Measurement of thermal neutron and 41Ar at KEK Linac

KEK

T. Oyama, H. Iwase, A. Toyoda, T. Sanami, and Y. NamitoRadiation science center, KEK, Japan

1. Purpose - Air activation by electron accelerator

2

- e- facilities have less air activation than hadron facilities- However modern e- accelerators will have higher power- More accurate evaluation of air activation is required

- Experimental study on 41Ar production

Nuclide Halflife Reaction Threshold (MeV). 12.3y

53.3d

20.4m

9.96m2.04m37.2m55.5m1.82h thermal neutron

a pair of pump and stack

ratio of evaluated air activation productionin whole linac tunnel

1.2 KEK Linac

3

e+ DumpingRing

Linace-

e-e+

e+ productiontarget

e-

e-

maximum beam powere- 10 GeV 625nA ~ 6.3kW

on target (future x1.5)e- 3.3 GeV 800nA ~ 2.6kW

SUPER

KEKB

PFAR6.5GeV

PF2.5Ge

V

1.3 KEK-Linac positron production target

4

Beam e- 3.3 GeV, 100 nATarget W 1cm and other components

(thick target)

targetyork

cabitye- by RF gun

T. Kamitani (KEK)

~15cm

lystron roomklystron room

concrete concrete

controlled(max 20 uSv/h)controlled

keep out

keep out

public(max

0.2 uSv/h)

KEK controlled (max 1.5 uSv/h)

Target shield 2015 (Radsynch15)Target shield final ver. (3.3 GeV 1250nA e-)

Distance 555cmConcrete 320cmIron 70cm

Distance 430cmConcrete 250cmIron 59cm

Distance 700cmConcrete 400cmIron 72cmground level

controlledcontrolled

KEKcontrolled

2. Methods

6

2.1 Experiment-A - 40Ar gas direct irradiation

7

40Ar gas packages were set in tunnel at positions of X, Y, and Z

3.3GeV e-X

(0m)

Target

Y(15m)

Z(35m)

2.2 Experiment-B - Thermal neutron fluence Φ

8

Thermal neutron fluence Φ in tunnel was measured by the Gold foil activation with/without Cadmium cover

JENDL-4.0JENDL-4.0

3.3GeV e- X(0m)

Target

Y(15m)

Z(35m

)

2.3 Experiment-C - Thermal neutron spatial distribution

9

targete-

30m

Thermal neutron fluence Φ in tunnel was measured by the Gold methodat different positions of upstream and downstream from the target

3 Result1 - Experiments A and B

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A: Ar gas(Bq/cm3/kW)

B: Gold foil(Bq/cm3/kW)

X(0m) 9.2±1.4 9.2±0.7

Y(15m) 0.73±0.14 0.56±0.034

Z(35m) 0.077±0.016 0.038±0.0015

3.3GeV e- X(0m)

Target

Y(15m)

Z(35m

)

kW)

3.2 Result2 - Thermal neutron spatial distribution

11

targete-

30m

±30m from target covers 99% of total

integrationrange

ratio to totalamount

±15m 91%

±20m 96%

±30m 99%

PHITS 3D Monte Carlo simulation

measurements

40Ar (n, γ) 41Ar reaction

12

capture

Neutron energy spectrum in tunnel around target

PHITSMonte Carlosimulation

41Ar productionsinduced below 1 eV neutron (10-6 MeV)account for 98.7% of total

Gold foil can be used instead of Gas?

• Experiment-C indicates that thermal neutrons distribute in certain range of ±20m (or ±30m)

• The discrepancy of the Experiment-A(Gas) and B (Gold foil) can negligible in case to obtain integral number of 41Ar production

• the answer is yes

Thermal neutron source around KEK Linac target

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thermal neutron (

Comparison to theoretical evaluation

15

(1) GDR neutron production Yn by Mao’s formula

Yn = 8 × 10−6 E0 (Z0.5 + 0.12Z1.5 − 0.001Z2.5 )

where Yn GDR neutron production per e-, E0 e- energy (MeV),

and Z atomic number of thick target.

If Z=74(W) Yn=1.0, and Z=29(Cu) Yn=0.52. => 0.76 (n/e-) in average

(2) GDR neutron reduction R by components (shielding effect)

It is difficult to predict by hand-calculation. For example

set R to 1/2. => R Yn = 0.76/2 = 0.38 (n/e-)

(3) Neutron thermalization formula by Patterson

Φ = C R Yn / S

where C is a constant of 1.25, and S is total area of the tunnel (cm2)

Φ = 2.1e4 (1/cm2/nA/sec)

=> 41Ar = 4.95e-3 (Bq/cm3/nA)

Standard method to estimate thermal neutron fluence for electron accelerator

compared to experiment (4.42e-3) calculation agrees well (due to (2) is estimated by experience)

5. Conclusion

16

投影片編號 11. Purpose - Air activation by electron accelerator1.2 KEK Linac1.3 KEK-Linac positron production target投影片編號 52. Methods2.1 Experiment-A - 40Ar gas direct irradiation2.2 Experiment-B - Thermal neutron fluence Φ2.3 Experiment-C - Thermal neutron spatial distribution3 Result1 - Experiments A and B3.2 Result2 - Thermal neutron spatial distribution40Ar (n, γ) 41Ar reactionGold foil can be used instead of Gas?Thermal neutron source �around KEK Linac targetComparison to theoretical evaluation5. Conclusion