MCNPX Monte Carlo code and applying the THOR-Y09 source of the BNCT beam. The THOR-Y09 source is a disk surface source located at the exit of the BNCT beam which was derived from a Monte Carlo calculation of the reactor core in couple with the beam shaping assembly for the BNCT beam. The calculated result was then adjusted for energy and angular distribution by deconvolution measurement using multiple activation foils and the indirect neutron radiography (Liu, 2009). The THOR-Y09 source consists of three neutron source terms, namely, thermal neutron, epithermal neutron, and fast neutron, and a gamma-ray source term. The source strengths of different source terms calibrated at 1.2-MW reactor power by using the on-line neutron monitoring system can be found in our previous work (Tsai et al., 2017). The geometry and materials of TLD-200 chips in PMMA phantom were modeled in details. The total number of histories to be run was 108. F6 tally was used to acquire the energy deposition in TLD chips for calculations with each source terms. The calculations were run on a PC with Intel Core i7-4820k, 3.70 GHz CPU. The
calculation times were 925, 874, 525, and 87 min for fast neutron, epithermal neutron, thermal neutron, and gamma-ray source term, respectively. The calculation results are listed in Table 3 and compared with the measurement data. Because TLD-200 does not respond to the neutron ionization with the same efficiency as it does to gamma-ray radiation we have also calculated the effective neutron dose by applying the effective neutron kerma conversion factor for TLD-400 cited in Sandia report (DePriest, 2002). In order to compare with the measured gamma-ray dose rate, the calculation gamma-ray dose rate included the contributions from effective neutron dose and induced and primary gamma-ray doses although the contribution from effective neutron dose was less than 0.3% and can be neglected. It is also noted from the calculation that more than 80% gamma-ray doses were contributed from induced gamma-rays with ~80% for TLD chips at 10-cm depth and ~88% at 5-cm. From the comparison between measurement and calculation in Table 3 it is evident that the agreement was excellent for data at 1- and 2-cm depths, however, there were relatively larger deviations for data at 5- and 10-cm depths. The reason for these larger deviations was suspected to be due to, among others, uncertainties in calibration and positioning. Also shown in Table 3 is the comparison of measured data with in situ calibration factors, which will be listed in the following Table 5, with the calculation ones. It is clear to see that the larger deviations at 5- and 10-cm depths improved themselves to some extent. It is good to compare the investigation results of this work with those of our previous study (Tsai et al., 2017); therefore, the results using TLD-400 were also included in Table 3. The calculated gamma-ray dose rates for TLD-200 were a little bit higher (2 to 5%) than those for TLD-400 with the maximum deviation appearing at the depth of 2 cm. It is evident due to that the atomic number of Dy(66) in TLD-200 is much higher than Mn(25) in TLD-400. It is also noted that the agreement between measurement and calculation was much better for TLD-400 than for TLD-200. In summary, the sensitivities of TLD-200 chips used in this study, even by applying a post-irradiation annealing to get rid of a large portion of lower-temperature TL signals, were higher than those of TLD-400 chips with 1.6% Mn used in our previous study (Tsai et al., 2017) by a factor of ~2 to 4 as mentioned in section 2.2. However, the self-irradiation percentage contribution of TLD-200 chips ,which ranged from ~5 to 12% even for a very small Dy concentration of 0.21%, as presented in the following Table 4, was about 50% higher than that of TLD-400 chips used in our previous work (Tsai et al., 2017), which ranged from ~3 to 8%. The 3-min 130 °C post-irradiation annealing for the TLD-200 chip operation process is indispensable to get rid of the low-temperature TL signal, which is larger than the remaining available signal. This post-irradiation annealing is an extra process which may indeed bring in additional uncertainty and inconvenience. In conclusion, the
usage of TLD-200 chips for the measurement of gamma-ray dose in the mixed neutron and gamma-ray field, such as BNCT beams, is feasible, however, it is not recommended with favor because of the potentially larger uncertainty and more complicated operation process. After irradiation at various depths in the PMMA phantom exposed to the BNCT beam each TLD-200 chips were measured for the activity of induced 165Dy by using an HPGe detector. From the measured activities of each TLD-200 chips and supported by the counting rate profile of the on-line monitor (fission chamber) the contents of Dy in each TLD chips were determined by applying the modified absolute calibration method of neutron activation analysis. The results are listed in Table 4. It can be seen that the Dy concentration in this batch of TLD-200 chips was determined to be 0.21% with a standard deviation of 3.6%. From the measured 165Dy activities of each TLD-200 chips, the energy deposition in the TLD-200 chip per disintegration of 165Dy and the read-out self-irradiation TL signals each TLD-200 chips were calibrated in situ. The in situ calibration factors of each TLD-200 chips were listed in Table 5 and compared with conventional calibration factors. It can be found that the agreement between both calibration factors was in general excellent although a little bit deviation existed for TLD-200 chips at 5- and 10-cm depths. It may need additional measurements to verify this discrepancy or more investigation to reveal the reason. 4.
