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Characterisation of neutron and gamma-ray emission from thick target Be(p,n) reaction for boron neutron capture therapy
Nuclear Instruments and Methods in Physics Research B 139 (1998) 471±475
Characterisation of neutron and gamma-ray emission from thick target Be(p,n) reaction for boron neutron capture therapy J. Guzek
a,*
, W.R. McMurray b, T. Mateva a, C.B. Franklyn c, U.A.S. Tapper
b
a
a De Beers Diamond Research Laboratory, P.O. Box 1770, Southdale 2135, South Africa National Accelerator Centre, Van de Graa Group, P.O. Box 72, Faure 7131, South Africa c Atomic Energy Corporation of South Africa, P.O. Box 582, Pretoria, South Africa
Abstract Low energy accelerator-based neutron sources have promising potential for use in a clinical treatment of cancer with boron neutron capture therapy (BNCT) and boron neutron capture synovectomy (BNCS). Such sources often utilise a thick target Be(p,n) reaction using incident proton energies from several hundred keV to 1±2 MeV above the reaction threshold of 2.06 MeV. The resulting neutron and gamma-ray beams require considerable moderation and ®ltration in order to obtain thermal and epithermal neutron ¯uxes for therapy. The detailed knowledge of neutron and gamma-ray spectra, yield and angular distribution are necessary in order to design eective moderators and ®lters to be used for the treatment. Thick and thin beryllium target neutron and gamma-ray spectra have been investigated in detail using the time-of-¯ight (TOF) technique, for incident proton energies from above threshold to 4 MeV. The results show characteristics of neutron and gamma-ray production of importance for the application of this neutron source for BNCT and BNCS. Ó 1998 Published by Elsevier Science B.V.
1. Introduction Accelerator-based neutron sources often utilise charge-particle-induced nuclear reactions between high energy projectiles of hydrogen isotopes and targets of lighter elements. The development of high power compact accelerators such as radio-frequency quadrupole linacs or cyclotrons allows for use of neutron techniques in applied research, medicine and industry, at reasonable cost. The Be(p,n) reaction with several MeV protons inci-
* Corresponding author. Tel.: ++27-11-490 6668; fax: ++2711-490 6483; e-mail: [email protected].
dent on a metallic beryllium target is very well suited for such applications. Although the neutron yield is less proli®c than for the well researched Li(p,n) reaction, the good thermal properties of beryllium make it attractive for applications where high beam currents are required. There has, recently, been considerable interest in the application of the Be(p,n) reaction at Ep 6 4 MeV in cancer treatment by boron neutron capture therapy (BNCT) [1] and boron neutron capture synovectomy (BNCS) [2]. Both procedures are based on the 10 B(n,a)7 Li reaction to selectively deliver highly ionising a particles and Li ions to malignant cells loaded with 10 B compounds. The accelerator-based neutron source
0168-583X/98/$19.00 Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 9 8 2 - 8
472
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475
utilises nuclear reactions, such as Be(p,n), to produce energetic neutrons, which are subsequently moderated and ®ltered to epithermal and thermal energies for BNCT and BNCS, respectively. In techniques such as BNCT and BNCS a detailed knowledge of the characteristics of neutron and gamma-ray emission is essential in order to design suitable moderators, ®lters and shielding assemblies. The Be(p,n) reaction has been investigated before, usually at incident proton energies well above 10 MeV [3]. Except for a recent publication by Howard et al. [4] there is very little published data from the Be(p,n) reaction at lower proton energies. The aim of this work was to characterise in detail neutron and gamma-ray emission resulting from bombardment of thin and thick beryllium targets with protons of energies from above the reaction threshold to 4 MeV.
