- Email: [email protected]
On accelerator-based neutron sources and neutron field characterization with low energy neutron spectrometer based on position sensitive 3He counter
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) S288–S291
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
On accelerator-based neutron sources and neutron field characterization with low energy neutron spectrometer based on position sensitive 3He counter I. Murata a,, H. Miyamaru a, I. Kato b, Y. Mori c a
Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Japan Department of Oral and Maxillofacial Surgery II, Graduate School of Dentistry, Osaka University, Japan c Kyoto University Research Reactor Institute, Kyoto University, Japan b
a r t i c l e in f o
Keywords: BNCT Accelerator-based neutron source Low energy neutron spectrometer Position sensitive proportional counter 3 He detector
a b s t r a c t The development of new neutron sources for BNCT applications, based on particle accelerators is currently underway all over the world. Though nuclear reactors were used for a long time as the only neutron source available having the requested flux levels, the accelerator-based ones have recently been investigated on the other hand due to its easy-to-use and acceptable performances. However, when using an accelerator, various secondary particles would be emitted which forms a troublesome background. Moreover, the neutrons produced have usually an energy spectrum somewhat different from the requested one and thus should be largely moderated. An additional issue to be taken into account is the patient positioning, which should be close to the neutron source, in order to take advantage of a neutron flux level high enough to limit the BNCT treatment time within 1 h. This implies that, inside a relatively narrow space, neutrons should be moderated, while unnecessary secondary particles should be shielded. Considering that a background-free neutron field from an acceleratordriven neutron source dedicated to BNCT application is generally difficult to be provided, the characterization of such a neutron field will have to be clearly assessed. In the present study, a low energy neutron spectrometer has been thus designed and is now being developed to measure the accelerator-based neutron source performance. The presently proposed spectrometer is based on a 3He proportional counter, which is 50 cm long and 5 cm in diameter, with a gas pressure of 0.5 MPa. It is quite unique that the spectrometer is set up in parallel with the incident neutron beam and a reaction depth distribution is measured by it as a position sensitive detector. Recently, a prototype detector has been developed and the signal test is now underway. In this paper, the feature of the accelerator-based neutron sources is outlined and importance of neutron field characterization is discussed. And the developed new low energy neutron spectrometer for the characterization is detailed. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Research aiming at boron neutron capture therapy (BNCT) is underway world-wide. As is well known, the BNCT is one of the most promising therapies for cancer comparing with other therapies like gamma (cyber) knife, ion beam radiotherapy and so on. Especially in Japan, the research and development have positively been carried out for years partly because traditionally the number of cancer patients is quite large in Japan. Recently, the therapy is beginning to be conducted more regularly by using a nuclear reactor. In Japan, there are two nuclear reactors available for BNCT: one is JRR-4 of Japan Atomic Energy Agency in Ibaraki prefecture, while the other one is KUR of Kyoto University in Osaka prefecture. These are experimental nuclear reactors for
Corresponding author. Tel.: +81 6 6879 7892; fax: +81 6 6879 7899.
E-mail address: [email protected] (I. Murata). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.03.066
nuclear engineering research, and thus there exist a lot of restrictions when used. For example, patients should reserve the machine time well in advance, but usually this is not flexible and an emergency treatment is normally not accepted. Moreover, because the nuclear reactors available are located only in Ibaraki and Osaka prefectures, patients and their doctors should go to one of the two facilities even if they live very far from the two facilities. Under these circumstances, investigation of accelerator-based neutron sources is in progress. It is obviously difficult to construct a nuclear reactor in a hospital because of a severe problem of public acceptance. However, accelerators are already accepted in hospitals as a radioisotope source for positron emission tomography (PET), for example. Since a fixed field alternating gradient (FFAG)-type accelerator for BNCT research in Japan has already been constructed in Kyoto University, named FFAGERIT (emittance recovery internal target; Mori, 2006), neutrons have already been produced, even as a test operation at present.
