Neutronic Design Studies on Small Accelerator 7 Li (p, n) Neutron - PowerPoint PPT Presentation

2009-07 Neutronic Design Studies on Small Accelerator 7 Li (p, n) Neutron Sources for Neutron Scattering Experiments Yoshiaki Kiyanagi, Fujio Hiraga, Takashi Kamiyama, Akira Homma, Fumiyuki Fujita and Michihiro Furusaka Hokkaido University 1

The technical background • Small accelerator neutron sources with an intensity of more than 10 12 [1/s] are necessary to facilitate the application of cold neutron scattering experiments such as transmission measurements. • Recently, an proton linear accelerator with a length of 3 m with a weight of 2.7 t with the proton energy of 2.5MeV has been put to practical use. • The neutron intensity of 10 12 [1/s] is expected to be produced by a Li target and the proton linear accelerator with the proton energy of 2.5MeV with the beam current of 1mA. 2

Neutronic advantages of the 7 Li(p, n) source using 2.5 MeV protons • The larger yield of neutrons than other methods with low energy protons around 3MeV. • The smaller energies of neutrons (less than 800keV ) than the evaporation neutrons. The cold moderator equipped with a 7 Li source may • moderation than a Bremsstrahlung ( γ γ γ γ , n) source . have the higher efficiency for the neutron • The decrease of the volume of the shield for neutrons around the source is expected. 3

The aim of this research • Examining the performance of practical models for the 7 Li (p, n) cold neutron source. • We studied the neutronic performance using the practical models (S-type and L-type) where the neutron absorption in the structures and the neutron streaming in the channels around the moderator were taken into account. neutrons and γ γ γ γ -rays to reduce the volume of the shield • We separately designed the shielding components for around the source. 4

Part -1 The study on the neutronic performance of a 7 Li (p, n) cold neutron source 5

Practical model and simplified model for 7 Li (p, n) cold neutron source of S-type Li target （ Li ） Li target （ Li ・ matrix ・ coolant water ） Vacuum vessel and channels The duct for neutrons Reflector (Be) Moderator The duct for (22K CH 4 ) protons Pre-moderator (polyethylene) Side view of the Side view of the simplified model practical model 6

Major differences between practical model and simplified model Li(t0.01) ・ Cu(t0.2) ・ Practical model Simplified model H 2 O(t0.1) ・ Cu(t0.2) , the （φ 3 ） Li(t0.01), installed Li target on the pre- distance to the pre- moderator moderator:1.9 Methane vessel (Al, t0.5), none Vacuum A channel 0.9, vessel （φ 32 or less ） Shield for heat (Al, t0.2), and channels A channel 0.9, unit ： cm Outer vessel (Al/Mg alloy, t0.6) 7

Practical model for 7 Li (p, n) cold neutron source of L-type unit ： cm 10 × 10 Li target ： φ 3 × 0.51 6 × 6 The duct for neutrons 1.9 111.2 The duct for 4.1(=Al:0.5+channel:0.9+Al: protons 0.2+channel:0.9+Al/Mg:0.6 +PE:1) 118.2 The view from the bottom The view from the bottom 8 (whole) (close-up)

Details of practical model for 7 Li (p, n) cold neutron source of L-type unit ： cm 118.2 118.2 φ 15 × 15 a : 11 × 4, Al φ 6 × 13.8 : 16.2 × 9.2, Al/Mg c a e d b b Channel: Methane vessel Channel: c Outer vessel d 111.2 e Flange: φ 32, Fe φ 11 × 12, Fe 18.2 × 11.2, PE : 13.2 × 6.2, Al Refrigerator: Pre-moderator: Shield for heat 116.1 Side view The view from the top 9

Method of neutronic calculation • Neutron yield: 8.8 × 10 11 [1/s] † • Energy of protons: 2.5MeV, Current: 1mA • Average energy of neutrons: 326 [keV] • The energy spectra and the angular distributions of source neutrons: LIYIELD † • Code: MCNPX † ： C.L. Lee, X.-L. Zhou, Nucl. Instr. Meth. B 152, 1 (1999) • Tally: 5 m from the moderator 10

Neutron energy spectra for 7 Li (p, n) and other reactions 0.01 "7Li(p, n), Ep=2MeV" "7Li(p, n), Ep=2.5MeV " "9Be(p, n), Ep=11MeV" 0.001 "Brems (gamma, n), Ee=35MeV" 0.0001 10 -5 2 /MeV/n] 10 -6 φ [1/cm 10 -7 10 -8 10 -9 0.001 0.01 0.1 1 10 100 E [MeV] 11

Optimal dimensions of the moderator, pre-moderator and reflector The dimensions shown below were found by parametrical calculations using the simplified model so that the intensity of cold neutrons (E<5meV) is maximized. The S-type The L-type The moderator 2.5 cm 3 cm thickness The pre-moderator 1.5 cm 1 cm thickness The reflector 40 cm 50 cm thickness 12

Comparison between the spectra of neutrons emitted from the moderator of the four models 1 Simplified S-type Simplified L-type Practical S-type Practical L-type 6.063e-10/E 0.01 2 /MeV/n] 0.0001 φ [1/cm 10 -6 10 -8 10 -10 10 -10 10 -8 10 -6 0.0001 0.01 1 E [MeV] 13