2. Experimental method The experiments were performed at the National Accelerator Centre, Faure, South Africa, 6 MV van de Graa facility. The 2 ns pulses of protons were incident on thin (7.5 lm) and thick (2 mm) beryllium targets with a repetition rate of 2 MHz. A 5 ´ 2.5 cm NE102A plastic scintillator coupled to a fast photomultiplier tube served as time-of-¯ight (TOF) detector. It was positioned at a distance of 1.5 and 2 m from the target at laboratory angles of 0±120°. The TOF detector was biased at 20 keV of gamma energy with respect to 137 Cs Compton edge (478 keV). A conventional TOF technique was used for measurement of neutron spectra [5]. For the measurement of gammaray spectra a large, eciency calibrated, intrinsic germanium detector was used. This detector was positioned at a distance of 5.1 m from the target at a laboratory angle of 40°. A TOF spectrum to germanium detector was also measured and the peak corresponding to gamma-ray emission from the target during the beam pulse was used for acquisition of TOF-gated gamma-ray energy spectra, for radiation generated in the target area [5].
Table 1 Neutron producing reaction channels in bombardment of beryllium with protons of energy 6 4 MeV Reaction
Q-value (MeV)
9
)1.57 )1.67 )1.85 )2.67 +0.89
Be(p,npa)4 He Be(p,np)8 Be 9 Be(p,n)9 B 9 Be(p,pa)5 He 5 He ) 4 He + n 9
3. Results and discussion As previously reported [3], at proton energies under consideration, several reaction channels may contribute to neutron production (Table 1). Neutrons resulting from multi-body reactions may have energies higher than the ground state neutrons of the 9 Be(p,n)9 B reaction because thresholds for break-up processes are lower [6]. The TOF spectra obtained by bombardment of 7.5 lm thick beryllium target with 3.0 and 3.8 MeV protons at 0° are presented in Fig. 1. The mono-energetic peaks arising from ground state neutrons occur at En 1.10 and 1.90 MeV for Ep 3.0 and 3.8 MeV, respectively. The broadening of the peaks is a result of slowing-down of the incident proton within the target. The calculations indicate an expected neutron energy spread of 115 and 138 keV for Ep 3.8 and 3.0 MeV, res-
Fig. 1. TOF spectra obtained in bombardment of 7.5 lm thick beryllium target with 3.0 and 3.8 MeV protons at 0°. Ground state neutron groups are labelled N0 and maximum energy of neutrons resulting from multi-body break-up (En 2.20 MeV for Ep 3.8 MeV) is also marked. The ¯ight path 2 m.
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475
473
pectively, and are in very good agreement with the measured values. Also clearly seen is a neutron continuum underlying the mono-energetic peaks and resulting from the multi-body break-up processes: 9 Be(p,np)8 Be and 9 Be(p,npa)4 He. The maximum energy of break-up neutrons for Ep 3.8 MeV is 2.20 MeV, and is higher than that of the ground state neutrons due to the lower thresholds for the break-up reactions. As can be seen, the relative contribution of the break-up neutrons to the overall yield is not signi®cant in this energy range. The thick target neutron spectra obtained with proton energies of 2.57, 3.00 and 4.00 MeV at 0° are presented in Fig. 2. The lower neutron energy cut-o at 0.25 MeV is a result of detector bias. The maximum energies of the ground state neutron groups are 0.65, 1.10 and 2.12 MeV for Ep 2.57, 3.00 and 4.00 MeV, respectively. The presence of neutrons of energies higher than the ground state neutron energy is clearly evident in the Ep 4.00 MeV spectrum. The neutron group corresponding to the ®rst excited state of 9 B residual nucleus is also seen in the same spectrum at 0.31 MeV. This level is not excited for Ep 2.57 and 3.00 MeV. The neutron peak at around 0.6 MeV seen in all spectra results from de-excitation of the 9 Be nucleus via 9 Be(p,p0 ) reactions listed in Table 1. Fig. 3(a) and (b) show the neutron spectral distribution from the thick target spectra obtained
with Ep 2.57 and 3.00 MeV at laboratory angles of 0°, 60° and 90°. The spectra measured for Ep 4.00 MeV and laboratory angles of 0±120° are presented in Ref. [5]. For all measured spectra the maximum energy of neutrons resulting from the 9 Be(p,n)9 B reaction decreases with increasing laboratory angle, as predicted by kinematics calculations, whilst the total neutron yield for all spectra is signi®cantly enhanced by the presence of break-up neutrons. The Be(p,n) thick target yield (for En P 0.25 MeV) is presented in Fig. 4. For all investigated proton energies the maximum yield is observed near 0°. It rapidly decreases with increasing laboratory angle to reach the minimum at around 80°, and subsequently increases in the 80±120° range.