ARTICLE IN PRESS I. Murata et al. / Applied Radiation and Isotopes 67 (2009) S288–S291
2. Nuclear reactor and accelerator as a neutron source for BNCT The fundamental difference between nuclear reactor and accelerator is the intensity of their neutron fluxes. In the nuclear reactor, even an experimental reactor, the thermal neutron flux of more than 1013 s 1 cm 2 can be achieved in the core. On the other hand, in the accelerator-based neutron source, even if integrated over the whole solid angle of 4p, the neutron yielding is about 1013 s 1. Moreover, in this case, the neutron energy is much higher than thermal neutrons, except for the one coming from the 7 Li(p,n) reaction using a proton beam having an energy slightly above the threshold level (1.889 MeV). The neutrons should, therefore, in the majority of the cases, be moderated down to thermal energy. Hence, the thermal neutron flux intensity should be around 109 s 1 cm 2 at the highest, which is known to be the lowest level applicable for BNCT. The above facts lead to the following discussion: (1) in the case of the nuclear reactor, the distance between the neutron source and a patient can be kept long enough to supply a large space for neutron filters and shielding material. A part of the space, being on the line between the neutron source and the patient, attenuates the direct high energy neutrons and gamma-rays from the neutron source. Also this enough space allows us to set up a sufficient amount of moderator materials ‘‘not’’ in the same place as the radiation shield, i.e., not on the line between the neutron source and the patient, so that neutrons are well moderated to supply an ideal thermal neutron irradiation field for BNCT, simultaneously shielding other background radiations. (2) In the case of the accelerator, because the neutron source intensity is not so high, the patient should be positioned close to the neutron source, typically within around 1 m. In this narrow space, a moderator system to moderate the incident neutron energy down to the thermal energy region should be set up. In addition a shielding system has to be placed ‘‘at the same place’’ as the moderator to attenuate high energy neutrons from the target and secondary neutrons and gamma-rays produced in the surrounding material. It is not trivial to combine moderating and shielding material in the same limited space. The problem to be solved is how to decrease the background radiation, because the thermal neutron intensity itself could be increased if approaching the neutron source. However, in return, the background intensity is also going up absolutely and relatively against the thermal neutron intensity. This is a principal difficulty in case of applying accelerators to BNCT. The FFAG-ERIT, which has been constructed in Kyoto University, would have an ability to
overcome it. The ERIT is an emittance recovery internal target. In this technique, a target is positioned inside the accelerator ring, meaning transmitted particles through the target are once again accelerated and focused by the ionization cooling technique. In other words, at the target region, there is no need to prepare a beam dump which normally produces a large amount of background radiations. In addition, using a thin target a large (cross section)/(stopping power) ratio can be achieved. This can facilitate the heat removal difficulty in the target. In any case, the point is to prepare an appropriate technique to characterize the neutron field in order to assess the applicability of the neutron source to BNCT in various neutron source facilities.
3. Low energy neutron spectrometer In the present study, a general-purpose low energy neutron spectrometer has been designed and is now being developed for characterization of the neutron field in accelerator-based neutron sources like FFAG-ERIT. In the following sections, the principle and brief description of the spectrometer is presented. The more details are found elsewhere (Murata and Miyamaru, 2008). 3.1. Principle of the spectrometer The spectral information about low energy neutrons are difficult to be obtained in a direct way. In this study, the neutron–nuclear cross sections are taken into account, which are directly related to the neutron energy. Consequently, the position of a reaction event in a neutron detector shall depend on the neutron energy. If the reaction cross section is larger, the distance from the detector entrance to the position at which the reaction occurs becomes smaller, and vice versa. In other words, the reaction position ‘‘distribution,’’ measured by the detector, varies with the incident neutron energy. If selecting 10B or 3He as the detection medium, the neutron energy and the reaction cross section share a one-to-one correspondence in the low energy region. Fig. 1 shows the reaction cross section of 3He, for example. Fortunately, 10B and 3He in gas are already available at present for a neutron detector. Fig. 2 shows the schematic design figure of the present neutron spectrometer. The neutron detector is a proportional counter having one wire in a cylindrical casing. Because we wish to
104 TOTAL ELASTIC
103
CAPTURE (n, p)
102 Cross section (barn)
After characterizing the neutron field, irradiation tests for BNCT are planned in the next phase. A new, low energy, neutron spectrometer aimed at the characterization measurement of FFAG-ERIT neutron beam has been designed and is now being developed. As well known, the neutron field characterization is a crucial issue even in a nuclear reactor facility, but, as described in Section 2, the characterization of an accelerator-driven neutron source is a more fundamental step. Such an achievement is, however, difficult to be done because no direct technique to measure the neutron energy spectrum particularly in the lower energy region is available. The new spectrometer being developed in the present study will thus be a general-purpose neutron spectrometer, which is able to be utilized in various applications, as well as in BNCT. In the present paper, the feature of the accelerator-based neutrons sources is outlined and importance of the neutron field characterization is discussed. Details of a new low energy neutron spectrometer based on a position sensitive 3He proportional counter being developed for the characterization are described.
