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Measurement of deep penetration of neutrons produced by 800-MeV proton beam through concrete and iron at ISIS
Nuclear Instruments and Methods in Physics Research B 179 (2001) 89±102
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Measurement of deep penetration of neutrons produced by 800-MeV proton beam through concrete and iron at ISIS T. Nunomiya a, N. Nakao b, P. Wright c, T. Nakamura a,*, E. Kim a,e, T. Kurosawa S. Taniguchi a,g, M. Sasaki a, H. Iwase a, Y. Uwamino d, T. Shibata b, S. Ito d, D.R. Perry c a
a,f
,
Department of Quantum Science and Energy Engineering, Tohoku University, Aza-aoba, Aramaki, Aoba-ku, Sendai-shi, Miyagi 980-8579, Japan b High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan c Rutherford Appleton Laboratory (RAL), Chilton, Didcot, Oxfordshire OX11 0QX, UK d Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama 351-0198, Japan e Japan Atomic Energy Research Institute (JAERI), Tokai, Naka, Ibaraki 319-1195, Japan f Electrotechnical Laboratory (ETL), 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan g Japan Synchrotron Radiation Research Institute (JASRI), Kouto 1-1-1, Mikazuki, Sayo, Hyogo 679-5198, Japan Received 26 October 2000; received in revised form 2 January 2001
Abstract A deep penetration experiment through a thick bulk shield was performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory (RAL), UK. ISIS is an 800 MeV±200 lA proton accelerator facility. Neutrons are produced from a tantalum target, which is shielded with approximately 3-m thick iron and 1-m thick ordinary concrete in the upward direction. On the top of the shield, we measured the neutron ¯ux attenuation through concrete and iron shields which were additionally placed up to 1.2-m and 0.6-m thicknesses, respectively, using the activation detectors of graphite, bismuth, aluminum and the multi-moderator spectrometer inserted indium. The attenuation lengths of concrete and iron for high-energy neutrons above 20 MeV produced at 90° to the proton beam were obtained from the 12 C n; 2n11 C reaction rates of graphite. The neutron spectra through concrete and iron were obtained by the unfolding analysis of the reaction rates of the 12 C n; 2n11 C, 27 Al n; a24 Na, 209 Bi n; xn210 x Bi x 4±10 and 115 In n; c116m In in the energy range of thermal to 400 MeV. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Spallation neutron; Activation detector; Attenuation length; Deep penetration; Neutron spectra
1. Introduction * Corresponding author. Tel.: +81-22-217-7805; fax: +81-22217-7809. E-mail address: [email protected] (T. Nakamura).
Shielding design is very important for the construction of an intense high-energy accelerator facility, since massive shields for high-energy
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 8 7 - 1
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neutrons having strong penetrability are required and then the cost for the radiation shielding contributes a considerable part of the total cost. Most shielding design for high-energy accelerators has been generally performed by using a point kernel method, that is the Moyer model, which is based on single exponential attenuation of neutron dose equivalent after penetrated through a thick shield to be in a spectral equilibrium state. In this deep penetration problem, the dose attenuation length, which is ruled by high-energy neutrons above about 100 MeV, is an essentially important parameter of the Moyer model. Neutron shielding experiments at several accelerator facilities have ever been performed to get the attenuation length, but the reliable experimental data penetrated through a very thick shield are still very scarce and dispersed [1±3]. Since 1992 the shielding experiments of deep penetration have been performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory (RAL) [4]. In these experiments, the neutrons penetrated through an additional shield of concrete and iron put upon the top of neutron source shield were measured to obtain the attenuation length of highenergy neutrons. But it was found that the additional shields were too small to decrease the background neutrons. In this succeeding study, we used the larger additional shields for better experimental geometry to decrease the background neutrons and measured the attenuation lengths and energy spectra of high-energy neutrons penetrated through concrete and iron at 90° to 800-MeV proton beam. This benchmark shielding experiment at an intense high-energy accelerator facility will give useful information for estimating the accuracy of the transport calculation for deep penetration.
800-MeV proton synchrotron and a spallation neutron target station. The beam intensity was about 170 lA at the target with 50-Hz repetition rate. A cross-sectional view around the target station along the 800-MeV proton beam axis is shown in Fig. 1. The tantalum target shown in Fig. 2 is assembled by the 90-mm diam tantalum disk of various thicknesses from 8.2 mm to 26.7 mm, and the gap between each disk for the cooling water (D2 O) is 1.75 mm [5]. This stopping-length tantalum target (total length of 296.5 mm) is placed at the center of the stainless-steel vessel. The moderators of heavy water and beryllium re¯ectors are placed around the target as seen in Fig. 2. The upward direction of the spallation target station is shielded with a shielding plug of 284-cm thick steel and 97-cm thick ordinary concrete given in Fig. 3. This experiment was performed at the top of the shield (shield top) just above the target station. As seen in Fig. 1, a big bent duct of about 40 cm 40 cm in which helium gas ¯ows for ®lling the target vessel reaches the shield top through the bulk shield downstream from the target. Neutrons that leaked from this duct became large background components in our measurement at the shield top. We therefore set an iron igloo (shown in Fig. 4) which is 60-cm thick, 120-cm inner diameter and 196-cm high on the top center of target station to reduce the background radiation, and
2. Experiment 2.1. ISIS facility The experiment was performed at the ISIS spallation neutron source facility, RAL. The ISIS facility consists of 70-MeV H linear accelerator,
Fig. 1. Cross-sectional view of neutron spallation target station with 800-MeV proton beam at ISIS.
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Fig. 2. Tantalum target and target assembly with re¯ectors and moderators.
additional shield of concrete or iron was piled up inside the igloo. 2.2. Additional shielding materials In this shielding experiment, the additional shielding blocks of ordinary concrete and iron were placed upon the top center of the bulk shield just above the target as shown in Fig. 4. Concrete shields of 20-, 40-, 60-, 80-, 100- and 120-cm thicknesses by using the 119-cm diam by 20-cm thick blocks of 2.36 g=cm3 density, and iron shields of 10-, 20-, 30-, 40-, 50- and 60-cm thicknesses were assembled by using the 119-cm diam by 10-cm thick blocks of 7.8 g=cm3 density. The concrete blocks contain the iron mesh for rein-
forcement. The major elements in additional concrete blocks analyzed by Kinno et al. [6] and in iron are shown in Tables 1 and 2, respectively. 2.3. Detectors The neutrons were produced from the target as the burst pulses corresponding to the 50-Hz synchrotron operation, and the pulse counters could not be used here because of the pulse pile-up problem. We therefore used the activation detectors for neutron spectrum measurement. The crays from the activation detectors after irradiation in this experiment were measured with two highpurity germanium (HPGe) detectors (GC-1818 and GC-2020, Canberra Industries).
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*DS
þ 9DFXXP 3ODWH 6WHHO
þ
&RQFUHWH
þ
*DS
þ
6WHHO
*DS
6WHHO
þ
)ODQJHRQ9RLG9HVVHO
XQLW FP
Fig. 3. Cross-sectional view of shielding plug.