Fig. 2. Neutron spectra at 0° obtained by bombardment of the thick beryllium with 2.57, 3.00 and 4.00 MeV protons. The low energy cut-o at 0.25 MeV is due to detector bias.
Fig. 3. Neutron spectra at 0°, 60° and 90° obtained in bombardment of the thick beryllium target with 2.57 (a) and 3.00 MeV (b) protons.
474
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475
Fig. 4. Neutron yield (En P 0.25 MeV) for the thick target Be(p,n) reaction for Ep 2.57, 3.00 and 4.00 MeV.
The TOF-gated and ungated gamma-ray spectra, obtained in bombardment of thick Be with 4.00 MeV protons at 40°, are presented in Fig. 5(a) and (b). The major gamma-ray lines are identi®ed. The lower energy cut-o at 260 keV is due to set ADC discrimination level. The TOF-gated spectrum, which represents gamma-ray emission from the target area during the proton beam pulse, is dominated by the Compton scatter background from high energy gamma-rays, strong 511 keV annihilation peak and the heavily Doppler-broadened peak centred around 3570 keV and its ®rst and second escape peaks. The Doppler-broadened line results from the 9 Be(p,a)6 Li reaction (Q 2.126 MeV) leaving 6 Li in the excited state at 3563 keV, which subsequently decays by gamma-ray emission. No other gamma-ray lines resulting directly from proton induced reactions in beryllium were identi®ed. Other major gammaray lines include proton induced prompt gammarays from the copper target holder (669, 691.3 and 962.2 keV), neutron induced lines in the germanium detector (596.4, 595.8 and 868.4 keV) and construction materials in the vicinity of the target and detector: 1778 keV from 27 Al(n,c)28 Al and 28 Si(n,n0 c)28 Al, 847.1 keV from 56 Fe(n,n0 c)56 Fe and notably 2223 keV line from 1 H(n,c)2 H. Also seen are high energy (Ec > 4 MeV) gamma-ray peaks resulting from neutron interactions with surrounding materials. These peaks, as resulting from time-uncorrelated interactions, are signi®cantly
Fig. 5. TOF-gated (a) and ungated (b) gamma-ray spectra at 40° obtained in bombardment of the thick beryllium target with 4.00 MeV protons. The major gamma-ray lines are labelled in keV. Spectra not corrected for detection eciency. The low energy cut-o at 260 keV is due to set ADC discrimination level.
