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

1

Neutronic Design Studies on Small Accelerator 7Li (p, n) Neutron Sources for Neutron Scattering Experiments

Yoshiaki Kiyanagi, Fujio Hiraga, Takashi Kamiyama, Akira Homma, Fumiyuki Fujita and Michihiro Furusaka Hokkaido University

2009-07

2

The technical background

• Small accelerator neutron sources with an intensity of

more than 1012 [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 1012 [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.

3

Neutronic advantages of the 7Li(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 7Li source may

have the higher efficiency for the neutron moderation than a Bremsstrahlung (γ γ γ γ, n) source.

• The decrease of the volume of the shield for neutrons

around the source is expected.

4

The aim of this research

• Examining the performance of practical models for

the7Li (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.

• We separately designed the shielding components for

neutrons and γ γ γ γ-rays to reduce the volume of the shield around the source.

5

Part -1

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

6

Practical model and simplified model for

7Li (p, n) cold neutron source of S-type

Side view of the practical model Side view of the simplified model Li target （Li・matrix・coolant water）

Moderator (22K CH4) Pre-moderator (polyethylene) Reflector (Be)

Li target （Li ）

Vacuum vessel and channels

The duct for neutrons The duct for protons

7

Major differences between practical model and simplified model

Simplified model Practical model

none Methane vessel (Al, t0.5), A channel 0.9, Shield for heat (Al, t0.2), A channel 0.9, Outer vessel (Al/Mg alloy, t0.6)

Vacuum vessel and channels

（φ32 or less） Li(t0.01), installed

• n the pre-

moderator Li(t0.01)・Cu(t0.2)・ H2O(t0.1) ・ Cu(t0.2) , the distance to the pre- moderator:1.9

Li target

（φ3） unit：cm

8

Practical model for 7Li (p, n) cold neutron source of L-type

The view from the bottom (whole) The view from the bottom (close-up) 10×10 6×6 118.2 111.2 1.9 4.1(=Al:0.5+channel:0.9+Al: 0.2+channel:0.9+Al/Mg:0.6 +PE:1) Li target ： φ3×0.51 unit：cm The duct for neutrons The duct for protons

9

Details of practical model for 7Li (p, n) cold neutron source of L-type

Side view a b c d e The view from the top 111.2 118.2 Channel: φ6×13.8 a b Channel: φ15×15 Refrigerator: φ11×12, Fe c Flange: φ32, Fe d Shield for heat : 13.2×6.2, Al Outer vessel : 16.2×9.2, Al/Mg 116.1 118.2 e Pre-moderator: 18.2×11.2, PE Methane vessel : 11×4, Al unit：cm

10

Method of neutronic calculation

• Energy of protons: 2.5MeV, Current: 1mA
• Neutron yield: 8.8×1011 [1/s]†
• Average energy of neutrons: 326 [keV]
• The energy spectra and the angular

distributions of source neutrons: LIYIELD†

• Code: MCNPX
• Tally: 5 m from the moderator

†： C.L. Lee, X.-L. Zhou, Nucl. Instr. Meth. B 152, 1 (1999)

11

Neutron energy spectra for 7Li (p, n) and other reactions

10-9 10-8 10-7 10-6 10-5 0.0001 0.001 0.01 0.001 0.01 0.1 1 10 100 "7Li(p, n), Ep=2MeV" "7Li(p, n), Ep=2.5MeV " "9Be(p, n), Ep=11MeV" "Brems (gamma, n), Ee=35MeV"

φ [1/cm

2/MeV/n]

E [MeV]

12

Optimal dimensions of the moderator, pre-moderator and reflector

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

13

Comparison between the spectra of neutrons emitted from the moderator of the four models

10-10 10-8 10-6 0.0001 0.01 1 10-10 10-8 10-6 0.0001 0.01 1 Simplified S-type Simplified L-type Practical S-type Practical L-type 6.063e-10/E

φ [1/cm

2/MeV/n]

E [MeV]

14

Comparison between the cold neutron fluxes per source neutron for the four models

0.196 2.66×10-8 Practical L-type 0.140 2.92×10-8 Practical S-type 0.169 7.26×10-8 Simplified L-type 0.148 8.63×10-8 Simplified S-type Ratio of neutron flux （ E<5meV/ total ） Neutron flux （E<5meV） [1/cm2/n]

15

Comparison of neutron spectra from moderators between 7Li(p, n) and Bremsstrahlung (γ, n) sources

10-10 10-8 10-6 0.0001 0.01 1 10-10 10-8 10-6 0.0001 0.01 1 S-type 7Li L-type 7Li S-type evaporation L-type evaporation