S289
10 1 10-1
(n, p) 3He
(n, p)
10-2 10-3 10-4
CAPTURE
10-5 10-6
10-1
1
10
102 104 103 Neutron energy (eV)
105
106
107
Fig. 1. Reaction cross section of 3He cited from JENDL-3.3. Neutrons are detected by 3He(n,p) reaction.
ARTICLE IN PRESS S290
I. Murata et al. / Applied Radiation and Isotopes 67 (2009) S288–S291
BNC connector for output signal (Most obliquely entering case) Collimator (to prevent obliquely entering neutron) Neutron
P 50 cm
2.5 cm Anode wire
h a Fig. 3. Photo of the prototype low energy neutron spectrometer. A BNC connector is seen in both ends of the spectrometer.
Covered with neutron absorber like B4C Anode wire
3He
P
gas
P cross section
BNC connector for output signal
Fig. 2. Schematic design figure of the spectrometer. a and h are determined so as to prevent neutrons from entering the side wall. (a) Side view; (b) top view.
measure the position where the neutron reaction in the detector occurs, the detector is operated as a position sensitive counter. This kind of detector is already available (Fischer, 1977). However, such a detector provides information on the one- or twodimensional position where an incident neutron enters. In other words, the detector is arranged in such a way that neutrons enter from a direction perpendicular to the detector axis. On the other hand, in the present spectrometer, the reaction position (depth) should be measured to finally determine the neutron energy. The detector should thus be fixed so that neutrons reach the detector, parallel to the detector axis, as shown in Fig. 2. This spectrometer is unique in this aspect. Neutrons enter the detector parallel to the detector axis from either side of it. Charged particles will be emitted because of nuclear reactions of neutrons with 10B or 3He. Their charges are collected by the electric field formed by the anode wire after appropriate electron multiplication. The detector consists of two output connectors on both ends. Generally, the difference (ratio) in the amount of charges collected at both ends indicates the reaction position in the detector. The obtained reaction position distribution can be converted into the energy spectrum by unfolding process using the detector response function. The detector response function is defined as the reaction position distribution in the detector for each neutron energy. As mentioned earlier, a one-to-one correspondence for the neutron energy and the reaction position distribution is a crucial requirement to properly unfold the distribution, because the unfolding process is equivalent to solving an inverse problem.
3.2. Spectrometer design As the detection gas, 3He and BF3 are a candidate suitable for the present spectrometer. 3He was finally employed taking into account an important requirement of BNCT that the measurable energy should be as high as possible. More practically, the ng discrimination problem is surely a drawback in the use of 3He. However, for 3He, a higher pressure is acceptable compared with 10 B and also the cross section is inherently larger by a factor of 1.4. Consequently, the detection sensitivity of 3He is much better than BF3. For the gas pressure, it is possible to utilize 3He gas up to around 1 MPa. Since the present spectrometer is a prototype, 0.5 MPa was adopted. For the diameter, 5 cm was employed with the same reason, though finally over 10 cm is aimed at. For convenience of use, a 50 cm long proportional counter is
employed, though a longer one is desired in order to improve the detection performance of higher energy neutrons. For BNCT, the dynamic range is set enough wide, i.e., from thermal to around 10 keV (6 orders of magnitude). However, the realization is not so straightforward, because the efficiency becomes very small around there. Nevertheless, as a final goal, unfolding up to 10 keV is aimed at for BNCT in the present study. As shown in the next section, for this purpose, various experimental approaches are planned. 3.3. Spectrometer fabrication A prototype spectrometer has been developed by the author’s group at Osaka University in collaboration with OHYO KOKEN KOGYO CO., LDT(http://www.oken.co.jp/web_oken/indexen.htm). Fig. 3 shows a photo of the developed spectrometer. As described earlier, the spectrometer has a BNC connector in both ends to measure the detection position distribution, and is placed so that neutrons reach the detector, parallel to the detector axis. The connector can thus hamper the neutron detection and distort the measured neutron spectrum. However, numerical correction of the distortion of the measured detection position distribution is possible. Though the present prototype spectrometer is 50 cm long, according to the manufacturer of the present spectrometer, a longer one could be produced; however, it seems to be easier to increase the gas pressure than making a longer counter. We have started neutron measurements with the present prototype spectrometer to test the spectrometer performance and the unfolding process. For the detection position identification a simple method of estimating the position from the ratio of charges detected at both ends. We have plans of neutron measurements in order to confirm feasibility of measuring overkeV neutrons, i.e., detector response measurements with monoenergetic neutrons of 8 keV (45Sc(p,n)) and 23 keV(9Be(g,n) by decay gamma-rays of 124Sb). After checking the whole spectrometer system, the neutron spectrum measurement is planned at the accelerator-based neutron source of FFAG-ERIT.