suppressed by the TOF-gate and are clearly visible in the ungated spectrum. The following lines are identi®ed: 3539 and 4934 keV from 28 Si, 6110.9 and 7413.8 keV from 35 Cl and 7278.8, 7631.1 and 7645.5 keV from 56 Fe, they are accompanied by the ®rst and second escape peaks. Similar gamma-ray lines are observed in gamma-ray spectra obtained in bombardment of thick beryllium with 2.57 and 3.00 MeV protons [5]. All gamma-ray spectra were corrected for detection eciency and the total gamma-ray yields (260 keV 6 Ec 6 8191 keV) were calculated. The results, presented in Table 2, indicate that there is less than one gamma-ray photon per neutron produced in the thick target Be(p,n) reaction, at the energy ranges investigated. This ratio changes if
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475 Table 2 Gamma-ray yield (260 keV 6 Ec 6 8191 keV) at laboratory angle of 40°, for the thick target Be(p,n) reaction for proton energies of 2.57, 3.00 and 4.00 MeV Ep (MeV)
2.57 3 4
Gamma-ray yield (c/lC/sr) ´ 10ÿ7 TOF-gated spectrum
Ungated spectrum
0.82 2.15 6.56
8.51 25.5 86.6
all gamma-rays, including secondary eects, are counted, and depends on the speci®c operating conditions of the experiment. In our experimental condition there were, in the energy ranges under consideration, approximately 10 gamma-ray photons produced per neutron. 4. Conclusions The characteristics of neutron and gamma-ray emission resulting from bombardment of thick beryllium targets with protons with kinetic energies below 4 MeV were investigated. The neutron yield results from the strong single neutron group corre-
475
sponding to the ®rst excited level in the residual nucleus in the 9 Be(p,n)9 B reaction and a neutron continuum resulting from multi-body break-up reactions. The break up neutrons have energies higher than the ground state neutrons due to lower reaction threshold. There is less than one gamma-ray photon per neutron produced in the reaction. The gamma-ray spectrum is dominated by the Doppler-broadened 3563 keV line resulting from 9 Be(p,a)6 Li reaction. This data is of importance for applications such as BNCT, BNCS or radiography. References [1] C.-K. Wang, B.R. Moore, Med. Phys. 21 (1994) 1633. [2] J.C. Yanch et al., Proc. Int. Conf. Neutrons in Research and Industry, Crete, Greece, 1996 SPIE 2867, 1997, p. 31. [3] M.A. Lone et al., Nucl. Instr. and Meth. 189 (1981) 515. [4] W.B. Howard et al., Proc. Int. Conf. Neutrons in Research and Industry, Crete, Greece, 1996 SPIE 2867, 1997, p. 84. [5] J. Guzek, Ph.D Thesis, University of the Witwatersrand, Johannesburg, South Africa, in preparation. [6] J. Guzek et al., Characterisation of neutron and gamma emission from bombardment of beryllium with protons with kinetic energies below 10 MeV, South African Institute of Physics Congress, Durban, South Africa, 1997.
Characterisation of neutron and gamma-ray emission from thick target Be(p,n) reaction for boron neutron capture therapy J. Guzek
a,*
, W.R. McMurray b, T. Mateva a, C.B. Franklyn c, U.A.S. Tapper
b
a
a De Beers Diamond Research Laboratory, P.O. Box 1770, Southdale 2135, South Africa National Accelerator Centre, Van de Graa Group, P.O. Box 72, Faure 7131, South Africa c Atomic Energy Corporation of South Africa, P.O. Box 582, Pretoria, South Africa
Abstract Low energy accelerator-based neutron sources have promising potential for use in a clinical treatment of cancer with boron neutron capture therapy (BNCT) and boron neutron capture synovectomy (BNCS). Such sources often utilise a thick target Be(p,n) reaction using incident proton energies from several hundred keV to 1±2 MeV above the reaction threshold of 2.06 MeV. The resulting neutron and gamma-ray beams require considerable moderation and ®ltration in order to obtain thermal and epithermal neutron ¯uxes for therapy. The detailed knowledge of neutron and gamma-ray spectra, yield and angular distribution are necessary in order to design eective moderators and ®lters to be used for the treatment. Thick and thin beryllium target neutron and gamma-ray spectra have been investigated in detail using the time-of-¯ight (TOF) technique, for incident proton energies from above threshold to 4 MeV. The results show characteristics of neutron and gamma-ray production of importance for the application of this neutron source for BNCT and BNCS. Ó 1998 Published by Elsevier Science B.V.