2.3.1. 12 C n; 2n11 C activation detector Two types of detectors were used, one is the disk type of 8-cm diam 3-cm thickness, and the other is the Marinelli-type as shown in Fig. 5, for measuring the spatial neutron ¯ux distribution on the shield top without additional shield and behind the additional concrete and iron shields with changing the thickness. The 12 C n; 2n11 C reaction has the threshold energy of about 20 MeV and the reaction cross-
Fig. 4. Horizontal and vertical cross-sectional view of iron igloo and additional shield. The ®ve positions of center, up, down, left and right at which the detectors were placed are shown as white circles.
Table 1 Major elements and water contents (w%) in the additional concrete shield blocks used in this studya
a
H
C
O
Na
Mg
Al
Si
S
K
Ca
Ti
Fe
1.08
6.01
51.34
0.12
0.28
0.76
12.56
0.19
0.28
21.99
0.03
5.36
(H2 O 9.74).
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Table 2 Major elements (w%) in the additional iron shield blocks C
Mn
Si
P
S
Other
Fe
0.14
1.0
0.32
0.02
0.008
0.02
98.5
2.3.2. 209 Bi n; xn activation detector The bismuth detector, 8-cm diam 1.1-cm thickness, was used to get the high-energy neutron spectrum by using the 209 Bi n; 4n206 Bi to 209 Bi n; 10n200 Bi reactions. The cross-section data have also been measured by our group [7] in the energy range of 20±150 MeV and the measured data are in good agreement with the ENDF/B-VI high-energy library data [9] calculated by Fukahori [10], as seen in Fig. 7. Their threshold energies regularly increase from 22 MeV of 209 Bi n; 4n to 70 MeV of 209 Bi n; 10n with the interval of 8 MeV corresponding to binding energy per nucleon. The half lives of 200 Bi to 206 Bi are between 36.4 min and 15.31 days. The physical properties of 209 Bi n; xn reaction are shown in Table 3, together with those of graphite, aluminum and indium reactions.
Fig. 5. Cross-sectional view of disk and Marinelli-type graphite activation detectors.
section has recently been measured up to 150 MeV by our group (see Table 3) [7,8], which has almost constant value of about 20 mb above 20 MeV as shown in Fig. 6. The half-life of 11 C is about 20 min, which is a shortly activated good neutron ¯ux monitor of energy above 20 MeV.
2.3.3. 27 Al n; a24 Na activation detector The aluminum detector is the Marinelli type of the same size as the graphite (see Fig. 5). The threshold energy of 27 Al n; a24 Na reaction is 3.25 MeV and the half-life of 24 Na is 15.02 h. The
Table 3 Physical parameters of activation detectors used in this study Activation detector
Reaction
Half-life
Threshold energy (MeV)
Main c-ray energy (keV)
Irradiation time
Graphite Bismuth
12
20.4 min 36.4 min 1.77 h 1.67 h 11.76 h 11.22 h 15.31 days 6.243 days 15.02 h 54.12 min
20.40 70.79 61.73 53.98 45.31 37.99 29.63 22.56 3.25 ±
511.0 462.4, 1026.5 629.1 422.2, 657.5, 960.7 820.2, 825.2, 1847.6 984.0, 899.2 703.4, 703.4 803.1, 881.0, 1718.7 1368.6 1293.5
40 min 2h 2h 2h 24 h 24 h 24 h 24 h 24 h 1h
Aluminum Indium
C n; 2n11 C Bi n; 10n200 Bi 209 Bi n; 9n201 Bi 209 Bi n; 8n202 Bi 209 Bi n; 7n203 Bi 209 Bi n; 6n204 Bi 209 Bi n; 5n205 Bi 209 Bi n; 4n206 Bi 27 Al n; a24 Na 115 In n; c116m In 209
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Fig. 6. Measured cross-section data of 12 C n; 2n reaction [7,8].
Fig. 7. Measured [7] and calculated [9] cross-section data of 209 Bi n; xn x 4±10 reaction.
cross-section data is given by the ENDF/B-VI high-energy library [9] calculated by Fukahori using the ALICE code [11] (Fig. 8). 2.3.4. Multi-moderator spectrometer inserted 115 In n; c116m In activation detector The 2.875-g In2 O3 powder is ®lled in a spherical cavity of 0.735-cm radius in a cylindrical acryl [12] which is placed in the the spherical polyethylene moderator. Five moderators of 9.8-, 5.5-, 3.2- and 2.0-cm radii, and 0-cm radius without moderator
Fig. 8. Cross-section data of 27 Al n; a24 Na reaction calculated by Fukahori using the ALICE code [10].
were used to get the neutron energy spectrum through the unfolding technique. The response functions of these multi-moderator spectrometers were calculated with the MCNPX-2.1.5 code [13]. The neutron cross-sections of 113 In, 115 In, 1 H, 12 C and 16 O used in the calculation were cited from the ENDF/B-VI data library [9] for thermal to 20 MeV and the LA-150 data library [14] above 20 MeV. The 115 In n; c116m In reaction cross-section data thus obtained were multiplied by a factor of 160= 160 42 [15], which is the branching ratio of producing 116m In when a thermal neutron is captured by 115 In. Fig. 9 shows the calculated response functions of this indium-loaded spectrometer, comparing with the experimental data measured by Uwamino et al. [12]. The agreement between calculated and measured results is pretty good. 2.4. Experimental procedure This experiment was performed during the beam time of about six weeks from 29 September to 7 November in 1998 by changing the thickness of concrete and iron shields with keeping the proton beam intensity at 170 lA. The graphite activation detectors were set at various positions called ``center'', ``up'', ``down'', ``left'' and ``right'' on the additional shield surface of each thickness
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95
times a day, and the photo-peak counts in the same condition were summed up to get the reaction rates. The neutron dose meter (Rem Ion Monitor, Harwell, UK) and the c-ray dose meter (RO2, Eberline, USA) were used in a current mode for measurements of neutron and c-ray dose equivalent data, respectively. 2.5. Analysis
Fig. 9. Response functions of indium-loaded multi-moderator spectrometer and comparison with the measured data [12].
as shown in Fig. 4. In order to investigate the spatial neutron ¯ux distribution above the shield top, the graphite detectors were also irradiated at once at these positions in the air without additional shield. The up and down positions mean upstream and downstream from the center just above the target along the proton beam direction, and the left and right mean the left and right sides perpendicular to the proton beam direction. The graphite detectors were used to get the attenuation length of neutron ¯ux above 20 MeV. The neutron energy spectrum through the bulk shield was measured using the aluminum, bismuth, graphite and indium-loaded multi-moderator detectors. These detectors were set at the center position on the shield surface for three conditions without additional shield (on the ¯oor), with 60cm thick concrete and with 30-cm thick iron shield. In order to get good statistics of photo-peak counts measured with two HPGe detectors, repeated irradiations and activity measurements were performed for each shield thickness several
2.5.1. Beam current monitor The neutron intensity from the tantalum target is proportional to the incident proton beam current. This proton beam current was continuously monitored by electromagnetic coil voltage just before the RIKEN (RAL Branch of Institute of Physical and Chemical Research) muon target during the experiment. This voltage was converted to ampere (Coulomb) with the correction of the beam loss in the muon target and used in the analysis of the activation detectors. Fig. 10 shows an example of the trend graph of the proton beam current. The errors of this beam current were assumed to be 1%. 2.5.2. Calculation of the reaction rates When the irradiation is repeated n times and the photo-peak counts are summed up m times, the reaction rate per beam current R r/ can be obtained as follows: ! 1 m Ck X R Aj atom 1 C 1 ; 1 ec j1
Fig. 10. A trend graph of proton beam current just before the RIKEN muon target (21 October 1998 8:00 a.m.±4:00 p.m.).