4. Conclusions A low energy neutron spectrometer, based on a 3He proportional counter has been designed and is now being developed to characterize the low energy neutron field of the accelerator-based neutron source, particularly for BNCT application. The proposed spectrometer is based on a 3He proportional counter and aims at covering the neutron energy from thermal to keV region, i.e., around 6 decades. The spectrometer is 50 cm long and 5 cm in diameter with the gas pressure of 0.5 MPa. Recently, the prototype detector has been completed and the signal test is now in progress. After checking the whole spectrometer system, the neutron spectrum measurement is planned at the acceleratorbased neutron source of FFAG-ERIT, Kyoto University. The present
ARTICLE IN PRESS I. Murata et al. / Applied Radiation and Isotopes 67 (2009) S288–S291
spectrometer is a general-purpose low energy neutron spectrometer, which would be utilized for various neutron measurements in nuclear reactors, accelerators and even in the environment.
Acknowledgment This study is being conducted in part with the support of grants-in-aid for Scientific Research by JSPS, Scientific Research (B) (Contract no. 19360429).
S291
References Fischer, B.E., 1977. A digital processor for position sensitive detectors. Nucl. Instrum. Meth. 141, 173. Mori, Y., 2006. Development of FFAG accelerators and their applications for intense secondary particle production. Nucl. Instrum. Meth. A 562, 591–595. Murata, I., Miyamaru, H., 2008. Low energy neutron spectrometer using position sensitive proportional counter—feasibility study based on numerical analysis’’. Nucl. Instrum. Meth. A 589, 445–454.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
On accelerator-based neutron sources and neutron field characterization with low energy neutron spectrometer based on position sensitive 3He counter I. Murata a,, H. Miyamaru a, I. Kato b, Y. Mori c a
Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Japan Department of Oral and Maxillofacial Surgery II, Graduate School of Dentistry, Osaka University, Japan c Kyoto University Research Reactor Institute, Kyoto University, Japan b
a r t i c l e in f o
Keywords: BNCT Accelerator-based neutron source Low energy neutron spectrometer Position sensitive proportional counter 3 He detector
a b s t r a c t The development of new neutron sources for BNCT applications, based on particle accelerators is currently underway all over the world. Though nuclear reactors were used for a long time as the only neutron source available having the requested flux levels, the accelerator-based ones have recently been investigated on the other hand due to its easy-to-use and acceptable performances. However, when using an accelerator, various secondary particles would be emitted which forms a troublesome background. Moreover, the neutrons produced have usually an energy spectrum somewhat different from the requested one and thus should be largely moderated. An additional issue to be taken into account is the patient positioning, which should be close to the neutron source, in order to take advantage of a neutron flux level high enough to limit the BNCT treatment time within 1 h. This implies that, inside a relatively narrow space, neutrons should be moderated, while unnecessary secondary particles should be shielded. Considering that a background-free neutron field from an acceleratordriven neutron source dedicated to BNCT application is generally difficult to be provided, the characterization of such a neutron field will have to be clearly assessed. In the present study, a low energy neutron spectrometer has been thus designed and is now being developed to measure the accelerator-based neutron source performance. The presently proposed spectrometer is based on a 3He proportional counter, which is 50 cm long and 5 cm in diameter, with a gas pressure of 0.5 MPa. It is quite unique that the spectrometer is set up in parallel with the incident neutron beam and a reaction depth distribution is measured by it as a position sensitive detector. Recently, a prototype detector has been developed and the signal test is now underway. In this paper, the feature of the accelerator-based neutron sources is outlined and importance of neutron field characterization is discussed. And the developed new low energy neutron spectrometer for the characterization is detailed. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Research aiming at boron neutron capture therapy (BNCT) is underway world-wide. As is well known, the BNCT is one of the most promising therapies for cancer comparing with other therapies like gamma (cyber) knife, ion beam radiotherapy and so on. Especially in Japan, the research and development have positively been carried out for years partly because traditionally the number of cancer patients is quite large in Japan. Recently, the therapy is beginning to be conducted more regularly by using a nuclear reactor. In Japan, there are two nuclear reactors available for BNCT: one is JRR-4 of Japan Atomic Energy Agency in Ibaraki prefecture, while the other one is KUR of Kyoto University in Osaka prefecture. These are experimental nuclear reactors for
Corresponding author. Tel.: +81 6 6879 7892; fax: +81 6 6879 7899.