1. Introduction Accelerator-based neutron sources often utilise charge-particle-induced nuclear reactions between high energy projectiles of hydrogen isotopes and targets of lighter elements. The development of high power compact accelerators such as radio-frequency quadrupole linacs or cyclotrons allows for use of neutron techniques in applied research, medicine and industry, at reasonable cost. The Be(p,n) reaction with several MeV protons inci-
* Corresponding author. Tel.: ++27-11-490 6668; fax: ++2711-490 6483; e-mail: [email protected].
dent on a metallic beryllium target is very well suited for such applications. Although the neutron yield is less proli®c than for the well researched Li(p,n) reaction, the good thermal properties of beryllium make it attractive for applications where high beam currents are required. There has, recently, been considerable interest in the application of the Be(p,n) reaction at Ep 6 4 MeV in cancer treatment by boron neutron capture therapy (BNCT) [1] and boron neutron capture synovectomy (BNCS) [2]. Both procedures are based on the 10 B(n,a)7 Li reaction to selectively deliver highly ionising a particles and Li ions to malignant cells loaded with 10 B compounds. The accelerator-based neutron source
0168-583X/98/$19.00 Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 9 8 2 - 8
472
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475
utilises nuclear reactions, such as Be(p,n), to produce energetic neutrons, which are subsequently moderated and ®ltered to epithermal and thermal energies for BNCT and BNCS, respectively. In techniques such as BNCT and BNCS a detailed knowledge of the characteristics of neutron and gamma-ray emission is essential in order to design suitable moderators, ®lters and shielding assemblies. The Be(p,n) reaction has been investigated before, usually at incident proton energies well above 10 MeV [3]. Except for a recent publication by Howard et al. [4] there is very little published data from the Be(p,n) reaction at lower proton energies. The aim of this work was to characterise in detail neutron and gamma-ray emission resulting from bombardment of thin and thick beryllium targets with protons of energies from above the reaction threshold to 4 MeV.
2. Experimental method The experiments were performed at the National Accelerator Centre, Faure, South Africa, 6 MV van de Graa facility. The 2 ns pulses of protons were incident on thin (7.5 lm) and thick (2 mm) beryllium targets with a repetition rate of 2 MHz. A 5 ´ 2.5 cm NE102A plastic scintillator coupled to a fast photomultiplier tube served as time-of-¯ight (TOF) detector. It was positioned at a distance of 1.5 and 2 m from the target at laboratory angles of 0±120°. The TOF detector was biased at 20 keV of gamma energy with respect to 137 Cs Compton edge (478 keV). A conventional TOF technique was used for measurement of neutron spectra [5]. For the measurement of gammaray spectra a large, eciency calibrated, intrinsic germanium detector was used. This detector was positioned at a distance of 5.1 m from the target at a laboratory angle of 40°. A TOF spectrum to germanium detector was also measured and the peak corresponding to gamma-ray emission from the target during the beam pulse was used for acquisition of TOF-gated gamma-ray energy spectra, for radiation generated in the target area [5].
Table 1 Neutron producing reaction channels in bombardment of beryllium with protons of energy 6 4 MeV Reaction
Q-value (MeV)
9
)1.57 )1.67 )1.85 )2.67 +0.89
Be(p,npa)4 He Be(p,np)8 Be 9 Be(p,n)9 B 9 Be(p,pa)5 He 5 He ) 4 He + n 9
3. Results and discussion As previously reported [3], at proton energies under consideration, several reaction channels may contribute to neutron production (Table 1). Neutrons resulting from multi-body reactions may have energies higher than the ground state neutrons of the 9 Be(p,n)9 B reaction because thresholds for break-up processes are lower [6]. The TOF spectra obtained by bombardment of 7.5 lm thick beryllium target with 3.0 and 3.8 MeV protons at 0° are presented in Fig. 1. The mono-energetic peaks arising from ground state neutrons occur at En 1.10 and 1.90 MeV for Ep 3.0 and 3.8 MeV, respectively. The broadening of the peaks is a result of slowing-down of the incident proton within the target. The calculations indicate an expected neutron energy spread of 115 and 138 keV for Ep 3.8 and 3.0 MeV, res-
Fig. 1. TOF spectra obtained in bombardment of 7.5 lm thick beryllium target with 3.0 and 3.8 MeV protons at 0°. Ground state neutron groups are labelled N0 and maximum energy of neutrons resulting from multi-body break-up (En 2.20 MeV for Ep 3.8 MeV) is also marked. The ¯ight path 2 m.