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Aj Nj
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102 n X Qij i1
Dt 1
1
e
e
kTm
j
kDt
e
k n iDt
;
e
j
DR
kTc
2
where R is the reaction rate (atom 1 C 1 ), C the total counts of c-ray peak area, Dt the measuring time bin ( 1 min), Qi the beam current during the irradiation time interval Dt (Coulomb), N the number of atoms in the sample (parent nucleus), r the reaction cross-section (cm2 ), / the neutron ¯uence (cm 2 ), k the decay constant (min 1 ), e the peak eciency (with the self-absorption and sumcoincident eect), c the branching ratio of c-rays, Tc the cooling time (min) and Tm is the counting time (min). The peak eciencies of the HPGe detectors e including the self-absorption eect and the sumcoincidence eect were estimated by using the EGS4 Monte Carlo code [16] and the SUMECC code developed by Torii et al. [17], respectively. The nucleus data necessary for this estimation were cited from the Table of Isotopes of eighth edition [18]. For graphite detector, the 511 keV crays from natural background were subtracted from the peak counts. 2.5.3. Estimation of the error The errors of the reaction rates can be estimated from the following equation:
Fig. 11. Spatial distribution of
12
q DQ2H Dc2 Dp2 :
3
DQH is the error of beam current which was assumed to be 1% (in Section 2.5.1). Dc is the statistical error of photo peak area, and Dp is the error of peak eciency of HPGe detector estimated by the EGS4 Monte Carlo code, which is within a few %. DR is dominated by Dc, which varies from 2% to 50% depending on the total peak counts.
3. Results and discussions 3.1. 12 C n; 2n11 C reaction rate distribution on the shield top The 12 C n; 2n reaction rates correspond to the neutron ¯ux above 20 MeV, since the 12 C n; 2n cross-section has almost constant value of 20 mb above 20 MeV (See Fig. 6). Fig. 11 shows the reaction rates (atom 1 C 1 ) of the 12 C n; 2n11 C reaction on the shield top with additional concrete shield of dierent thickness. Fig. 12 shows those with iron shield. The errors of the results are the statistical errors of the photopeak counts of the 511-keV c-rays. Figs. 11(a) and 12(a) give the reaction rate distributions along the direction of the proton beam axis, where the center is a position on the shield top just above the target,
C n; 2n11 C reaction rate with the thickness of additional concrete shield.
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Fig. 12. Spatial distribution of
12
C n; 2n11 C reaction rate with the thickness of additional iron shield.
and Figs. 11(b) and 12(b) give those along the direction perpendicular to the proton beam axis. In Figs. 11(a) and 12(a), the reaction rates increase from the upstream to the downstream direction although the reaction rates along the transverse direction in Figs. 11(b) and 12(b) are almost uniform. This means that the neutron streaming eect from the helium duct that situates in about 4-m downstream from the center of the shield top cannot be neglected (see Fig. 1). From the comparison of Figs. 11(a) and 12(a), the reaction rate distribution through concrete becomes more uniform than that through iron with increasing the shield thickness. This means that the neutron spectrum changes on the boundary between the additional concrete shield and the iron igloo surrounding the shield (see Fig. 4). In Figs. 11(b) and 12(b) the reaction rates at the 50-cm left and right positions are larger than those at the center position for thin additional shield by re¯ecting some streaming from the circular gap of about 0.5 and 1.5 cm around the shielding plug which exists at the respective radius of 26 and 74 cm from the center of the shielding plug (see Fig. 3). 3.2. Attenuation pro®les of reaction rates through shield 12
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Fig. 13 shows the attenuation pro®les of the C n; 2n11 C reaction rates with the thickness of
concrete and iron at ®ve positions on the shield top as shown in Fig. 4; Fig. 13(a) at the center position on the shield top center, Fig. 13(b) at the 50-cm upstream position, up, Fig. 13(c) at the 50-cm downstream position, down, Fig. 13(d) at the 50cm left side from the center, left, Fig. 13(e) at 50cm right side from the center, right, along the beam direction. These ®gures also give the spatial distribution of the reaction rates in the air along the vertical axis of the bulk shield upward from the tantalum target without any additional shield on the shield top. We veri®ed that the data without additional shield decreased approximately with the square of the distance from the tantalum target which are shown in solid line, except at the down position where the data might be in¯uenced still by the streaming from the helium duct. With additional concrete and iron shields, the reaction rate can be ®tted well using the least-square method to the following equation: R
R0 e r2
t=k
;
4
where r is a distance from the tantalum target, t is an additional shield thickness and k is an attenuation length corresponding to neutron ¯ux above 20 MeV. This ®tting was done excluding the data on the ¯oor and 20-cm concrete thickness, because the neutron attenuation length may change at the shallow position. From this ®tting, we could
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Fig. 13. Attenuation pro®les of 12 C n; 2n11 C reaction rates as a function of the distance from the shield top at the center, up, down, left and right positions.
obtain the attenuation length k of neutron ¯ux above 20 MeV for concrete and iron as shown in Table 4. It is found that the k values at ®ve positions behind the shield show some dierence each other and there are large errors at the right, left
and up positions, owing to the in¯uence of streaming from the helium duct and the gap of beam shielding plug, and of boundary eect between the additional shield and iron igloo. The k values at these ®ve positions are averaged, and are
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102 Table 4 Attenuation length of neutrons above 20 MeV for concrete and iron at ®ve positions on the shield surface in g=cm2 Position
Concrete (2.36 g=cm3 )
Iron (7.8 g=cm3 )
Center Up Down Left Right
125.4 5.1 111.3 25.7 124.5 9.2 126.6 11.2 130.7 43.4
161.1 2.1 170.5 11.3 157.0 6.1 141.5 17.5 154.6 2.6
Average
123.7 52.7
156.9 22.0
Finally selected
125.4 5.1
161.1 2.1
shown in Table 4 together with the large errors. We therefore decided to select ®nally the values at the center position of 125:4 5:1 g=cm2 for con3 2 crete (2:36 g=cm ), 161:1 2:1 g=cm for iron 3 (7:8 g=cm ), because they have most probable values with the smallest errors coming from the spurious in¯uence described above, by comparing with the values at other positions. The attenuation length thus obtained for concrete is compared with other experimental and calculated results [1±3, 19±22] in Fig. 14. The present value of 2 125:4 g=cm is just between the values of 2 120 g=cm for 25-GeV proton by Stevenson et al. 2 [1] and 143 g=cm for 12-GeV proton by Ban et al. [2]. For the attenuation length of iron the present 2 value of 161:1 g=cm is a little larger than that of
Fig. 14. Comparison of various data on neutron attenuation length for concrete as a function of incident proton energy [1± 3,19±22].