Comparison between the cold neutron fluxes per source neutron for the four models （ E<5meV ） flux （ E<5meV/ total ） Neutron flux Ratio of neutron 8.63 × 10 -8 [1/cm 2 /n] Simplified S-type 0.148 7.26 × 10 -8 Simplified L-type 0.169 2.92 × 10 -8 Practical S-type 0.140 2.66 × 10 -8 Practical L-type 0.196 14

Comparison of neutron spectra from Bremsstrahlung ( γ , n) sources moderators between 7 Li(p, n) and 1 S-type 7Li L-type 7Li S-type evaporation 0.01 L-type evaporation 0.0001 2 /MeV/n] φ [1/cm 10 -6 10 -8 10 -10 10 -10 10 -8 10 -6 0.0001 0.01 1 E [MeV] 15 E<5meV

Cold neutron fluxes at 5 m from the L- type moderators with the four types of sources, by using simplified models （ E<5meV ） Type of source The neutron yield at the Neutron flux source [1/s/mA] and the 1.10 × 10 11 , E n ~75keV 8.87 × 10 -8 [1/cm 2 /n] average energy of neutrons 7 Li(p, n), E p =2MeV 8.80 × 10 11 , E n ~326keV 7.26 × 10 -8 7 Li(p, n), E p =2.5MeV 2.15 × 10 13 , E n ~2.04MeV 5.12 × 10 -8 9 Be(p, n), E p =11MeV Bremsstrahlung ( γ , n), 5.60 × 10 13 , E n ~2.52MeV 4.50 × 10 -8 E e =35MeV 16

Cold neutron flux at 5 m from the L-type moderator, by using the practical models • The 7 Li (p, n) cold neutron source of 2.5 KW cold neutrons (E<5meV) of 2.4 × 10 4 [1/cm 2 /s]. (E p =2.5MeV, I=1mA) produces the intensity of ( γ , n) cold neutron source of 0.8 KW • This corresponds to that for the Bremsstrahlung (E e =35MeV, I=0.023mA). 17

Part-2 The design for the shielding components around the 7 Li (p, n) cold neutron source 18

The radiation sources used for the design of the shielding components Neutron source: Energy of protons: 2.5MeV, Current: 1mA Neutron yield: 8.8e11 [1/s] † Average energy of neutrons: 326 [keV] Source neutrons: LIYIELD † 4.1 × 10 10 †† 2.1 × 10 11 † 5.7 × 10 6 ☆ Energy of 0.429 0.478 14 ☆ ☆ ☆ photons [MeV] Photon yield † ： C.L. Lee, X.-L. Zhou, Nucl. Instr. Meth. B 152, 1 (1999) , [1/s] † † ： A. Z. KISS et al, 1984, ☆ ☆ ☆ ☆ ： C. L. LEE et al, 2000. 19

Photon spectrum at 3.5cm from a Li target 1 0.1 normalized 478keV 429keV 0.01 0.001 2 /p] φ [1/cm 0.0001 10 -5 10 -6 14MeV 10 -7 0.1 1 10 100 E [MeV] 20

shield for γ -rays around the source Models for the necessary thickness of the unit ： cm z z 111.2 118.2 116.1 The duct for The duct for neutrons protons y x The upper limit of the surface photon dose = 10 [ μ μ Sv/h] μ μ The iron slabs or lead slabs The side view of the L-type moderator 21

Position of tallies x-y(+) ： on the upper shield The duct for The duct for neutrons protons y-z(-) ： on the side shield facing x-z(+) ： on the side shield facing 22 the proton entrance the neutron exit

Maximum photon dose depending on the thickness of the iron slab 10 4 10 4 case of Fe shield case of Fe shield case of Fe shield case of Fe shield (without 14MeV photons) (without 14MeV photons) (without 14MeV photons) (without 14MeV photons) case of Fe sheild case of Fe sheild case of Fe sheild case of Fe sheild 1000 1000 x-y(+) x-y(+) maximum surface dose [ μ Sv/h] y-z(-) maximum surface dose [ μ Sv/h] y-z(-) x-z(+) x-z(+) 100 100 10 10 The upper limit of dose 1 1 5 6 7 8 9 10 11 12 13 5 6 7 8 9 10 11 12 13 x-y(+) ： on the upper shield, y-z(-) ： on the side shield facing the proton entrance, thickness of photon shield [cm] thickness of photon shield [cm] x-z(+) ： on the side shield facing the neutron exit 23

Maximum photon dose depending on the thickness of the lead slab 50 case of Pb shield case of Pb shield case of Pb shield case of Pb shield x-y(+) 40 y-z(-) x-z(+) maximum surface dose [ μ Sv/h] 30 20 10 The upper limit of dose 0 5 6 7 8 9 10 11 12 13 x-y(+) ： on the upper shield, y-z(-) ： on the side shield facing the proton entrance, x-z(+) ： on the side shield facing the neutron exit thickness of photon shield [cm] 24

Study of the effective shielding materials for neutrons from 7 Li(p, n) reactions 2 × T z Bremsstrahlung ( γ , n) reactions A point neutron source of the 7 Li(p, n) reactions or the 2 × T 2 × T y “T” means a shortest distance to the surfaces from the point A cube of source, i.e. the thickness of the x concrete or water shielding material. 25