SHIELDING MATERIALS FOR HIGH-ENERGY NEUTRONS

Transcription

1 SHELDNG MATERALS FOR HGH-ENERGY NEUTRONS Hsiao-Hua Hsu Health Physis Measurements Group Los Alamos National Laboratory Los Alamos, New Mexio, USA Abstrat We used the Monte Carlo transport ode Los Alamos High-Energy Transport (LAHET) to study the shielding effetiveness of ommon Shielding materials for high-energy neutrons. The soure neutron spetrum was fkom the interation of an 800-MeV proton beam and iron target. n a normal inident, the neutrons ollided with walls made of six ommon shielding materials: water, onrete, iron, lead, polyethylene, and soil. The walls were of four different thiknesses: 25,50,75 and 100 m. We then tallied the neutron spetra on the other side of the shielding wall and alulated the neutron doses. For the high-z materials-iron and lead-we find that many neutrons with energies between 1-10 MeV are reated when high-energy neutrons interat with shielding materials. For materials ontaining low-z elements-water, soil, onrete, and polyethylens-the spetra show higher energy peaks at about 100 MeV. Our studies show that for a given wall thikness, onrete is more effetive than the other materials. We also studied the effetiveness of ombinations of materials, suh as onrete and water, onrete and soil, iron and polyethylene, or iron polyethylene and onrete. ntrodution Neutron shielding for high-energy aelerators is an important issue in health physis for, in general, massive shielding is required to redue neutron dose.the purpose of this simple study is to find out the relative effetiveness of ommon shielding materials by using Monte Carlo simulations. f we know the shielding effetiveness of materials, we will be able to properly design neutron sheilding. While we know atoms of materials ontaining low-z elements will transfer most of the neutron energy during a ollision proess, highenergy nulear reations may have different harateristis. Monte Carlo simulations 1 For our simulations, we used the Los Alamos High-Energy Transport ode (LAHET).The neutron soure simulates the interation of an 800-MeV proton beam with an iron target. Figure 1 shows the spetrum that results.

2 DSCLAMER This report was prepared as an aount of work sponsored by an ageny of the United States Government. Neither the United States Government nor any ageny thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the auray, ompleteness, or usefulness of any information, apparatus, produt, or proess dislosed, or represents that its use would not infringe privately owned rights. Referene herein to any speifi ommerial produt, proess, or servie by trade name, trademark, manufaturer, or otherwise does not neessarily onstitute or imply its endorsement, reommendation. or favoring by the United States Government or any ageny thereof. The views and opinions of authors expressed herein do not neessarily state or reflet those of the United States Government or any ageny thereof.

3 Poxtiom of this doument may be illegible in eletroni image produts. mages are produed from the best available original doament,

4 E6 1E-02 1E-01 1E+00 1E+01 1E+02 1E+03 1E+04 Fig. 1. Neutron spetrum 800-MeV proton on iron target n a normal inident, the neutron ollides with a wall. For the wall, we used water, onrete, iron, lead, polyethylene, and soil, eah at four different thiknesses: 25,50,75 and 100 m. We then tallied the neutron spetra on other side of the wall and alulated the neutron doses. n LAHET alulations, when the energyzofa neutron reahes 20 MeV, the ode reords all information on a tape. We used the tape as input for a MCNP alulation, whih provides more detailed neutron interation ross setions. Results and disussion n figures 2-7, we show the neutron spetra for six materials and four wall thikness.

5 8E-02 l 8E-02 l.$' 4E-02 C a, S.s 3 ([ 2E-02 4 Fig. 2. Neutron spetra CH, slab Fig. 3. Neutron spetra water slab 8E-02 6E-02.$' 4E-02 E a, -S a,. 1 lu 2 2E-02 Fig. 4. Neutron spetra soil slab 14 OE+N f Fig. 5. Neutron spetra onrete slab

6 le-o1 2,, 1s E+02 ie+04 Fig. 6. Neutron spetra iron slab Fig. 7. Neutron spetra lead slab For water and polyethylene, the spetra maintain a single peak at about 100 MeV. For soil and onrete, two extra peaks between 5 and 8 MeV derive from silion (both ontain about 30% by weight). For iron and lead, the disontinuity at 20 MeV is not real but omes from the differenes in ross setions when we link LAHET to MCNP.However, the neutron peaks at about 1 MeV are real, the result of (n, xn) reations. We then fold the resulting spetra to the neutron fluene-to-dose funtion, alulating neutron doses on the other side of the walls. Table 1 is a summary of our results, whih show that materials onsisting of low-z elements are more effetively to redue neutron doses. The material density is also an important fator; the alulation for water and for onrete with a density of 1 gm/m3shows almost idential results in neutron spetrum and neutron dose. Table 1. Neutron doses with different walls Combinations of materials We onsidered the following ombinations of two or three materials: (1) 50-m iron + 50-m CH,, (2) 25-m iron + 75-m CH,, (3) 25-m iron + 50-m CH, + 25-m onrete, (4) 25-m iron + 25-m CH m onrete, 4

7 ' (5) 25-m onrete + 50-m water + 25-m onrete, (6) 25-m onrete + 25-m water + 50-m onrete, (7) 25-m onrete + 50-m soil + 25-m onrete, (8) 25-m onrete + 25-m soil + 50-m onrete. Figures 8 and 9 show neutron spetra. Table 2 summarizes dose results E Conrete Case 3 ase4 Case 2 Case s U Q 2E-02 e m v) 3 0 G.L. 1E-02 v) E B C -9 -m, 5E-03 OEM0 1E-04 1E+OO 1E Fig. 9. Neutron spetra ombination slab Fig. 8. Neutron spetra ombination slab DOSE CASE 1E-02 (psv per soure neutron) e e e-2 48.% e-2 They onfirm further that higher Z elements, suh as iron, are not as effetive as the onrete shown in ases 1-4 or as ompared with ases 5-8; and higher density materials are more effetive, as shown in ases 3 and 4, than the onrete ompared with CH2 in ases 1 and 2; and also more effetive, as shown in ases 7 and 8, than the soil as ompared with water in ases 5 and 6. 5

8 r Conlusion For high-energy neutron shielding, we should use high-density materials that onsist of low-z elements. Conrete is a good hoie; in some ases, an aeptable hoie is in replaing part of the onrete with soil. Referenes 1. R E. Prael and H. Lihtenstein, The LAHET Code System, Los Alamos National Laboratory, LA-UR , J. F. Briesmeister, (ed) MCNP-A General Monte Carlo N-Partile Transport Code, Version 4A, Los Alamos National Laboratory, LA M, 1993.