Conclusion Measurements of gamma-ray doses at 1-, 2-, 5- and 10-cm depths in a PMMA phantom exposed to the BNCT beam at THOR have been made by using TLD-200 chips. The concentration of Dy in the TLD-200 chips was determined by applying the modified absolute calibration method of neutron activation analysis to be 0.21%. In additional to the conventional calibration in a reference 60Co gamma-ray field each TLD-200 chips were calibrated in situ by applying the read-out self-irradiation TL signal, the measured 165Dy activity in TLD-200 chips and the energy deposition in the TLD-200 chip per disintegration of 165Dy. It shows a well agreement between both calibrations. It concluded through this study that the usage of TLD-200 chips for the measurement of gamma-ray dose in the mixed neutron and gamma-ray field, such as BNCT beams, is feasible, however, it is not recommended with favor because of the potentially larger uncertainty and more complicated operation process. Acknowledgements This work was supported by the National Science Council of the Republic of
China under contract numbers NSC92-2745-P-007-002 and NSC94-2212-E-007-069 and -135 and partially supported by the Accelerator Based BNCT project funded by Industrial Technology Research Institute. References Gambarini, G., Klamert, V., Agosteo, S., Birattari, C, Gay, S., Rosi, G., Scolari, L., 2004. Study of a Method Based on TLD Detectors for In-Phantom Dosimetry in BNCT. Radiat. Prot. Dosimetry 110, 631-636. Gambarini, G., Gallivanone, F., Carrara, M., Nagels, S., Vogtlander, L., Hampel, G., Pirola, L. 2008. Study of reliability of TLDs for the photon dose mapping in reactor neutron fields for BNCT. Radiation Measurements 43, 1118-1122. Huang, C. K., Jiang, S. H., 2017. Neutron activation analysis using a modified absolute calibration method. J. Radioanal. Nucl. Chem 311, 1201-1207. IAEA, 2017. IAEA Isotope Browser App. https://play.google.com/store/apps. Liu, Y.-H., 2009. The neutronic characterization of an epithermal neutron beam for boron neutron capture therapy. Doctoral dissertation, National Tsing Hua University, Hsinchu, Taiwan. Martsolf, S. W., Johson, J. E., Vostmyer, C. E. D., Albertson, B. D., Binney, S. E., 1995. Practical Considerations for TLD-400 /700-Based Gamma Ray Dosimetry for BNCT Applications in a High Thermal Neutron Fluence. Health Phys.,69, 966-970. Pelowitz, D.B. (Ed.), 2008. MCNPXTM user’s manual version 2.6.0.Los Alamos National Laboratory, LA-CP-07-1473, April. Raaijmakersa, C. P. J., Verhagen, H. W., Mijnheer, B. J., 1995. Determination Of Dose Components in Phantoms Irradiated with an Epithermal Neutron Beam for Boron Neutron Capture Therapy. Med. Phys. 22, 321-329. Raaijmakersa, C. P. J., Watkins, P. R. D., Nottelman, E. L., Verhagen, H. W., Jansen, J. T. M., Zoetelief, J., Mijnheer, B. J., 1996. The Neutron Sensitivity of Dosimeters Applied to Boron Neutron Capture Therapy. Med. Phys., 23, 1581-1589. Tsai, W.C., Huang, C.K., and Jiang, S.H., 2018. QA Measurement of Gamma-ray Dose and Neutron Activation Using TLD-400 for BNCT Beam. Appl. Radiat. Isot. 137, 73-79.
Figure Captions Fig. 1 TLD chips in PMMA phantom exposed to the BNCT beam at THOR. Fig. 2 Glow curves of TLD-200 (CaF2:Dy) with and without post-irradiation annealing compared with TLD-400 (CaF2:Mn). Fig. 3 Beta and gamma-ray spectra of Dy-165 decay.
Table List Table 1 Energy deposition distribution in TLD-200 chip per disintegration of Dy-165 in the SS304 storage tray. Table 2 Measured data for gamma-ray dose in the PMMA phantom. Table 3 Comparison of gamma-ray dose rate between measurements and calculations. Table 4 Dy content of TLD-200 determined using the modified absolute calibration method of neutron activation analysis. Table 5 In situ calibration factors of TLD-200 chips compared with conventional calibration.
Table 1 Energy deposition distribution in TLD-200 chip per disintegration of Dy-165 in the SS304 storage tray. Depth (mm)
β (MeV)
γ (MeV)
Total (MeV)
Fraction
0.000 - 0.178
5.50E-02a
3.41E-05
5.50E-02
0.19
0.178 - 0.356 0.356 - 0.534 0.534 - 0.712 0.712 - 0.890
6.50E-02 6.71E-02 6.31E-02 4.68E-02
3.52E-05 3.63E-05 3.51E-05 3.02E-05
6.50E-02 6.71E-02 6.32E-02 4.68E-02
0.22 0.23 0.21 0.16
0.000 – 0.890
2.97E-01
1.71E-04
2.97E-01
1.00
a
-2
Read as 5.50 x 10 .
Table 2 Measured data for gamma-ray dose in the PMMA phantom. ID Depth D(t1) Dn(>t1) fa (cm)
DSI/D(t1) Dγ (%) (Gy)
(Gy/s)
γ
(Gy)
(Gy)
5.65
0.45
1.49
12.0
4.98
1.75E-03
5.73
0.46
1.51
12.0
5.04
1.78E-03
203
5.89
0.47
1.53
12.1
5.17
1.82E-03
204
6.94
0.55
1.55
12.3
6.09
2.15E-03
6.62
0.51
1.57
12.2
5.81
2.05E-03
206
6.56
0.51
1.59
12.3
5.76
2.03E-03
207
5.79
0.36
1.61
10.0
5.21
1.84E-03
5.97
0.36
1.63
9.9
5.38
1.90E-03
209
6.06
0.37
1.65
10.1
5.45
1.92E-03
210
1.88
0.06
1.67
5.6
1.77
6.25E-04
1.92
0.06
1.69
5.4
1.82
6.40E-04
1.95
0.06
1.71
5.5
1.84
6.49E-04
201 202
205
208
211 212 a b c
1
2
5
10
The term in brackets of Eq. (1). Read as 1.79 x 10-3. Standard deviation.