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148 g=cm2 obtained by Jerey et al. [23] using a 800-MeV proton accelerator at Los Alamos National Laboratry. Stevenson [1] and Ban [2] have 2 also given the values of 147 10 g=cm for 252 GeV proton and 188 12 g=cm for 12-GeV proton, respectively. Fig. 15 shows the attenuation pro®les of the 209 Bi n; xn210 x Bi x 4±10 reaction rates at the center position with increasing the thickness of concrete and iron shields. All experimental data can be well ®tted to a solid line of Eq. (4) having the same attenuation length k as that ®tted to the 12 C n; 2n reaction data, in spite of dierent threshold energies of these reactions. This clari®ed that the neutron energy spectrum after penetrated through the deep shield is in an equilibrium state. 3.3. Attenuation pro®les of the dose equivalent Fig. 16 shows the attenuation pro®les of the neutron and c-ray dose equivalents measured with the REM-counter and c-ray dose meter at the center position of additional concrete and iron shields on the shield top. This REM-counter is a conventional ionization-type detector which has low sensitivity to neutrons of energies higher than 20 MeV. The measured data could also be ®tted to the same equation (Eq. (4)) and got the attenuation lengths of dose equivalents with the least-square method. For concrete shield, neutron dose equivalent decreases quickly up to 60-cm thickness but beyond 60-cm thickness it decreases with the similar slope as that of 12 C n; 2n11 C reaction 2 rate (' 125 g=cm ). This may be partly because 3.6-cm thick iron plate under the additional concrete shield will produce lower-energy neutrons below MeV region and this neutrons can be easily attenuated by concrete shield and approach a spectrum equilibrium state. The c-ray dose equivalent has the same slope as the 12 C n; 2n11 C reaction rate, because the c-rays are generated by the high-energy neutron reactions and have no direct components in this deep penetration geometry. For iron shield, on the other hand, the attenuation of neutron dose equivalent is much slower than that of 12 C n; 2n11 C reaction rate. This
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Fig. 15. Attenuation pro®les of
209
Bi n; xn210 x Bi x 4±10 reactions for concrete and iron at the center position.
means that the neutron slowing-down accompanying the production of lower energy neutrons continues in iron and the neutron spectrum changes with increasing the iron thickness. The atten-
uation of c-ray dose equivalent is a little larger than that of 12 C n; 2n11 C reaction rate. This may be explained from the addition of c-rays produced from 56 Fe n; c reaction mainly by low-energy
Fig. 16. Attenuation pro®les of neutron and c-ray dose equivalents for concrete and iron at the center position compared with the 12 C n; 2n reaction rate.
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neutrons to those produced by high-energy neutrons. 3.4. Neutron energy spectra The neutron energy spectra were measured at three conditions, on the ¯oor without additional shield, behind 60-cm thick additional concrete shield and 30-cm thick additional iron shield. Fig. 17 shows the neutron energy spectra in lethargy unit on the ¯oor of shield top and on the surfaces of the additional concrete and iron shields which were obtained with the activation detectors and the indium-loaded multi-moderator spectrometer through the unfolding by the SAND-II code [24]. The initial guess spectra for the SAND-II unfolding were used from the spectra calculated with the ANISN code [25] by Nakao and Uwamino [26]. The response functions of these detectors are given in Fig. 9. The obtained neutron spectrum on the ¯oor of shield top has two components; high-energy component of a peak around 100 MeV in the energy range above 10 MeV and low-energy component having a broad peak around 500 keV below 10 MeV. The former is the direct component of neutrons penetrated at 90° from the tantalum target bombarded by 800-MeV protons and
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the latter is the neutrons slowed down in iron and concrete shields. The neutron spectrum through 60-cm thick concrete shield placed additionally on the shield top shows a rather ¯attened lethargy spectrum down to thermal energy especially in the low-energy component below 10 MeV. This re¯ects the slowing-down eect due to the elastic collision of light elements like hydrogen in the concrete. This spectrum has a typical 1=E slowing-down spectrum after penetrating the concrete. The neutron spectrum through 30-cm thick iron shield has also a typical spectrum after penetrated the iron, where high-energy neutron components above a few MeV are slowed down to less than 1 MeV with the inelastic collision of iron and a broad peak over the region of 10±500 keV is remarkable due to relatively low inelastic cross section of iron in this energy region. 4. Conclusion The deep penetration experiment was performed at the ISIS neutron source using the 800MeV proton synchrotron. The attenuation pro®les through concrete and iron shields were clari®ed and the attenuation lengths of high-energy neutrons produced at 90° by 800-MeV protons were estimated. The neutron energy spectra were obtaind on the ¯oor of the shield top, through 60-cm thick additional concrete shield and through 30cm thick additional iron shield. The shielding con®guration is well de®ned and the measured attenuation lengths of concrete and iron will be the good benchmark data for investigating the accuracy of deep penetration calculation. Acknowledgements
Fig. 17. Neutron energy spectra on the shield top ¯oor, 60-cm thick additional concrete shield and 30-cm thick additional iron shield.
The authors wish to thank the machine group of RAL for synchrotron operation, RIKEN branch oce at RAL for monitoring the beam pro®les and various arrangements during this shielding experiment. This experiment is partly ®nancially supported by Japan Atomic Energy Research Institute.
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References [1] G.R. Stevenson, K.L. Liu, R.H. Thomas, Health Phys. 43 (1982) 13. [2] S. Ban, H. Hirayama, K. Kondo, S. Miura, K. Hozumi, M. Taino, A. Yamamoto, H. Hirabayashi, K. Katoh, Nucl. Instr. and Meth. 174 (1980) 271. [3] S. Ban, H. Hirayama, K. Katoh, Nucl. Instr. and Meth. 184 (1981) 409. [4] Y. Uwamino, T. Shibata, T. Ohkubo, S. Sato, D. Perry, OECD/NEA/NSC The Specialists' Meeting on Shielding Aspects of Accelerators, Targets, and Irradiation Facilities, Arlington, Texas, April 1994, p. 185. [5] A. Carne, G.H. Eaton, F. Atchison, T.A. Broome, D.J. Clarke, B.R. Diplock, B.H. Poulten, K.H. Roberts, SNS Target Station Safety Assessment (Support Document to SNS Target Station Hazard Survey SNS/ENV/N1/83 Rev.1) SNSPC/P6/82, 1983. [6] M. Kinno, S. Yokosuka, Fujita Co., Japan, 1999, Private communication. [7] E. Kim, T. Nakamura, A. Konno, Nucl. Sci. Eng. 129 (1998) 209. [8] Y. Uno, Y. Uwamino, T.S. Soewarsono, T. Nakamura, Nucl. Sci. Eng. 122 (1996) 247. [9] Evaluated Nuclear Data File, ENDF/B-VI, National Neutron Cross Section Center, Brookhaven National Laboratory, 1990. [10] T. Fukahori, in: Proceeding of the 1990 Symposium on Nuclear Data, Tokai, Japan, JAERI-M 91-032, 1991, p. 106. [11] M. Blann, Code ALICE/89, 1989, Private communication. [12] Y. Uwamino, T. Nakamura, A. Hara, Nucl. Instr. and Meth. A 239 (1985) 299. [13] H.G. Hughes et. al., MCNPX for neutron±proton transport, in: Proceedings of the International Conference on Mathematics and Computation, Reactor Physics & Environmental Analysis in Nuclear Applications, American Nuclear Society, Madrid Spain, September 1999.