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475
473
pectively, and are in very good agreement with the measured values. Also clearly seen is a neutron continuum underlying the mono-energetic peaks and resulting from the multi-body break-up processes: 9 Be(p,np)8 Be and 9 Be(p,npa)4 He. The maximum energy of break-up neutrons for Ep 3.8 MeV is 2.20 MeV, and is higher than that of the ground state neutrons due to the lower thresholds for the break-up reactions. As can be seen, the relative contribution of the break-up neutrons to the overall yield is not signi®cant in this energy range. The thick target neutron spectra obtained with proton energies of 2.57, 3.00 and 4.00 MeV at 0° are presented in Fig. 2. The lower neutron energy cut-o at 0.25 MeV is a result of detector bias. The maximum energies of the ground state neutron groups are 0.65, 1.10 and 2.12 MeV for Ep 2.57, 3.00 and 4.00 MeV, respectively. The presence of neutrons of energies higher than the ground state neutron energy is clearly evident in the Ep 4.00 MeV spectrum. The neutron group corresponding to the ®rst excited state of 9 B residual nucleus is also seen in the same spectrum at 0.31 MeV. This level is not excited for Ep 2.57 and 3.00 MeV. The neutron peak at around 0.6 MeV seen in all spectra results from de-excitation of the 9 Be nucleus via 9 Be(p,p0 ) reactions listed in Table 1. Fig. 3(a) and (b) show the neutron spectral distribution from the thick target spectra obtained
with Ep 2.57 and 3.00 MeV at laboratory angles of 0°, 60° and 90°. The spectra measured for Ep 4.00 MeV and laboratory angles of 0±120° are presented in Ref. [5]. For all measured spectra the maximum energy of neutrons resulting from the 9 Be(p,n)9 B reaction decreases with increasing laboratory angle, as predicted by kinematics calculations, whilst the total neutron yield for all spectra is signi®cantly enhanced by the presence of break-up neutrons. The Be(p,n) thick target yield (for En P 0.25 MeV) is presented in Fig. 4. For all investigated proton energies the maximum yield is observed near 0°. It rapidly decreases with increasing laboratory angle to reach the minimum at around 80°, and subsequently increases in the 80±120° range.
Fig. 2. Neutron spectra at 0° obtained by bombardment of the thick beryllium with 2.57, 3.00 and 4.00 MeV protons. The low energy cut-o at 0.25 MeV is due to detector bias.
Fig. 3. Neutron spectra at 0°, 60° and 90° obtained in bombardment of the thick beryllium target with 2.57 (a) and 3.00 MeV (b) protons.
474
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475
Fig. 4. Neutron yield (En P 0.25 MeV) for the thick target Be(p,n) reaction for Ep 2.57, 3.00 and 4.00 MeV.
The TOF-gated and ungated gamma-ray spectra, obtained in bombardment of thick Be with 4.00 MeV protons at 40°, are presented in Fig. 5(a) and (b). The major gamma-ray lines are identi®ed. The lower energy cut-o at 260 keV is due to set ADC discrimination level. The TOF-gated spectrum, which represents gamma-ray emission from the target area during the proton beam pulse, is dominated by the Compton scatter background from high energy gamma-rays, strong 511 keV annihilation peak and the heavily Doppler-broadened peak centred around 3570 keV and its ®rst and second escape peaks. The Doppler-broadened line results from the 9 Be(p,a)6 Li reaction (Q 2.126 MeV) leaving 6 Li in the excited state at 3563 keV, which subsequently decays by gamma-ray emission. No other gamma-ray lines resulting directly from proton induced reactions in beryllium were identi®ed. Other major gammaray lines include proton induced prompt gammarays from the copper target holder (669, 691.3 and 962.2 keV), neutron induced lines in the germanium detector (596.4, 595.8 and 868.4 keV) and construction materials in the vicinity of the target and detector: 1778 keV from 27 Al(n,c)28 Al and 28 Si(n,n0 c)28 Al, 847.1 keV from 56 Fe(n,n0 c)56 Fe and notably 2223 keV line from 1 H(n,c)2 H. Also seen are high energy (Ec > 4 MeV) gamma-ray peaks resulting from neutron interactions with surrounding materials. These peaks, as resulting from time-uncorrelated interactions, are signi®cantly
Fig. 5. TOF-gated (a) and ungated (b) gamma-ray spectra at 40° obtained in bombardment of the thick beryllium target with 4.00 MeV protons. The major gamma-ray lines are labelled in keV. Spectra not corrected for detection eciency. The low energy cut-o at 260 keV is due to set ADC discrimination level.