γ, ave
(Gy/s) 1.79E-03b (1.6%)c
2.08E-03 (2.5%)
1.89E-03 (1.9%)
6.38E-04 (1.5%)
Table 3 Comparison of gamma-ray dose rate (Gy/s) between measurements and calculations. Depth (cm)
1
2
5
10
TLD-200 (0.21%Dy) 4.38E-05a
1.0%b
3.34E-05
1.1%
1.61E-05
1.6%
4.09E-06
4.3%
0.11
0.5%
0.12
0.5%
0.13
0.8%
0.14
2.3%
Effective neutron
4.82E-06
1.1%
4.00E-06
1.2%
2.09E-06
1.8%
5.72E-07
4.9%
Induced gamma
1.50E-03
1.7%
1.76E-03
1.6%
1.48E-03
1.6%
5.91E-04
2.3%
2.90E-04
0.6%
2.71E-04
0.6%
2.03E-04
0.7%
1.46E-04
0.8%
Calculation total
1.80E-03
2.2%
2.04E-03
2.1%
1.68E-03
2.6%
7.37E-04
5.4%
Measurement(conv.)
1.79E-03
1.6%
2.08E-03
2.5%
1.89E-03
1.9%
6.38E-04
1.5%
Neutron Neutron effectiveness
Primary gamma c
(M-C) / C
-0.6%
Measurement(in situ) (M-C) / C
1.79E-03
2.0% 0.5%
-0.5%
2.06E-03
12.0% 3.6%
1.3%
1.80E-03
-13.4% 1.3%
6.9%
6.82E-04
0.9%
-7.5%
TLD-400 (1.6%Mn) Calculation totalc
1.73E-03
0.4%
1.94E-03
0.4%
1.64E-03
0.4%
7.23E-04
0.6%
Measurement(conv.)
1.71E-03
1.1%
1.98E-03
1.7%
1.68E-03
2.4%
7.39E-04
3.1%
(M-C) / C a
-1.1% -5
1.9%
2.5%
Read as 4.38 x 10 . Standard deviation. c The sum of effective neutron, induced gamma and primary gamma. b
2.2%
Table 4 Dy content of TLD-200 determined using the modified absolute calibration method of neutron activation analysis. ID 201 202
Depth (cm) A (Bq)
Dy(mg)
50967 50417
1.63E-02 1.62E-02
203
51302
204 205 206
1
2
207 208 209 210 211 212
5
10
28.06 28.14
0.21% 0.20%
1.64E-02
28.28
0.21%
62597 57449 57123
1.68E-02 1.54E-02 1.53E-02
28.15 28.23 28.17
0.21% 0.19% 0.19%
40131
1.65E-02
28.34
0.21%
40080 41517
1.65E-02 1.71E-02
27.93 28.34
0.21% 0.21%
7843 7921 7799
1.69E-02 1.71E-02 1.68E-02
27.45 28.20 28.24
0.22% 0.21% 0.21%
Ave. Std. a
TLD (mg) w% a
Read as 1.63 x 10-2.
0.21% 3.6%
Table 5 In situ calibration factors of TLD-200 chips compared with conventional calibration. ID 201 202
Depth (cm) TL (LRU)
A (Bq)
TL Calibration factor (Gy/LRU) In situ
a
Conventional (I-C)/C (%)
6.33E+01 1.85E+04 5.99E-03 6.05E+01 1.83E+04 6.18E-03
5.84E-03 6.19E-03
2.5% -0.1%
203
6.50E+01 1.86E+04 5.82E-03
5.93E-03
-1.8%
204 205 206
2
8.25E+01 2.27E+04 5.62E-03 9.32E+01 2.08E+04 4.56E-03 9.55E+01 2.07E+04 4.43E-03
5.57E-03 4.64E-03 4.50E-03
0.9% -1.7% -1.4%
5
5.91E+01 1.45E+04 5.00E-03 5.05E+01 1.45E+04 5.94E-03 4.65E+01 1.51E+04 6.57E-03
5.20E-03 6.21E-03 6.94E-03
-3.8% -4.4% -5.3%
8.19E+00 2.84E+03 7.28E-03
6.67E-03
9.1%
8.65E+00 2.87E+03 6.77E-03 8.48E+00 2.83E+03 6.80E-03
6.29E-03 6.54E-03
7.6% 3.9%
207 208 209
1
210 211 212 a
10
Read as 6.33 x 101.
Fig. 1 TLD chips in PMMA phantom exposed to the BNCT beam at THOR.
2.0
400 TLD-200 o without 130 C 3 min post-irradiation annealing
1.5
300
1.0
200 TLD-200 with post-irradiation annealing
0.5
100 TLD-400
0.0 0
10
20
30
0 40
Time (S)
Fig. 2 Glow curves of TLD-200 (CaF2:Dy) with and without post-irradiation annealing compared with TLD-400 (CaF2:Mn).
o
500
Temperature C
TL Intensity (LRU)
2.5
0.10
Probability
β
0.05
γ-ray
0.00 0.0
0.5
1.0
Energy (MeV)
Fig. 3 Beta and gamma-ray spectra of Dy-165 decay.
1.5
Highlights Each TLD-200 chip was treated as an individual detector. A 3-min 130°C post-irradiation annealing was applied. The Dy content was determined by the modified absolute calibration method. The measured self-irradiation TL signals have been applied for in situ calibration. The usage of TLD-200 chips is feasible for QA measurement for BNCT beam.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsaia,*, Zi-Yi Yanga, Shao-Chun Leea and Shiang-Huei Jiang.a,b, a
Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013 , Taiwan, R.O.C. b Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013 Taiwan, R.O.C.
S0969-8043(19)30964-9
DOI:
https://doi.org/10.1016/j.apradiso.2020.109146
Reference:
ARI 109146
To appear in:
Applied Radiation and Isotopes
Received Date: 28 August 2019 Revised Date:
4 February 2020
Accepted Date: 23 March 2020
Please cite this article as: Tsai, W.-C., Yang, Z.-Y., Lee, S.-C., Jiang, S.-H., Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200, Applied Radiation and Isotopes (2020), doi: https://doi.org/10.1016/j.apradiso.2020.109146. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
CRediT author statement
Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsai: Writing-Original draft preparation, Methodology, Investigation. Zi-Yi Yang: Investigation. Shao-Chun Lee: Investigation. Shiang-Huei Jiang: Methodology, Investigation.
Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsaia,*, Zi-Yi Yanga, Shao-Chun Leea and Shiang-Huei Jiang.a,b, a
Institute of Nuclear Engineering and Science, National Tsing Hua University,
Hsinchu 30013 , Taiwan, R.O.C. b Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013 Taiwan, R.O.C.
*Corresponding author: Wen-Chyi Tsai Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013 Taiwan, R.O.C. Fax: +886-3-5739522 E-mail: [email protected]
Keywords: TLD-200; BNCT beam; Gamma-ray dose; Neutron activation; QA measurement
Abstract In this work TLD-200 (CaF2:Dy) chips were used to measure the gamma-ray doses in a PMMA phantom exposed to the BNCT beam at Tsing Hua Open-pool Reactor (THOR). The self-irradiation component induced by the decay of Dy-165 was corrected. The neutron dose contamination was less than 0.3%. The Dy content in the TLD-200 chip was determined by using the modified absolute calibration method of NAA. The self-irradiation TL signal was also applied for the in situ calibration.
1.
Introduction BNCT beams are mixed neutron and gamma-ray field where epithermal neutrons
predominate the radiation flux. Due to the relatively low reproducibility of the TL reading and the relatively large thermal neutron sensitivity TLD-700 (7LiF:Mg,Ti) was reported to be not suitable for accurate in-phantom BNCT dosimetry (Raaijmakers et al. 1995, 1996). CaF2 TLDs in comparison with 7LiF have very low thermal neutron sensitivity and therefore have obvious advantage for gamma-ray dose measurement in BNCT beams. There are three types of CaF2 with different activators, namely, CaF2:Dy (TLD-200), CaF2:Tm (TLD-300) and CaF2:Mn (TLD-400). When irradiated in BNCT beams all these activators became non-negligibly radioactive owing to their significant capture cross sections. As a consequence, The CaF2 TLD will receive additional internal beta and gamma-ray irradiation through the decay of the radioactive activators. Gambarini et al. (2004, 2008) successfully applied TLD-300 for the gamma-ray dose measurement in thermal and epithermal columns of a nuclear reactor. They found that the self-irradiation does not cause problems for the utilization of TLD-300 in high fluxes of thermal neutrons because its contribution is always less than 3% of the measured dose. Martsolf et al. (1995) applied TLD-400 for the gamma-ray dose measurement in the BNCT irradiation facility consisting of bismuth shielding block in the thermal column of the Oregan State TRIGA Reactor. The effect of thermal neutron induced manganese activation in the TLD chips was generally comparable in magnitude with the response of gamma-ray exposure. The manganese content in their TLD-400 chips was 2.44 ± 0.11 wt%. In our previous work (Tsai et al., 2018) TLD-400 chips with 1.6 % Mn activator were also used for the QA measurement of gamma-ray dose and neutron activation in a PMMA phantom exposed to the BNCT beam at Tsing Hua Open-pool Reactor (THOR). The self-irradiation contribution by 56Mn decay to the measured gamma-ray dose was found ranging from 3 to 8%. In parallel the measured self-irradiation response was applied for an in situ calibration of the irradiated TLD-400 chips. TLD-200 is more sensitive than TLD-400 by a factor of ~2-18 (JRT Associates Inc.), however, because Dy presents a huge thermal neutron cross section (σ=2700 barns) and a short half-life (2.35 h) radioactive product, the self-irradiation effect may cause significant contribution to the measured TL response. In this work we, therefore, intend to investigate the feasibility of the usage of TLD-200 for the measurement of gamma-ray dose and neutron activation in the BNCT beam. 2. Materials and methods 2.1 Experimental setup
Measurements were carried out in a Polymethylmethacrylate (PMMA) phantom with dimensions of 21 × 21 × 21 cm3 in direct contact to the exit of the BNCT beam at THOR as shown in Fig. 1. TLD chips were put along the central axis of the PMMA phantom at depths of 1, 2, 5, and 10 cm behind the beam entrance surface. THOR is a swimming pool type research reactor with a thermal power of 2 MW. The characterization of the BNCT beam at THOR has already well been established (Liu, 2009) and the source strength of different components have been presented in the literature (Tsai et al., 2018). An on-line neutron monitoring system consisting of three miniature fission chambers was installed around the beam collimator to monitor the time variation of the intensity of the neutron beam. 2.2 Annealing and TL readout process for TLD-200 In this study TLD-200 (CaF2:Dy) chips with dimensions of 3.2 x 3.2 x 0.89 mm3 from Thermo Fisher Scientific Inc. were adopted. The UL-320 TLD reader is a direct contact planchet heating type from Rexon Components & TLD Systems, Inc.. All the annealing were carried out on a specially designed hot plate, which can be heated up to 400 °C precisely (Tsai and Jiang, 2011). Before exposure TLD-200 chips on a 2-mm thick stainless steel (SS) storage tray underwent a 400 °C annealing for 3 min and followed a fast cooling by transferring the SS storage tray directly to an aluminum quenching block stored in the frozen drawer of a refrigerator. After exposure TLD-200 chips received a 3-min 130 °C post-irradiation annealing to get rid of the low-temperature signal, which is prone to fading, so as to improve the signal reproducibility. Fig. 2 shows the glow curves of TLD-200 with and without post-irradiation annealing. Also shown in Fig. 2 is the glow curve of TLD-400 for comparison. It can be seen from Fig. 2 that the glow curve after post-irradiation annealing shows a peak at 200 °C and the integral area is significantly reduced. However, the sensitivity is still higher than that of TLD-400, for which the glow curve peaks at 260 °C, by a factor of around 2 to 4. For each readout process, firstly, the reader will read a two-second dark current counts and then a two-second light reference counts. The TLD on the planchet was heated from room temperature at a heating rate of 10 °C/s to 300 °C at 30th second and then kept at 300 °C for 10 seconds. TL signal was collected from 5th to 26th second during the heating process. All the read-out counts were expressed in terms of light reference unit (LRU) that is the net counts of the two-second light reference reading. It is emphasized that each TLD-200 chip was treated as an individual detector with a given ID number in this work to improve the reproducibility of the readout signal. 2.3 Gamma-ray dose measurement