[14] M.B. Chadwick, L.J. Cox, P.G. Young, A.S. Meigooni, Nucl. Sci. Eng. 123 (1996) 17. [15] G.F. Knoll, in: Radiation Detection and Measurement, Wiley, New York, 1979. [16] W.R. Nelson, H. Hirayama, D.W.O. Rogers, The EGS4 Code System, SLAC-265, Stanford Linear Accelerator Center, Stanford University, 1985. [17] A. Torii, Y. Uwamino, T. Nakamura, Computer Code for Activation Data Analysis, Coincidence Summing Correction Code: SUMECC and Decay Correction Code: DECFIT, INS-T-468, Institute for Nuclear Study, University of Tokyo, 1987. [18] R.B. Firestone, V.S. Shirley (Eds.), Table of Isotopes, 8th ed., Wiley, New York, 1996. [19] R.G. Alsmiller Jr., R.T. Santro, J. Barish, Particle Accl. 7 (1975) 1. [20] L.L. Carter, Nucl. Technol. 3 (1983) 165. [21] T.H. Braid, R.F. Rapid, R.H. Siemssen, J.W. Tippie, IEEE Trans. Nucl. Sci. 18 (1971) 821. [22] K. O'Brien, Quoted in Accelerator Helth Physics, Academic Press, New York, 1973, p. 463. [23] J.S. Bull, J.B. Donahue, R.L. Burman, in: Proceedings of the Fourth Workshop on Simulating Accelerator Radiation Environments, Knoxvill, Tennessee, September 1998, p. 201. [24] W.N. McElroy, S. Berg, T. Crockett, R.G. Hawkins, A computer automated iterative method for neutron ¯ux spectra determination by foil activation, AFWL-TR-67-41, Air Force Weapons Laboratory, Kirtland Air Force Base, 1967. [25] W.A. Engle Jr., A User's Manual for ANISN, A OneDimensional Discrete Ordinates Transport Code With Anisotropic Scattering, USAEC Report K-1693, 1967. [26] N. Nakao, Y. Uwamino, in: Proceedings of Ninth International Conference on Radiation Shielding. J. Nucl. Sci. Technol. March (Suppl. 1) (2000) 162.
www.elsevier.nl/locate/nimb
Measurement of deep penetration of neutrons produced by 800-MeV proton beam through concrete and iron at ISIS T. Nunomiya a, N. Nakao b, P. Wright c, T. Nakamura a,*, E. Kim a,e, T. Kurosawa S. Taniguchi a,g, M. Sasaki a, H. Iwase a, Y. Uwamino d, T. Shibata b, S. Ito d, D.R. Perry c a
a,f
,
Department of Quantum Science and Energy Engineering, Tohoku University, Aza-aoba, Aramaki, Aoba-ku, Sendai-shi, Miyagi 980-8579, Japan b High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan c Rutherford Appleton Laboratory (RAL), Chilton, Didcot, Oxfordshire OX11 0QX, UK d Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama 351-0198, Japan e Japan Atomic Energy Research Institute (JAERI), Tokai, Naka, Ibaraki 319-1195, Japan f Electrotechnical Laboratory (ETL), 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan g Japan Synchrotron Radiation Research Institute (JASRI), Kouto 1-1-1, Mikazuki, Sayo, Hyogo 679-5198, Japan Received 26 October 2000; received in revised form 2 January 2001
Abstract A deep penetration experiment through a thick bulk shield was performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory (RAL), UK. ISIS is an 800 MeV±200 lA proton accelerator facility. Neutrons are produced from a tantalum target, which is shielded with approximately 3-m thick iron and 1-m thick ordinary concrete in the upward direction. On the top of the shield, we measured the neutron ¯ux attenuation through concrete and iron shields which were additionally placed up to 1.2-m and 0.6-m thicknesses, respectively, using the activation detectors of graphite, bismuth, aluminum and the multi-moderator spectrometer inserted indium. The attenuation lengths of concrete and iron for high-energy neutrons above 20 MeV produced at 90° to the proton beam were obtained from the 12 C n; 2n11 C reaction rates of graphite. The neutron spectra through concrete and iron were obtained by the unfolding analysis of the reaction rates of the 12 C n; 2n11 C, 27 Al n; a24 Na, 209 Bi n; xn210 x Bi x 4±10 and 115 In n; c116m In in the energy range of thermal to 400 MeV. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Spallation neutron; Activation detector; Attenuation length; Deep penetration; Neutron spectra
1. Introduction * Corresponding author. Tel.: +81-22-217-7805; fax: +81-22217-7809. E-mail address: [email protected] (T. Nakamura).
Shielding design is very important for the construction of an intense high-energy accelerator facility, since massive shields for high-energy
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 8 7 - 1
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neutrons having strong penetrability are required and then the cost for the radiation shielding contributes a considerable part of the total cost. Most shielding design for high-energy accelerators has been generally performed by using a point kernel method, that is the Moyer model, which is based on single exponential attenuation of neutron dose equivalent after penetrated through a thick shield to be in a spectral equilibrium state. In this deep penetration problem, the dose attenuation length, which is ruled by high-energy neutrons above about 100 MeV, is an essentially important parameter of the Moyer model. Neutron shielding experiments at several accelerator facilities have ever been performed to get the attenuation length, but the reliable experimental data penetrated through a very thick shield are still very scarce and dispersed [1±3]. Since 1992 the shielding experiments of deep penetration have been performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory (RAL) [4]. In these experiments, the neutrons penetrated through an additional shield of concrete and iron put upon the top of neutron source shield were measured to obtain the attenuation length of highenergy neutrons. But it was found that the additional shields were too small to decrease the background neutrons. In this succeeding study, we used the larger additional shields for better experimental geometry to decrease the background neutrons and measured the attenuation lengths and energy spectra of high-energy neutrons penetrated through concrete and iron at 90° to 800-MeV proton beam. This benchmark shielding experiment at an intense high-energy accelerator facility will give useful information for estimating the accuracy of the transport calculation for deep penetration.
800-MeV proton synchrotron and a spallation neutron target station. The beam intensity was about 170 lA at the target with 50-Hz repetition rate. A cross-sectional view around the target station along the 800-MeV proton beam axis is shown in Fig. 1. The tantalum target shown in Fig. 2 is assembled by the 90-mm diam tantalum disk of various thicknesses from 8.2 mm to 26.7 mm, and the gap between each disk for the cooling water (D2 O) is 1.75 mm [5]. This stopping-length tantalum target (total length of 296.5 mm) is placed at the center of the stainless-steel vessel. The moderators of heavy water and beryllium re¯ectors are placed around the target as seen in Fig. 2. The upward direction of the spallation target station is shielded with a shielding plug of 284-cm thick steel and 97-cm thick ordinary concrete given in Fig. 3. This experiment was performed at the top of the shield (shield top) just above the target station. As seen in Fig. 1, a big bent duct of about 40 cm 40 cm in which helium gas ¯ows for ®lling the target vessel reaches the shield top through the bulk shield downstream from the target. Neutrons that leaked from this duct became large background components in our measurement at the shield top. We therefore set an iron igloo (shown in Fig. 4) which is 60-cm thick, 120-cm inner diameter and 196-cm high on the top center of target station to reduce the background radiation, and
2. Experiment 2.1. ISIS facility The experiment was performed at the ISIS spallation neutron source facility, RAL. The ISIS facility consists of 70-MeV H linear accelerator,
Fig. 1. Cross-sectional view of neutron spallation target station with 800-MeV proton beam at ISIS.