φ [1/cm

2/MeV/n]

E [MeV]

E<5meV

16

Cold neutron fluxes at 5 m from the L- type moderators with the four types of sources, by using simplified models

4.50×10-8 5.60×1013, En~2.52MeV Bremsstrahlung (γ, n), Ee=35MeV 5.12×10-8 2.15×1013, En~2.04MeV

9Be(p, n), Ep=11MeV

7.26×10-8 8.80×1011, En~326keV

7Li(p, n), Ep=2.5MeV

8.87×10-8 1.10×1011, En~75keV

7Li(p, n), Ep=2MeV

Neutron flux （E<5meV） [1/cm2/n] The neutron yield at the source [1/s/mA] and the average energy of neutrons Type of source

17

Cold neutron flux at 5 m from the L-type moderator, by using the practical models

• The 7Li (p, n) cold neutron source of 2.5 KW

(Ep=2.5MeV, I=1mA) produces the intensity of cold neutrons (E<5meV) of 2.4×104 [1/cm2/s].

• This corresponds to that for the Bremsstrahlung

(γ, n) cold neutron source of 0.8 KW (Ee=35MeV, I=0.023mA).

18

Part-2

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

19

The radiation sources used for the design of the shielding components

5.7×106 ☆

☆ ☆ ☆

2.1×1011† 4.1×1010†† Photon yield [1/s] 14 0.478 0.429 Energy of photons [MeV] †： C.L. Lee, X.-L. Zhou, Nucl. Instr. Meth. B 152, 1 (1999) , † † ：A. Z. KISS et al, 1984, ☆ ☆ ☆ ☆：C. L. LEE et al, 2000. Neutron source: Energy of protons: 2.5MeV, Current: 1mA Neutron yield: 8.8e11 [1/s]† Average energy of neutrons: 326 [keV] Source neutrons: LIYIELD†

20

Photon spectrum at 3.5cm from a Li target

10-7 10-6 10-5 0.0001 0.001 0.01 0.1 1 0.1 1 10 100 normalized

E [MeV] φ [1/cm

2/p]

14MeV 429keV 478keV

21

Models for the necessary thickness of the shield for γ-rays around the source

y z x z 116.1 111.2 118.2 The side view of the L-type moderator unit：cm The duct for neutrons The duct for protons The iron slabs or lead slabs The upper limit of the surface photon dose = 10 [μ μ μ μSv/h]

22

Position of tallies

The duct for neutrons The duct for protons 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

23

Maximum photon dose depending on the thickness of the iron slab

1 10 100 1000 104 5 6 7 8 9 10 11 12 13

case of Fe sheild case of Fe sheild case of Fe sheild case of Fe sheild

x-y(+) y-z(-) x-z(+)

maximum surface dose [μSv/h] thickness of photon shield [cm]

1 10 100 1000 104 5 6 7 8 9 10 11 12 13

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)

x-y(+) y-z(-) x-z(+)

maximum surface dose [μSv/h] thickness of photon shield [cm]

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

The upper limit of dose

24

Maximum photon dose depending on the thickness of the lead slab

10 20 30 40 50 5 6 7 8 9 10 11 12 13

case of Pb shield case of Pb shield case of Pb shield case of Pb shield

x-y(+) y-z(-) x-z(+)

maximum surface dose [μSv/h] thickness of photon shield [cm]

The upper limit of dose

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

25

Study of the effective shielding materials for neutrons from 7Li(p, n) reactions

x y z 2×T 2×T 2×T A point neutron source of the

7Li(p, n) reactions or the

Bremsstrahlung (γ, n) reactions A cube of concrete or water “T” means a shortest distance to the surfaces from the point source, i.e. the thickness of the shielding material.

26

Maximum neutron dose depending on the thickness of the ordinary concrete slab

1 10 100 1000 104 105 106 20 40 60 80 100 120 evaporation [μSv/h ] "2MeV p, n [μSv/h ]"

maximum surface dose [μSv/h ] thickness [cm]

The concrete slab with a thickness of more than 1 meter is needed to decrease the surface neutron dose on the slab down to 10 [μSv/h].