Prior to carrying out gamma-ray dose measurement TLD-200 chips were send to National Ionizing Radiation Standard Laboratory for a conventional calibration using a reference Co-60 gamma-ray source, where they were exposed at 5 mm beneath the surface of a 10 x 10 x 1 cm3 PMMA phantom with a SAD of 100 cm for 3 Gy air kerma. The air kerma to TLD-200 dose conversion factor (DTLD / Kair) for TLD in the phantom was calculated by the MCNPX code (Pelowitz, 2008) to be 0.965; hence each of TLD chips received a 2.895-Gy absorbed dose. Prior to the readout of the TL signals by the TLD reader with high voltage (HV) set at 450 V each TLD-200 chip underwent a 3-min 130 °C post-irradiation annealing with a fast cooling. For QA measurement of gamma-ray dose TLD-200 chips were put at 1-, 2-, 5-, and 10-cm depths along the central axis of the PMMA phantom in direct contact with the exit of the BNCT beam at THOR. There were three chips at each depth. The irradiation time was around 20 to 40 minutes for THOR operated at 1.2 MW. After the irradiation each TLD-200 chips were measured for activities of the induced 165Dy by using an HPGe detector. Following the activity measurement each TLD-200 chips underwent a post-irradiation annealing with fast cooling and then the TL signals were read out with HV set at 450 V at the time denoted by t1. This measured gamma-ray dose, D(t1), contained the self-irradiation contribution originating from the beta decay of the induced 165Dy activity in the TLD chip in addition to the gamma-ray dose in the PMMA phantom. The self-irradiation component can be corrected by applying an additional readout, with HV set at 600 V due to much lower TL signal, of each irradiated TLD chip after the induced 165Dy has completely decayed, normally at one day after the first readout at t1.This secondly read-out dose was denoted by Dn(>t1). Note that after the first readout at t1 each TLD-200 chip underwent a 3-min 400 °C main annealing with fast cooling to clean out the existing signal and get ready to accumulate the self-irradiation signal, while prior to the second readout a post-irradiation annealing with fast cooling was applied to each TLD-200 chip. The gamma-ray dose in the PMMA phantom, Dγ, was then determined by subtracting Dn(>t1) from D(t1) using Eq. (2) derived by Martsolf et al. (1995). D =D
−D >
− 1 ,……………………………………(1)
where λ is the decay constant of 165Dy and ti is the irradiation time. It is worthwhile to note that the secondly read-out dose Dn(>t1) in Eq. (1) can also be acquired from the activity of 165Dy measured using the HPGe detector and the energy deposition in the TLD-200 chip per disintegration of 165Dy calculated by using MCNPX code as will be described in the following section 2.5. The second term in the right hand side of Eq. (1) is the contribution due to self-irradiation and is denoted as DSI.
By applying the counting rate profile recorded by the on-line neutron monitoring system the measured gamma-ray doses were expressed in terms of gamma-ray dose rate and normalized to 1.2 MW reactor power. 2.4 Determination of Dy concentration in TLD-200 The Dy concentration which is the ratio of dysprosium (Dy) mass doped in the TLD-200 chip to the mass of TLD-200 chip and is expressed in terms of percentage was determined by using the modified absolute calibration method of neutron activation analysis (Huang and Jiang, 2017). According to this method the mass of Dy can be determined by the following equation: =
,
(
*
[ !" #
$
%
#
"∆ % ]
∙ +, ………………………………(2) -.
where, mx: mass of the irradiated element, g A: Activity of 165Dy at end of irradiation, Bq λ: decay constant, s-1 ti: irradiation time, s Ra,s: reaction rate per atom per source strength F: fission chamber calibration factor in terms of source strength per counting rate FCj: counting rate of fission chamber at jth time interval ∆t: =ti/J, time interval where the neutron fluence rate is assumed to be constant Ma: atomic mass, g mol-1 θ: isotopic abundance of the target isotope NAV: Avogadro’s number, mol-1 One of the key point of this method is that Ra,s (reaction rate per atom per source strength at the measurement depth in the PMMA phantom ) in equation (2) was acquired from the MCNPX (Pelowitz, 2008) calculation applying the source term of the BNCT beam. The activity A of 165Dy was obtained by measuring the gamma rays emitted from 165Dy decay using an HPGe (high purity Germanium) detector. After the mass of Dy in the TLD-200 has been determined, the TLD chip can be applied to verify the fission chamber calibration factor, F, in the QA measurement of neutron activation for the BNCT beam. 2.5 In situ calibration of TLD-200
After TLD-200 chips in the PMMA phantom being exposed to the BNCT beam for a time period, normally 20 to 40 min, the TL signal of the TLD chip was read out for gamma-ray dose estimation and the activity of each TLD chip was measured using an HPGe detector. After complete decay of 165Dy, normally one day after the activity measurement, the TL signal due to 165Dy decay was read out. This self-irradiation signal can be used not only for the correction of the measured gamma-ray dose during beam irradiation but also for the calibration of the TLD chip in situ. Fig. 3 shows the beta-particle and gamma-ray energy distributions per disintegration of 165Dy taken from RADAR On-Line Decay data (RADAR Home Page, 2003) and IAEA Isotope Browser App (IAEA, 2017), respectively. MCNPX was applied to calculate the energy deposition in TLD-200 chips per 165Dy disintegration for TLD chip on the SS-304 storage stay. The result is listed in Table 1. It is clear to note that the energy deposition is essentially caused by beta particles while the contribution of gamma-rays is less than 0.1%. From the read-out TL signal due to 165Dy decay, the measured 165Dy activity and the energy deposition per disintegration of 165Dy, the TL calibration factor of theTLD-200 chip can be determined in situ. 3.