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
91
Fig. 2. Tantalum target and target assembly with re¯ectors and moderators.
additional shield of concrete or iron was piled up inside the igloo. 2.2. Additional shielding materials In this shielding experiment, the additional shielding blocks of ordinary concrete and iron were placed upon the top center of the bulk shield just above the target as shown in Fig. 4. Concrete shields of 20-, 40-, 60-, 80-, 100- and 120-cm thicknesses by using the 119-cm diam by 20-cm thick blocks of 2.36 g=cm3 density, and iron shields of 10-, 20-, 30-, 40-, 50- and 60-cm thicknesses were assembled by using the 119-cm diam by 10-cm thick blocks of 7.8 g=cm3 density. The concrete blocks contain the iron mesh for rein-
forcement. The major elements in additional concrete blocks analyzed by Kinno et al. [6] and in iron are shown in Tables 1 and 2, respectively. 2.3. Detectors The neutrons were produced from the target as the burst pulses corresponding to the 50-Hz synchrotron operation, and the pulse counters could not be used here because of the pulse pile-up problem. We therefore used the activation detectors for neutron spectrum measurement. The crays from the activation detectors after irradiation in this experiment were measured with two highpurity germanium (HPGe) detectors (GC-1818 and GC-2020, Canberra Industries).
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T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
*DS
þ 9DFXXP 3ODWH 6WHHO
þ
&RQFUHWH
þ
*DS
þ
6WHHO
*DS
6WHHO
þ
)ODQJHRQ9RLG9HVVHO
XQLW FP
Fig. 3. Cross-sectional view of shielding plug.
2.3.1. 12 C n; 2n11 C activation detector Two types of detectors were used, one is the disk type of 8-cm diam 3-cm thickness, and the other is the Marinelli-type as shown in Fig. 5, for measuring the spatial neutron ¯ux distribution on the shield top without additional shield and behind the additional concrete and iron shields with changing the thickness. The 12 C n; 2n11 C reaction has the threshold energy of about 20 MeV and the reaction cross-
Fig. 4. Horizontal and vertical cross-sectional view of iron igloo and additional shield. The ®ve positions of center, up, down, left and right at which the detectors were placed are shown as white circles.
Table 1 Major elements and water contents (w%) in the additional concrete shield blocks used in this studya
a
H
C
O
Na
Mg
Al
Si
S
K
Ca
Ti
Fe
1.08
6.01
51.34
0.12
0.28
0.76
12.56
0.19
0.28
21.99
0.03
5.36
(H2 O 9.74).
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93
Table 2 Major elements (w%) in the additional iron shield blocks C
Mn
Si
P
S
Other
Fe
0.14
1.0
0.32
0.02
0.008
0.02
98.5
2.3.2. 209 Bi n; xn activation detector The bismuth detector, 8-cm diam 1.1-cm thickness, was used to get the high-energy neutron spectrum by using the 209 Bi n; 4n206 Bi to 209 Bi n; 10n200 Bi reactions. The cross-section data have also been measured by our group [7] in the energy range of 20±150 MeV and the measured data are in good agreement with the ENDF/B-VI high-energy library data [9] calculated by Fukahori [10], as seen in Fig. 7. Their threshold energies regularly increase from 22 MeV of 209 Bi n; 4n to 70 MeV of 209 Bi n; 10n with the interval of 8 MeV corresponding to binding energy per nucleon. The half lives of 200 Bi to 206 Bi are between 36.4 min and 15.31 days. The physical properties of 209 Bi n; xn reaction are shown in Table 3, together with those of graphite, aluminum and indium reactions.
Fig. 5. Cross-sectional view of disk and Marinelli-type graphite activation detectors.
section has recently been measured up to 150 MeV by our group (see Table 3) [7,8], which has almost constant value of about 20 mb above 20 MeV as shown in Fig. 6. The half-life of 11 C is about 20 min, which is a shortly activated good neutron ¯ux monitor of energy above 20 MeV.
2.3.3. 27 Al n; a24 Na activation detector The aluminum detector is the Marinelli type of the same size as the graphite (see Fig. 5). The threshold energy of 27 Al n; a24 Na reaction is 3.25 MeV and the half-life of 24 Na is 15.02 h. The
Table 3 Physical parameters of activation detectors used in this study Activation detector
Reaction
Half-life
Threshold energy (MeV)
Main c-ray energy (keV)
Irradiation time
Graphite Bismuth
12
20.4 min 36.4 min 1.77 h 1.67 h 11.76 h 11.22 h 15.31 days 6.243 days 15.02 h 54.12 min
20.40 70.79 61.73 53.98 45.31 37.99 29.63 22.56 3.25 ±
511.0 462.4, 1026.5 629.1 422.2, 657.5, 960.7 820.2, 825.2, 1847.6 984.0, 899.2 703.4, 703.4 803.1, 881.0, 1718.7 1368.6 1293.5
40 min 2h 2h 2h 24 h 24 h 24 h 24 h 24 h 1h
Aluminum Indium
C n; 2n11 C Bi n; 10n200 Bi 209 Bi n; 9n201 Bi 209 Bi n; 8n202 Bi 209 Bi n; 7n203 Bi 209 Bi n; 6n204 Bi 209 Bi n; 5n205 Bi 209 Bi n; 4n206 Bi 27 Al n; a24 Na 115 In n; c116m In 209
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T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
Fig. 6. Measured cross-section data of 12 C n; 2n reaction [7,8].
Fig. 7. Measured [7] and calculated [9] cross-section data of 209 Bi n; xn x 4±10 reaction.
cross-section data is given by the ENDF/B-VI high-energy library [9] calculated by Fukahori using the ALICE code [11] (Fig. 8). 2.3.4. Multi-moderator spectrometer inserted 115 In n; c116m In activation detector The 2.875-g In2 O3 powder is ®lled in a spherical cavity of 0.735-cm radius in a cylindrical acryl [12] which is placed in the the spherical polyethylene moderator. Five moderators of 9.8-, 5.5-, 3.2- and 2.0-cm radii, and 0-cm radius without moderator
Fig. 8. Cross-section data of 27 Al n; a24 Na reaction calculated by Fukahori using the ALICE code [10].
were used to get the neutron energy spectrum through the unfolding technique. The response functions of these multi-moderator spectrometers were calculated with the MCNPX-2.1.5 code [13]. The neutron cross-sections of 113 In, 115 In, 1 H, 12 C and 16 O used in the calculation were cited from the ENDF/B-VI data library [9] for thermal to 20 MeV and the LA-150 data library [14] above 20 MeV. The 115 In n; c116m In reaction cross-section data thus obtained were multiplied by a factor of 160= 160 42 [15], which is the branching ratio of producing 116m In when a thermal neutron is captured by 115 In. Fig. 9 shows the calculated response functions of this indium-loaded spectrometer, comparing with the experimental data measured by Uwamino et al. [12]. The agreement between calculated and measured results is pretty good. 2.4. Experimental procedure This experiment was performed during the beam time of about six weeks from 29 September to 7 November in 1998 by changing the thickness of concrete and iron shields with keeping the proton beam intensity at 170 lA. The graphite activation detectors were set at various positions called ``center'', ``up'', ``down'', ``left'' and ``right'' on the additional shield surface of each thickness
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
95
times a day, and the photo-peak counts in the same condition were summed up to get the reaction rates. The neutron dose meter (Rem Ion Monitor, Harwell, UK) and the c-ray dose meter (RO2, Eberline, USA) were used in a current mode for measurements of neutron and c-ray dose equivalent data, respectively. 2.5. Analysis
Fig. 9. Response functions of indium-loaded multi-moderator spectrometer and comparison with the measured data [12].