7Li(p, n) [μSv/h]

The upper limit of dose

27

Maximum neutron dose depending on the thickness of the water slab

1 10 100 1000 104 105 20 40 60 80 100 120 evaporation [μSv/h ] "2MeV p, n [μSv/h ]"

maximum surface dose [μSv/h ] thickness [cm]

7Li(p, n) [μSv/h]

The upper limit of dose

28

Maximum neutron dose depending on the thickness and the density of the boric acid water slab

0.1 1 10 100 1000 104 105 106 107 10 20 30 40 50 pure [μSv/h ] 0.2% [μSv/h ] 0.4% [μSv/h ] 1.0% [μSv/h ]

maximum surface dose [μSv/h ] thickness [cm]

The upper limit

• f dose

We should use the material including hydrogen and boron as the shielding components for neutrons from the 7Li(p, n) reactions.

29

Model for the necessary thickness of the shield for neutrons around the source

y z x z 140.1 135.2 142.2 The duct for protons The duct for neutrons The lead slabs of 12 cm thick The boric acid resin slabs The side view of the L-type moderator unit：cm The upper limit of the surface neutron dose = 10 [μ μ μ μSv/h]

30

Maximum neutron dose depending on the thickness of the boric acid resin slab

100 200 300 400 500 600 19 20 21 22 23 24 25 26 27 x-y(+) y-z(-) x-z(+)

maximum surface dose [μSv/h] thickness of neutron shield [cm]

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

The upper limit

• f dose

31

Models for the necessary thickness of the shield for γ-rays around the sample and the duct

y= 400cm, 3.8μSv/h

• y

z 605 95.6 unit：cm The side view of the L-type moderator The lead slab of 3 cm thick decreases the surface photon dose to 3.8 [μSv/h].

The sample space for scattering experiments, 106×108×106 The neutron duct, 16×401.4×16 The exit of neutrons, 34×28×34

32

Models for the necessary thickness of the shield for γ-rays around the proton liner accelerator

The lead slab of 12 cm thick decreases the surface photon dose on the shield around the Li target to 3.7 [μSv/h] and the lead slab of 5 cm thick decreases the surface photon dose on the shield around the proton liner accelerator to 2.5 [μSv/h]. 404.1 99.1

• x

z 95.6 x= -404.1cm, 2.5μSv/h x= 99.1cm, 3.7μSv/h The side view of the L-type moderator unit：cm

The space for the proton liner accelerator, 305×106×106 The entrance of protons, 28×30×30

33

The whole of shielding components for γ-rays

y x 605 404.1 99.1 95.6

• Slabs around the neutron

duct, t3, Pb, 16×401.4×16 Slabs around the proton liner accelerator t3, Pb, 305×106×106

• Ex. t5
• Ex. t5

Slabs around the sample for scattering experiments t3, Pb, 106×108×106 Exit of neutrons t12, Pb, 34×28×34 Entrance of protons t12, Pb, 28×30×30

The view from the bottom (whole) unit：cm

34

Models for the necessary thickness of the shield for neutrons around the sample and the duct

y z 728.6 196.1 The boric acid resin slabs of 28 cm thick decreases the surface neutron dose to zero [μSv/h] . (In the case of 24 cm thick, the dose was 56μSv/h at y = 230cm.)

• unit：cm

The side view of the L-type moderator

The sample space for scattering experiments, 154×160×154 The neutron duct, 72×377.4×72

y = 230cm

35

Models for the necessary thickness of the shield for neutrons around the proton liner accelerator

x z 196.1 531.2

• The boric acid resin slabs of 28 cm thick decreases the surface neutron

dose to zero [μSv/h] . (In the case of 24 cm thick, the dose was 15μSv/h at x = -180cm. ) unit：cm The side view of the L-type moderator

The space for the proton liner accelerator, 329×162×162

x = -180cm

36

The whole of shielding components for neutrons

x y 728.6 531.2

Slabs around the sample for scattering experiments t28, BAR, 154×160×154 Slabs around the duct for neutrons t28, BAR, 72×377.4×72 Slabs around the proton liner accelerator t28, BAR, 329×162×162

• The view from the bottom (whole)

unit：cm Abbreviation: BAR: boron acid resin

37

Conclusion

• The 7Li (p, n) cold neutron source of 2.5 KW produces

the cold neutrons of the same ratio as a Bremsstrahlung (γ, n) cold neutron source of 0.8 KW and greatly decreases the volume of the shielding components around the neutron source.

• The 7Li (p, n) cold neutron source of 2.5 KW has

capability to be used in many places in manner of a moving trailer, since a 2.5MeV proton linear accelerator having the surface radiation dose of less than 20 μSv/h

• n the shielding components is permitted by the law in

Japan to be transported for a use in a field.

38

Further study

• Since the photons with an energy larger than

14MeV dominate the surface photon dose on the shielding components, we should study the yield and the spectrum of the high energy photons.

• Feasibility studies of the Li target such as the

removal of heat by the water flow are also needed.