Results Table 2 lists the measured data of each TLD-200 chips. It is worthwhile to
mention that the self-irradiation percentage contributions ranged from 5.4 to 12.3%. For TLD chips at 1- and 2-cm depths they were about the same. This percentage contributions decreased by ~20% for TLD chips at 5-cm depth and ~50% at 10-cm depth. In order to verify the measured data we have carried out a detailed radiation transport calculation of different dose rate components for the experimental setup by using MCNPX Monte Carlo code and applying the THOR-Y09 source of the BNCT beam. The THOR-Y09 source is a disk surface source located at the exit of the BNCT beam which was derived from a Monte Carlo calculation of the reactor core in couple with the beam shaping assembly for the BNCT beam. The calculated result was then adjusted for energy and angular distribution by deconvolution measurement using multiple activation foils and the indirect neutron radiography (Liu, 2009). The THOR-Y09 source consists of three neutron source terms, namely, thermal neutron, epithermal neutron, and fast neutron, and a gamma-ray source term. The source strengths of different source terms calibrated at 1.2-MW reactor power by using the on-line neutron monitoring system can be found in our previous work (Tsai et al., 2017). The geometry and materials of TLD-200 chips in PMMA phantom were modeled in details. The total number of histories to be run was 108. F6 tally was used to acquire the energy deposition in TLD chips for calculations with each source terms. The calculations were run on a PC with Intel Core i7-4820k, 3.70 GHz CPU. The
calculation times were 925, 874, 525, and 87 min for fast neutron, epithermal neutron, thermal neutron, and gamma-ray source term, respectively. The calculation results are listed in Table 3 and compared with the measurement data. Because TLD-200 does not respond to the neutron ionization with the same efficiency as it does to gamma-ray radiation we have also calculated the effective neutron dose by applying the effective neutron kerma conversion factor for TLD-400 cited in Sandia report (DePriest, 2002). In order to compare with the measured gamma-ray dose rate, the calculation gamma-ray dose rate included the contributions from effective neutron dose and induced and primary gamma-ray doses although the contribution from effective neutron dose was less than 0.3% and can be neglected. It is also noted from the calculation that more than 80% gamma-ray doses were contributed from induced gamma-rays with ~80% for TLD chips at 10-cm depth and ~88% at 5-cm. From the comparison between measurement and calculation in Table 3 it is evident that the agreement was excellent for data at 1- and 2-cm depths, however, there were relatively larger deviations for data at 5- and 10-cm depths. The reason for these larger deviations was suspected to be due to, among others, uncertainties in calibration and positioning. Also shown in Table 3 is the comparison of measured data with in situ calibration factors, which will be listed in the following Table 5, with the calculation ones. It is clear to see that the larger deviations at 5- and 10-cm depths improved themselves to some extent. It is good to compare the investigation results of this work with those of our previous study (Tsai et al., 2017); therefore, the results using TLD-400 were also included in Table 3. The calculated gamma-ray dose rates for TLD-200 were a little bit higher (2 to 5%) than those for TLD-400 with the maximum deviation appearing at the depth of 2 cm. It is evident due to that the atomic number of Dy(66) in TLD-200 is much higher than Mn(25) in TLD-400. It is also noted that the agreement between measurement and calculation was much better for TLD-400 than for TLD-200. In summary, the sensitivities of TLD-200 chips used in this study, even by applying a post-irradiation annealing to get rid of a large portion of lower-temperature TL signals, were higher than those of TLD-400 chips with 1.6% Mn used in our previous study (Tsai et al., 2017) by a factor of ~2 to 4 as mentioned in section 2.2. However, the self-irradiation percentage contribution of TLD-200 chips ,which ranged from ~5 to 12% even for a very small Dy concentration of 0.21%, as presented in the following Table 4, was about 50% higher than that of TLD-400 chips used in our previous work (Tsai et al., 2017), which ranged from ~3 to 8%. The 3-min 130 °C post-irradiation annealing for the TLD-200 chip operation process is indispensable to get rid of the low-temperature TL signal, which is larger than the remaining available signal. This post-irradiation annealing is an extra process which may indeed bring in additional uncertainty and inconvenience. In conclusion, the
usage of TLD-200 chips for the measurement of gamma-ray dose in the mixed neutron and gamma-ray field, such as BNCT beams, is feasible, however, it is not recommended with favor because of the potentially larger uncertainty and more complicated operation process. After irradiation at various depths in the PMMA phantom exposed to the BNCT beam each TLD-200 chips were measured for the activity of induced 165Dy by using an HPGe detector. From the measured activities of each TLD-200 chips and supported by the counting rate profile of the on-line monitor (fission chamber) the contents of Dy in each TLD chips were determined by applying the modified absolute calibration method of neutron activation analysis. The results are listed in Table 4. It can be seen that the Dy concentration in this batch of TLD-200 chips was determined to be 0.21% with a standard deviation of 3.6%. From the measured 165Dy activities of each TLD-200 chips, the energy deposition in the TLD-200 chip per disintegration of 165Dy and the read-out self-irradiation TL signals each TLD-200 chips were calibrated in situ. The in situ calibration factors of each TLD-200 chips were listed in Table 5 and compared with conventional calibration factors. It can be found that the agreement between both calibration factors was in general excellent although a little bit deviation existed for TLD-200 chips at 5- and 10-cm depths. It may need additional measurements to verify this discrepancy or more investigation to reveal the reason. 4.