as shown in Fig. 4. In order to investigate the spatial neutron ¯ux distribution above the shield top, the graphite detectors were also irradiated at once at these positions in the air without additional shield. The up and down positions mean upstream and downstream from the center just above the target along the proton beam direction, and the left and right mean the left and right sides perpendicular to the proton beam direction. The graphite detectors were used to get the attenuation length of neutron ¯ux above 20 MeV. The neutron energy spectrum through the bulk shield was measured using the aluminum, bismuth, graphite and indium-loaded multi-moderator detectors. These detectors were set at the center position on the shield surface for three conditions without additional shield (on the ¯oor), with 60cm thick concrete and with 30-cm thick iron shield. In order to get good statistics of photo-peak counts measured with two HPGe detectors, repeated irradiations and activity measurements were performed for each shield thickness several
2.5.1. Beam current monitor The neutron intensity from the tantalum target is proportional to the incident proton beam current. This proton beam current was continuously monitored by electromagnetic coil voltage just before the RIKEN (RAL Branch of Institute of Physical and Chemical Research) muon target during the experiment. This voltage was converted to ampere (Coulomb) with the correction of the beam loss in the muon target and used in the analysis of the activation detectors. Fig. 10 shows an example of the trend graph of the proton beam current. The errors of this beam current were assumed to be 1%. 2.5.2. Calculation of the reaction rates When the irradiation is repeated n times and the photo-peak counts are summed up m times, the reaction rate per beam current R r/ can be obtained as follows: ! 1 m Ck X R Aj atom 1 C 1 ; 1 ec j1
Fig. 10. A trend graph of proton beam current just before the RIKEN muon target (21 October 1998 8:00 a.m.±4:00 p.m.).
96
Aj Nj
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102 n X Qij i1
Dt 1
1
e
e
kTm
j
kDt
e
k n iDt
;
e
j
DR
kTc
2
where R is the reaction rate (atom 1 C 1 ), C the total counts of c-ray peak area, Dt the measuring time bin ( 1 min), Qi the beam current during the irradiation time interval Dt (Coulomb), N the number of atoms in the sample (parent nucleus), r the reaction cross-section (cm2 ), / the neutron ¯uence (cm 2 ), k the decay constant (min 1 ), e the peak eciency (with the self-absorption and sumcoincident eect), c the branching ratio of c-rays, Tc the cooling time (min) and Tm is the counting time (min). The peak eciencies of the HPGe detectors e including the self-absorption eect and the sumcoincidence eect were estimated by using the EGS4 Monte Carlo code [16] and the SUMECC code developed by Torii et al. [17], respectively. The nucleus data necessary for this estimation were cited from the Table of Isotopes of eighth edition [18]. For graphite detector, the 511 keV crays from natural background were subtracted from the peak counts. 2.5.3. Estimation of the error The errors of the reaction rates can be estimated from the following equation:
Fig. 11. Spatial distribution of
12
q DQ2H Dc2 Dp2 :
3
DQH is the error of beam current which was assumed to be 1% (in Section 2.5.1). Dc is the statistical error of photo peak area, and Dp is the error of peak eciency of HPGe detector estimated by the EGS4 Monte Carlo code, which is within a few %. DR is dominated by Dc, which varies from 2% to 50% depending on the total peak counts.
3. Results and discussions 3.1. 12 C n; 2n11 C reaction rate distribution on the shield top The 12 C n; 2n reaction rates correspond to the neutron ¯ux above 20 MeV, since the 12 C n; 2n cross-section has almost constant value of 20 mb above 20 MeV (See Fig. 6). Fig. 11 shows the reaction rates (atom 1 C 1 ) of the 12 C n; 2n11 C reaction on the shield top with additional concrete shield of dierent thickness. Fig. 12 shows those with iron shield. The errors of the results are the statistical errors of the photopeak counts of the 511-keV c-rays. Figs. 11(a) and 12(a) give the reaction rate distributions along the direction of the proton beam axis, where the center is a position on the shield top just above the target,
C n; 2n11 C reaction rate with the thickness of additional concrete shield.
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
Fig. 12. Spatial distribution of
12
C n; 2n11 C reaction rate with the thickness of additional iron shield.
and Figs. 11(b) and 12(b) give those along the direction perpendicular to the proton beam axis. In Figs. 11(a) and 12(a), the reaction rates increase from the upstream to the downstream direction although the reaction rates along the transverse direction in Figs. 11(b) and 12(b) are almost uniform. This means that the neutron streaming eect from the helium duct that situates in about 4-m downstream from the center of the shield top cannot be neglected (see Fig. 1). From the comparison of Figs. 11(a) and 12(a), the reaction rate distribution through concrete becomes more uniform than that through iron with increasing the shield thickness. This means that the neutron spectrum changes on the boundary between the additional concrete shield and the iron igloo surrounding the shield (see Fig. 4). In Figs. 11(b) and 12(b) the reaction rates at the 50-cm left and right positions are larger than those at the center position for thin additional shield by re¯ecting some streaming from the circular gap of about 0.5 and 1.5 cm around the shielding plug which exists at the respective radius of 26 and 74 cm from the center of the shielding plug (see Fig. 3). 3.2. Attenuation pro®les of reaction rates through shield 12
97
Fig. 13 shows the attenuation pro®les of the C n; 2n11 C reaction rates with the thickness of
concrete and iron at ®ve positions on the shield top as shown in Fig. 4; Fig. 13(a) at the center position on the shield top center, Fig. 13(b) at the 50-cm upstream position, up, Fig. 13(c) at the 50-cm downstream position, down, Fig. 13(d) at the 50cm left side from the center, left, Fig. 13(e) at 50cm right side from the center, right, along the beam direction. These ®gures also give the spatial distribution of the reaction rates in the air along the vertical axis of the bulk shield upward from the tantalum target without any additional shield on the shield top. We veri®ed that the data without additional shield decreased approximately with the square of the distance from the tantalum target which are shown in solid line, except at the down position where the data might be in¯uenced still by the streaming from the helium duct. With additional concrete and iron shields, the reaction rate can be ®tted well using the least-square method to the following equation: R
R0 e r2
t=k
;
4
where r is a distance from the tantalum target, t is an additional shield thickness and k is an attenuation length corresponding to neutron ¯ux above 20 MeV. This ®tting was done excluding the data on the ¯oor and 20-cm concrete thickness, because the neutron attenuation length may change at the shallow position. From this ®tting, we could
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T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
Fig. 13. Attenuation pro®les of 12 C n; 2n11 C reaction rates as a function of the distance from the shield top at the center, up, down, left and right positions.