E-mail address: [email protected] (I. Murata). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.03.066
nuclear engineering research, and thus there exist a lot of restrictions when used. For example, patients should reserve the machine time well in advance, but usually this is not flexible and an emergency treatment is normally not accepted. Moreover, because the nuclear reactors available are located only in Ibaraki and Osaka prefectures, patients and their doctors should go to one of the two facilities even if they live very far from the two facilities. Under these circumstances, investigation of accelerator-based neutron sources is in progress. It is obviously difficult to construct a nuclear reactor in a hospital because of a severe problem of public acceptance. However, accelerators are already accepted in hospitals as a radioisotope source for positron emission tomography (PET), for example. Since a fixed field alternating gradient (FFAG)-type accelerator for BNCT research in Japan has already been constructed in Kyoto University, named FFAGERIT (emittance recovery internal target; Mori, 2006), neutrons have already been produced, even as a test operation at present.
ARTICLE IN PRESS I. Murata et al. / Applied Radiation and Isotopes 67 (2009) S288–S291
2. Nuclear reactor and accelerator as a neutron source for BNCT The fundamental difference between nuclear reactor and accelerator is the intensity of their neutron fluxes. In the nuclear reactor, even an experimental reactor, the thermal neutron flux of more than 1013 s 1 cm 2 can be achieved in the core. On the other hand, in the accelerator-based neutron source, even if integrated over the whole solid angle of 4p, the neutron yielding is about 1013 s 1. Moreover, in this case, the neutron energy is much higher than thermal neutrons, except for the one coming from the 7 Li(p,n) reaction using a proton beam having an energy slightly above the threshold level (1.889 MeV). The neutrons should, therefore, in the majority of the cases, be moderated down to thermal energy. Hence, the thermal neutron flux intensity should be around 109 s 1 cm 2 at the highest, which is known to be the lowest level applicable for BNCT. The above facts lead to the following discussion: (1) in the case of the nuclear reactor, the distance between the neutron source and a patient can be kept long enough to supply a large space for neutron filters and shielding material. A part of the space, being on the line between the neutron source and the patient, attenuates the direct high energy neutrons and gamma-rays from the neutron source. Also this enough space allows us to set up a sufficient amount of moderator materials ‘‘not’’ in the same place as the radiation shield, i.e., not on the line between the neutron source and the patient, so that neutrons are well moderated to supply an ideal thermal neutron irradiation field for BNCT, simultaneously shielding other background radiations. (2) In the case of the accelerator, because the neutron source intensity is not so high, the patient should be positioned close to the neutron source, typically within around 1 m. In this narrow space, a moderator system to moderate the incident neutron energy down to the thermal energy region should be set up. In addition a shielding system has to be placed ‘‘at the same place’’ as the moderator to attenuate high energy neutrons from the target and secondary neutrons and gamma-rays produced in the surrounding material. It is not trivial to combine moderating and shielding material in the same limited space. The problem to be solved is how to decrease the background radiation, because the thermal neutron intensity itself could be increased if approaching the neutron source. However, in return, the background intensity is also going up absolutely and relatively against the thermal neutron intensity. This is a principal difficulty in case of applying accelerators to BNCT. The FFAG-ERIT, which has been constructed in Kyoto University, would have an ability to
overcome it. The ERIT is an emittance recovery internal target. In this technique, a target is positioned inside the accelerator ring, meaning transmitted particles through the target are once again accelerated and focused by the ionization cooling technique. In other words, at the target region, there is no need to prepare a beam dump which normally produces a large amount of background radiations. In addition, using a thin target a large (cross section)/(stopping power) ratio can be achieved. This can facilitate the heat removal difficulty in the target. In any case, the point is to prepare an appropriate technique to characterize the neutron field in order to assess the applicability of the neutron source to BNCT in various neutron source facilities.