Conclusion Measurements of gamma-ray doses at 1-, 2-, 5- and 10-cm depths in a PMMA phantom exposed to the BNCT beam at THOR have been made by using TLD-200 chips. The concentration of Dy in the TLD-200 chips was determined by applying the modified absolute calibration method of neutron activation analysis to be 0.21%. In additional to the conventional calibration in a reference 60Co gamma-ray field each TLD-200 chips were calibrated in situ by applying the read-out self-irradiation TL signal, the measured 165Dy activity in TLD-200 chips and the energy deposition in the TLD-200 chip per disintegration of 165Dy. It shows a well agreement between both calibrations. It concluded through this study that the usage of TLD-200 chips for the measurement of gamma-ray dose in the mixed neutron and gamma-ray field, such as BNCT beams, is feasible, however, it is not recommended with favor because of the potentially larger uncertainty and more complicated operation process. Acknowledgements This work was supported by the National Science Council of the Republic of
China under contract numbers NSC92-2745-P-007-002 and NSC94-2212-E-007-069 and -135 and partially supported by the Accelerator Based BNCT project funded by Industrial Technology Research Institute. References Gambarini, G., Klamert, V., Agosteo, S., Birattari, C, Gay, S., Rosi, G., Scolari, L., 2004. Study of a Method Based on TLD Detectors for In-Phantom Dosimetry in BNCT. Radiat. Prot. Dosimetry 110, 631-636. Gambarini, G., Gallivanone, F., Carrara, M., Nagels, S., Vogtlander, L., Hampel, G., Pirola, L. 2008. Study of reliability of TLDs for the photon dose mapping in reactor neutron fields for BNCT. Radiation Measurements 43, 1118-1122. Huang, C. K., Jiang, S. H., 2017. Neutron activation analysis using a modified absolute calibration method. J. Radioanal. Nucl. Chem 311, 1201-1207. IAEA, 2017. IAEA Isotope Browser App. https://play.google.com/store/apps. Liu, Y.-H., 2009. The neutronic characterization of an epithermal neutron beam for boron neutron capture therapy. Doctoral dissertation, National Tsing Hua University, Hsinchu, Taiwan. Martsolf, S. W., Johson, J. E., Vostmyer, C. E. D., Albertson, B. D., Binney, S. E., 1995. Practical Considerations for TLD-400 /700-Based Gamma Ray Dosimetry for BNCT Applications in a High Thermal Neutron Fluence. Health Phys.,69, 966-970. Pelowitz, D.B. (Ed.), 2008. MCNPXTM user’s manual version 2.6.0.Los Alamos National Laboratory, LA-CP-07-1473, April. Raaijmakersa, C. P. J., Verhagen, H. W., Mijnheer, B. J., 1995. Determination Of Dose Components in Phantoms Irradiated with an Epithermal Neutron Beam for Boron Neutron Capture Therapy. Med. Phys. 22, 321-329. Raaijmakersa, C. P. J., Watkins, P. R. D., Nottelman, E. L., Verhagen, H. W., Jansen, J. T. M., Zoetelief, J., Mijnheer, B. J., 1996. The Neutron Sensitivity of Dosimeters Applied to Boron Neutron Capture Therapy. Med. Phys., 23, 1581-1589. Tsai, W.C., Huang, C.K., and Jiang, S.H., 2018. QA Measurement of Gamma-ray Dose and Neutron Activation Using TLD-400 for BNCT Beam. Appl. Radiat. Isot. 137, 73-79.
Figure Captions Fig. 1 TLD chips in PMMA phantom exposed to the BNCT beam at THOR. Fig. 2 Glow curves of TLD-200 (CaF2:Dy) with and without post-irradiation annealing compared with TLD-400 (CaF2:Mn). Fig. 3 Beta and gamma-ray spectra of Dy-165 decay.
Table List Table 1 Energy deposition distribution in TLD-200 chip per disintegration of Dy-165 in the SS304 storage tray. Table 2 Measured data for gamma-ray dose in the PMMA phantom. Table 3 Comparison of gamma-ray dose rate between measurements and calculations. Table 4 Dy content of TLD-200 determined using the modified absolute calibration method of neutron activation analysis. Table 5 In situ calibration factors of TLD-200 chips compared with conventional calibration.
Table 1 Energy deposition distribution in TLD-200 chip per disintegration of Dy-165 in the SS304 storage tray. Depth (mm)
β (MeV)
γ (MeV)
Total (MeV)
Fraction
0.000 - 0.178
5.50E-02a
3.41E-05
5.50E-02
0.19
0.178 - 0.356 0.356 - 0.534 0.534 - 0.712 0.712 - 0.890
6.50E-02 6.71E-02 6.31E-02 4.68E-02
3.52E-05 3.63E-05 3.51E-05 3.02E-05
6.50E-02 6.71E-02 6.32E-02 4.68E-02
0.22 0.23 0.21 0.16
0.000 – 0.890
2.97E-01
1.71E-04
2.97E-01
1.00
a
-2
Read as 5.50 x 10 .
Table 2 Measured data for gamma-ray dose in the PMMA phantom. ID Depth D(t1) Dn(>t1) fa (cm)
DSI/D(t1) Dγ (%) (Gy)
(Gy/s)
γ
(Gy)
(Gy)
5.65
0.45
1.49
12.0
4.98
1.75E-03
5.73
0.46
1.51
12.0
5.04
1.78E-03
203
5.89
0.47
1.53
12.1
5.17
1.82E-03
204
6.94
0.55
1.55
12.3
6.09
2.15E-03
6.62
0.51
1.57
12.2
5.81
2.05E-03
206
6.56
0.51
1.59
12.3
5.76
2.03E-03
207
5.79
0.36
1.61
10.0
5.21
1.84E-03
5.97
0.36
1.63
9.9
5.38
1.90E-03
209
6.06
0.37
1.65
10.1
5.45
1.92E-03
210
1.88
0.06
1.67
5.6
1.77
6.25E-04
1.92
0.06
1.69
5.4
1.82
6.40E-04
1.95
0.06
1.71
5.5
1.84
6.49E-04
201 202
205
208
211 212 a b c
1
2
5
10
The term in brackets of Eq. (1). Read as 1.79 x 10-3. Standard deviation.