obtain the attenuation length k of neutron ¯ux above 20 MeV for concrete and iron as shown in Table 4. It is found that the k values at ®ve positions behind the shield show some dierence each other and there are large errors at the right, left
and up positions, owing to the in¯uence of streaming from the helium duct and the gap of beam shielding plug, and of boundary eect between the additional shield and iron igloo. The k values at these ®ve positions are averaged, and are
T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102 Table 4 Attenuation length of neutrons above 20 MeV for concrete and iron at ®ve positions on the shield surface in g=cm2 Position
Concrete (2.36 g=cm3 )
Iron (7.8 g=cm3 )
Center Up Down Left Right
125.4 5.1 111.3 25.7 124.5 9.2 126.6 11.2 130.7 43.4
161.1 2.1 170.5 11.3 157.0 6.1 141.5 17.5 154.6 2.6
Average
123.7 52.7
156.9 22.0
Finally selected
125.4 5.1
161.1 2.1
shown in Table 4 together with the large errors. We therefore decided to select ®nally the values at the center position of 125:4 5:1 g=cm2 for con3 2 crete (2:36 g=cm ), 161:1 2:1 g=cm for iron 3 (7:8 g=cm ), because they have most probable values with the smallest errors coming from the spurious in¯uence described above, by comparing with the values at other positions. The attenuation length thus obtained for concrete is compared with other experimental and calculated results [1±3, 19±22] in Fig. 14. The present value of 2 125:4 g=cm is just between the values of 2 120 g=cm for 25-GeV proton by Stevenson et al. 2 [1] and 143 g=cm for 12-GeV proton by Ban et al. [2]. For the attenuation length of iron the present 2 value of 161:1 g=cm is a little larger than that of
Fig. 14. Comparison of various data on neutron attenuation length for concrete as a function of incident proton energy [1± 3,19±22].
99
148 g=cm2 obtained by Jerey et al. [23] using a 800-MeV proton accelerator at Los Alamos National Laboratry. Stevenson [1] and Ban [2] have 2 also given the values of 147 10 g=cm for 252 GeV proton and 188 12 g=cm for 12-GeV proton, respectively. Fig. 15 shows the attenuation pro®les of the 209 Bi n; xn210 x Bi x 4±10 reaction rates at the center position with increasing the thickness of concrete and iron shields. All experimental data can be well ®tted to a solid line of Eq. (4) having the same attenuation length k as that ®tted to the 12 C n; 2n reaction data, in spite of dierent threshold energies of these reactions. This clari®ed that the neutron energy spectrum after penetrated through the deep shield is in an equilibrium state. 3.3. Attenuation pro®les of the dose equivalent Fig. 16 shows the attenuation pro®les of the neutron and c-ray dose equivalents measured with the REM-counter and c-ray dose meter at the center position of additional concrete and iron shields on the shield top. This REM-counter is a conventional ionization-type detector which has low sensitivity to neutrons of energies higher than 20 MeV. The measured data could also be ®tted to the same equation (Eq. (4)) and got the attenuation lengths of dose equivalents with the least-square method. For concrete shield, neutron dose equivalent decreases quickly up to 60-cm thickness but beyond 60-cm thickness it decreases with the similar slope as that of 12 C n; 2n11 C reaction 2 rate (' 125 g=cm ). This may be partly because 3.6-cm thick iron plate under the additional concrete shield will produce lower-energy neutrons below MeV region and this neutrons can be easily attenuated by concrete shield and approach a spectrum equilibrium state. The c-ray dose equivalent has the same slope as the 12 C n; 2n11 C reaction rate, because the c-rays are generated by the high-energy neutron reactions and have no direct components in this deep penetration geometry. For iron shield, on the other hand, the attenuation of neutron dose equivalent is much slower than that of 12 C n; 2n11 C reaction rate. This
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T. Nunomiya et al. / Nucl. Instr. and Meth. in Phys. Res. B 179 (2001) 89±102
Fig. 15. Attenuation pro®les of
209
Bi n; xn210 x Bi x 4±10 reactions for concrete and iron at the center position.
means that the neutron slowing-down accompanying the production of lower energy neutrons continues in iron and the neutron spectrum changes with increasing the iron thickness. The atten-
uation of c-ray dose equivalent is a little larger than that of 12 C n; 2n11 C reaction rate. This may be explained from the addition of c-rays produced from 56 Fe n; c reaction mainly by low-energy
Fig. 16. Attenuation pro®les of neutron and c-ray dose equivalents for concrete and iron at the center position compared with the 12 C n; 2n reaction rate.
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neutrons to those produced by high-energy neutrons. 3.4. Neutron energy spectra The neutron energy spectra were measured at three conditions, on the ¯oor without additional shield, behind 60-cm thick additional concrete shield and 30-cm thick additional iron shield. Fig. 17 shows the neutron energy spectra in lethargy unit on the ¯oor of shield top and on the surfaces of the additional concrete and iron shields which were obtained with the activation detectors and the indium-loaded multi-moderator spectrometer through the unfolding by the SAND-II code [24]. The initial guess spectra for the SAND-II unfolding were used from the spectra calculated with the ANISN code [25] by Nakao and Uwamino [26]. The response functions of these detectors are given in Fig. 9. The obtained neutron spectrum on the ¯oor of shield top has two components; high-energy component of a peak around 100 MeV in the energy range above 10 MeV and low-energy component having a broad peak around 500 keV below 10 MeV. The former is the direct component of neutrons penetrated at 90° from the tantalum target bombarded by 800-MeV protons and
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the latter is the neutrons slowed down in iron and concrete shields. The neutron spectrum through 60-cm thick concrete shield placed additionally on the shield top shows a rather ¯attened lethargy spectrum down to thermal energy especially in the low-energy component below 10 MeV. This re¯ects the slowing-down eect due to the elastic collision of light elements like hydrogen in the concrete. This spectrum has a typical 1=E slowing-down spectrum after penetrating the concrete. The neutron spectrum through 30-cm thick iron shield has also a typical spectrum after penetrated the iron, where high-energy neutron components above a few MeV are slowed down to less than 1 MeV with the inelastic collision of iron and a broad peak over the region of 10±500 keV is remarkable due to relatively low inelastic cross section of iron in this energy region. 4. Conclusion The deep penetration experiment was performed at the ISIS neutron source using the 800MeV proton synchrotron. The attenuation pro®les through concrete and iron shields were clari®ed and the attenuation lengths of high-energy neutrons produced at 90° by 800-MeV protons were estimated. The neutron energy spectra were obtaind on the ¯oor of the shield top, through 60-cm thick additional concrete shield and through 30cm thick additional iron shield. The shielding con®guration is well de®ned and the measured attenuation lengths of concrete and iron will be the good benchmark data for investigating the accuracy of deep penetration calculation. Acknowledgements
Fig. 17. Neutron energy spectra on the shield top ¯oor, 60-cm thick additional concrete shield and 30-cm thick additional iron shield.
The authors wish to thank the machine group of RAL for synchrotron operation, RIKEN branch oce at RAL for monitoring the beam pro®les and various arrangements during this shielding experiment. This experiment is partly ®nancially supported by Japan Atomic Energy Research Institute.
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