3. Low energy neutron spectrometer In the present study, a general-purpose low energy neutron spectrometer has been designed and is now being developed for characterization of the neutron field in accelerator-based neutron sources like FFAG-ERIT. In the following sections, the principle and brief description of the spectrometer is presented. The more details are found elsewhere (Murata and Miyamaru, 2008). 3.1. Principle of the spectrometer The spectral information about low energy neutrons are difficult to be obtained in a direct way. In this study, the neutron–nuclear cross sections are taken into account, which are directly related to the neutron energy. Consequently, the position of a reaction event in a neutron detector shall depend on the neutron energy. If the reaction cross section is larger, the distance from the detector entrance to the position at which the reaction occurs becomes smaller, and vice versa. In other words, the reaction position ‘‘distribution,’’ measured by the detector, varies with the incident neutron energy. If selecting 10B or 3He as the detection medium, the neutron energy and the reaction cross section share a one-to-one correspondence in the low energy region. Fig. 1 shows the reaction cross section of 3He, for example. Fortunately, 10B and 3He in gas are already available at present for a neutron detector. Fig. 2 shows the schematic design figure of the present neutron spectrometer. The neutron detector is a proportional counter having one wire in a cylindrical casing. Because we wish to
104 TOTAL ELASTIC
103
CAPTURE (n, p)
102 Cross section (barn)
After characterizing the neutron field, irradiation tests for BNCT are planned in the next phase. A new, low energy, neutron spectrometer aimed at the characterization measurement of FFAG-ERIT neutron beam has been designed and is now being developed. As well known, the neutron field characterization is a crucial issue even in a nuclear reactor facility, but, as described in Section 2, the characterization of an accelerator-driven neutron source is a more fundamental step. Such an achievement is, however, difficult to be done because no direct technique to measure the neutron energy spectrum particularly in the lower energy region is available. The new spectrometer being developed in the present study will thus be a general-purpose neutron spectrometer, which is able to be utilized in various applications, as well as in BNCT. In the present paper, the feature of the accelerator-based neutrons sources is outlined and importance of the neutron field characterization is discussed. Details of a new low energy neutron spectrometer based on a position sensitive 3He proportional counter being developed for the characterization are described.
S289
10 1 10-1
(n, p) 3He
(n, p)
10-2 10-3 10-4
CAPTURE
10-5 10-6
10-1
1
10
102 104 103 Neutron energy (eV)
105
106
107
Fig. 1. Reaction cross section of 3He cited from JENDL-3.3. Neutrons are detected by 3He(n,p) reaction.
ARTICLE IN PRESS S290
I. Murata et al. / Applied Radiation and Isotopes 67 (2009) S288–S291
BNC connector for output signal (Most obliquely entering case) Collimator (to prevent obliquely entering neutron) Neutron
P 50 cm
2.5 cm Anode wire
h a Fig. 3. Photo of the prototype low energy neutron spectrometer. A BNC connector is seen in both ends of the spectrometer.
Covered with neutron absorber like B4C Anode wire
3He
P
gas
P cross section
BNC connector for output signal
Fig. 2. Schematic design figure of the spectrometer. a and h are determined so as to prevent neutrons from entering the side wall. (a) Side view; (b) top view.
measure the position where the neutron reaction in the detector occurs, the detector is operated as a position sensitive counter. This kind of detector is already available (Fischer, 1977). However, such a detector provides information on the one- or twodimensional position where an incident neutron enters. In other words, the detector is arranged in such a way that neutrons enter from a direction perpendicular to the detector axis. On the other hand, in the present spectrometer, the reaction position (depth) should be measured to finally determine the neutron energy. The detector should thus be fixed so that neutrons reach the detector, parallel to the detector axis, as shown in Fig. 2. This spectrometer is unique in this aspect. Neutrons enter the detector parallel to the detector axis from either side of it. Charged particles will be emitted because of nuclear reactions of neutrons with 10B or 3He. Their charges are collected by the electric field formed by the anode wire after appropriate electron multiplication. The detector consists of two output connectors on both ends. Generally, the difference (ratio) in the amount of charges collected at both ends indicates the reaction position in the detector. The obtained reaction position distribution can be converted into the energy spectrum by unfolding process using the detector response function. The detector response function is defined as the reaction position distribution in the detector for each neutron energy. As mentioned earlier, a one-to-one correspondence for the neutron energy and the reaction position distribution is a crucial requirement to properly unfold the distribution, because the unfolding process is equivalent to solving an inverse problem.