suppressed by the TOF-gate and are clearly visible in the ungated spectrum. The following lines are identi®ed: 3539 and 4934 keV from 28 Si, 6110.9 and 7413.8 keV from 35 Cl and 7278.8, 7631.1 and 7645.5 keV from 56 Fe, they are accompanied by the ®rst and second escape peaks. Similar gamma-ray lines are observed in gamma-ray spectra obtained in bombardment of thick beryllium with 2.57 and 3.00 MeV protons [5]. All gamma-ray spectra were corrected for detection eciency and the total gamma-ray yields (260 keV 6 Ec 6 8191 keV) were calculated. The results, presented in Table 2, indicate that there is less than one gamma-ray photon per neutron produced in the thick target Be(p,n) reaction, at the energy ranges investigated. This ratio changes if
J. Guzek et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 471±475 Table 2 Gamma-ray yield (260 keV 6 Ec 6 8191 keV) at laboratory angle of 40°, for the thick target Be(p,n) reaction for proton energies of 2.57, 3.00 and 4.00 MeV Ep (MeV)
2.57 3 4
Gamma-ray yield (c/lC/sr) ´ 10ÿ7 TOF-gated spectrum
Ungated spectrum
0.82 2.15 6.56
8.51 25.5 86.6
all gamma-rays, including secondary eects, are counted, and depends on the speci®c operating conditions of the experiment. In our experimental condition there were, in the energy ranges under consideration, approximately 10 gamma-ray photons produced per neutron. 4. Conclusions The characteristics of neutron and gamma-ray emission resulting from bombardment of thick beryllium targets with protons with kinetic energies below 4 MeV were investigated. The neutron yield results from the strong single neutron group corre-
475
sponding to the ®rst excited level in the residual nucleus in the 9 Be(p,n)9 B reaction and a neutron continuum resulting from multi-body break-up reactions. The break up neutrons have energies higher than the ground state neutrons due to lower reaction threshold. There is less than one gamma-ray photon per neutron produced in the reaction. The gamma-ray spectrum is dominated by the Doppler-broadened 3563 keV line resulting from 9 Be(p,a)6 Li reaction. This data is of importance for applications such as BNCT, BNCS or radiography. References [1] C.-K. Wang, B.R. Moore, Med. Phys. 21 (1994) 1633. [2] J.C. Yanch et al., Proc. Int. Conf. Neutrons in Research and Industry, Crete, Greece, 1996 SPIE 2867, 1997, p. 31. [3] M.A. Lone et al., Nucl. Instr. and Meth. 189 (1981) 515. [4] W.B. Howard et al., Proc. Int. Conf. Neutrons in Research and Industry, Crete, Greece, 1996 SPIE 2867, 1997, p. 84. [5] J. Guzek, Ph.D Thesis, University of the Witwatersrand, Johannesburg, South Africa, in preparation. [6] J. Guzek et al., Characterisation of neutron and gamma emission from bombardment of beryllium with protons with kinetic energies below 10 MeV, South African Institute of Physics Congress, Durban, South Africa, 1997.