γ, ave
(Gy/s) 1.79E-03b (1.6%)c
2.08E-03 (2.5%)
1.89E-03 (1.9%)
6.38E-04 (1.5%)
Table 3 Comparison of gamma-ray dose rate (Gy/s) between measurements and calculations. Depth (cm)
1
2
5
10
TLD-200 (0.21%Dy) 4.38E-05a
1.0%b
3.34E-05
1.1%
1.61E-05
1.6%
4.09E-06
4.3%
0.11
0.5%
0.12
0.5%
0.13
0.8%
0.14
2.3%
Effective neutron
4.82E-06
1.1%
4.00E-06
1.2%
2.09E-06
1.8%
5.72E-07
4.9%
Induced gamma
1.50E-03
1.7%
1.76E-03
1.6%
1.48E-03
1.6%
5.91E-04
2.3%
2.90E-04
0.6%
2.71E-04
0.6%
2.03E-04
0.7%
1.46E-04
0.8%
Calculation total
1.80E-03
2.2%
2.04E-03
2.1%
1.68E-03
2.6%
7.37E-04
5.4%
Measurement(conv.)
1.79E-03
1.6%
2.08E-03
2.5%
1.89E-03
1.9%
6.38E-04
1.5%
Neutron Neutron effectiveness
Primary gamma c
(M-C) / C
-0.6%
Measurement(in situ) (M-C) / C
1.79E-03
2.0% 0.5%
-0.5%
2.06E-03
12.0% 3.6%
1.3%
1.80E-03
-13.4% 1.3%
6.9%
6.82E-04
0.9%
-7.5%
TLD-400 (1.6%Mn) Calculation totalc
1.73E-03
0.4%
1.94E-03
0.4%
1.64E-03
0.4%
7.23E-04
0.6%
Measurement(conv.)
1.71E-03
1.1%
1.98E-03
1.7%
1.68E-03
2.4%
7.39E-04
3.1%
(M-C) / C a
-1.1% -5
1.9%
2.5%
Read as 4.38 x 10 . Standard deviation. c The sum of effective neutron, induced gamma and primary gamma. b
2.2%
Table 4 Dy content of TLD-200 determined using the modified absolute calibration method of neutron activation analysis. ID 201 202
Depth (cm) A (Bq)
Dy(mg)
50967 50417
1.63E-02 1.62E-02
203
51302
204 205 206
1
2
207 208 209 210 211 212
5
10
28.06 28.14
0.21% 0.20%
1.64E-02
28.28
0.21%
62597 57449 57123
1.68E-02 1.54E-02 1.53E-02
28.15 28.23 28.17
0.21% 0.19% 0.19%
40131
1.65E-02
28.34
0.21%
40080 41517
1.65E-02 1.71E-02
27.93 28.34
0.21% 0.21%
7843 7921 7799
1.69E-02 1.71E-02 1.68E-02
27.45 28.20 28.24
0.22% 0.21% 0.21%
Ave. Std. a
TLD (mg) w% a
Read as 1.63 x 10-2.
0.21% 3.6%
Table 5 In situ calibration factors of TLD-200 chips compared with conventional calibration. ID 201 202
Depth (cm) TL (LRU)
A (Bq)
TL Calibration factor (Gy/LRU) In situ
a
Conventional (I-C)/C (%)
6.33E+01 1.85E+04 5.99E-03 6.05E+01 1.83E+04 6.18E-03
5.84E-03 6.19E-03
2.5% -0.1%
203
6.50E+01 1.86E+04 5.82E-03
5.93E-03
-1.8%
204 205 206
2
8.25E+01 2.27E+04 5.62E-03 9.32E+01 2.08E+04 4.56E-03 9.55E+01 2.07E+04 4.43E-03
5.57E-03 4.64E-03 4.50E-03
0.9% -1.7% -1.4%
5
5.91E+01 1.45E+04 5.00E-03 5.05E+01 1.45E+04 5.94E-03 4.65E+01 1.51E+04 6.57E-03
5.20E-03 6.21E-03 6.94E-03
-3.8% -4.4% -5.3%
8.19E+00 2.84E+03 7.28E-03
6.67E-03
9.1%
8.65E+00 2.87E+03 6.77E-03 8.48E+00 2.83E+03 6.80E-03
6.29E-03 6.54E-03
7.6% 3.9%
207 208 209
1
210 211 212 a
10
Read as 6.33 x 101.
Fig. 1 TLD chips in PMMA phantom exposed to the BNCT beam at THOR.
2.0
400 TLD-200 o without 130 C 3 min post-irradiation annealing
1.5
300
1.0
200 TLD-200 with post-irradiation annealing
0.5
100 TLD-400
0.0 0
10
20
30
0 40
Time (S)
Fig. 2 Glow curves of TLD-200 (CaF2:Dy) with and without post-irradiation annealing compared with TLD-400 (CaF2:Mn).
o
500
Temperature C
TL Intensity (LRU)
2.5
0.10
Probability
β
0.05
γ-ray
0.00 0.0
0.5
1.0
Energy (MeV)
Fig. 3 Beta and gamma-ray spectra of Dy-165 decay.
1.5
Highlights Each TLD-200 chip was treated as an individual detector. A 3-min 130°C post-irradiation annealing was applied. The Dy content was determined by the modified absolute calibration method. The measured self-irradiation TL signals have been applied for in situ calibration. The usage of TLD-200 chips is feasible for QA measurement for BNCT beam.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Measurement of gamma-ray dose and neutron activation in BNCT beams using TLD-200 Wen-Chyi Tsaia,*, Zi-Yi Yanga, Shao-Chun Leea and Shiang-Huei Jiang.a,b, a
Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013 , Taiwan, R.O.C. b Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013 Taiwan, R.O.C.