3.2. Spectrometer design As the detection gas, 3He and BF3 are a candidate suitable for the present spectrometer. 3He was finally employed taking into account an important requirement of BNCT that the measurable energy should be as high as possible. More practically, the ng discrimination problem is surely a drawback in the use of 3He. However, for 3He, a higher pressure is acceptable compared with 10 B and also the cross section is inherently larger by a factor of 1.4. Consequently, the detection sensitivity of 3He is much better than BF3. For the gas pressure, it is possible to utilize 3He gas up to around 1 MPa. Since the present spectrometer is a prototype, 0.5 MPa was adopted. For the diameter, 5 cm was employed with the same reason, though finally over 10 cm is aimed at. For convenience of use, a 50 cm long proportional counter is
employed, though a longer one is desired in order to improve the detection performance of higher energy neutrons. For BNCT, the dynamic range is set enough wide, i.e., from thermal to around 10 keV (6 orders of magnitude). However, the realization is not so straightforward, because the efficiency becomes very small around there. Nevertheless, as a final goal, unfolding up to 10 keV is aimed at for BNCT in the present study. As shown in the next section, for this purpose, various experimental approaches are planned. 3.3. Spectrometer fabrication A prototype spectrometer has been developed by the author’s group at Osaka University in collaboration with OHYO KOKEN KOGYO CO., LDT(http://www.oken.co.jp/web_oken/indexen.htm). Fig. 3 shows a photo of the developed spectrometer. As described earlier, the spectrometer has a BNC connector in both ends to measure the detection position distribution, and is placed so that neutrons reach the detector, parallel to the detector axis. The connector can thus hamper the neutron detection and distort the measured neutron spectrum. However, numerical correction of the distortion of the measured detection position distribution is possible. Though the present prototype spectrometer is 50 cm long, according to the manufacturer of the present spectrometer, a longer one could be produced; however, it seems to be easier to increase the gas pressure than making a longer counter. We have started neutron measurements with the present prototype spectrometer to test the spectrometer performance and the unfolding process. For the detection position identification a simple method of estimating the position from the ratio of charges detected at both ends. We have plans of neutron measurements in order to confirm feasibility of measuring overkeV neutrons, i.e., detector response measurements with monoenergetic neutrons of 8 keV (45Sc(p,n)) and 23 keV(9Be(g,n) by decay gamma-rays of 124Sb). After checking the whole spectrometer system, the neutron spectrum measurement is planned at the accelerator-based neutron source of FFAG-ERIT.
4. Conclusions A low energy neutron spectrometer, based on a 3He proportional counter has been designed and is now being developed to characterize the low energy neutron field of the accelerator-based neutron source, particularly for BNCT application. The proposed spectrometer is based on a 3He proportional counter and aims at covering the neutron energy from thermal to keV region, i.e., around 6 decades. The spectrometer is 50 cm long and 5 cm in diameter with the gas pressure of 0.5 MPa. Recently, the prototype detector has been completed and the signal test is now in progress. After checking the whole spectrometer system, the neutron spectrum measurement is planned at the acceleratorbased neutron source of FFAG-ERIT, Kyoto University. The present
ARTICLE IN PRESS I. Murata et al. / Applied Radiation and Isotopes 67 (2009) S288–S291
spectrometer is a general-purpose low energy neutron spectrometer, which would be utilized for various neutron measurements in nuclear reactors, accelerators and even in the environment.
Acknowledgment This study is being conducted in part with the support of grants-in-aid for Scientific Research by JSPS, Scientific Research (B) (Contract no. 19360429).
S291
References Fischer, B.E., 1977. A digital processor for position sensitive detectors. Nucl. Instrum. Meth. 141, 173. Mori, Y., 2006. Development of FFAG accelerators and their applications for intense secondary particle production. Nucl. Instrum. Meth. A 562, 591–595. Murata, I., Miyamaru, H., 2008. Low energy neutron spectrometer using position sensitive proportional counter—feasibility study based on numerical analysis’’. Nucl. Instrum. Meth. A 589, 445–454.