Production & Measurement of Thermal Neutron at RCNP

Chhom SakboreyChhom SakboreyNguyen Thi Duyen AnNguyen Thi Duyen An

Tran Hoai NamTran Hoai NamLi ChunjuanLi ChunjuanWang MianWang Mian

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Outline

• Introduction

• Methodology

• Experiments arrangement

• γmeasurement and results

• β-γcoincidence measurement and results

• Conclusion

3

Outline

• Introduction

• Methodology

• Experiments

• γmeasurement and results

• β-γcoincidence measurement and results

• Conclusion

4

Introduction (1)About thermal neutrons:• Discovered by Enrico Fermi

(1938 Nobel prize was awarded for his work on thermal neutrons).

• Produced when fast neutron enter and are slowed down in material with large concentration of hydrogen such paraffin or water.

• More readily absorbed by atomic nucleus (large reaction cross section)

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Introduction (2)

–Application of neutrons:•Therapy

•Neutron Activation Analysis

•Material structure

•Nuclear reaction …

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Purpose of our experiments

• To produce neutrons by Be(p,n) reaction with 53MeV protron beams from the cyclotron accellerator, and then thermalize them in the water-drum.

• To measure the space distribution of the thermal neutron in the water-

drum.

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Methodology

• Activation method to detect neutron

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1) 27Al + 1n [28Al*]

γ + 28Al (n, γ) reaction

1H + 27Mg (n, p) reaction

4He + 24Na (n, α) reaction

2 1n + 26Al (n, 2n) reaction1n + 27Al elastic scattering

2) 197Au + 1n [ 198Au* ] γ + 198Au (n, γ) reaction

Methodology

– In our experiments,we choose 197Au foils and 27Al foils

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C : Counting rate (s-1)

N0 : Number of nuclei

Φ : Neutron flux (cm-2s-1)

σ : Cross section (1barn = 10-24 cm2)

λ : Decay constant ( = Ln2 / T1/2)

ti : Irradiation time ( h)

tw : Waiting time (h)

tm : Measurement time (h)

Iγ : Relative intensity (%)

ε : Detector efficiency (%)

g : Geometry efficiency (%)

• Activation equation

gIeeeN

C mwi ttt

)1()1(0

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Experiment(1)• Target preparation for 9Be(p,n)9B reaction• Set the position and make sure that the beam is

in the center of the target. • Proton beam:

– E = 53 MeV– I = 80 nA

Beryllium target

Collimator

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Experiment(2)

• Moderate fast neutron with water

Water tank

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Experiment(3)• Set some kind of foils into the water-

drum– Gold foils– Gold foils with cadmium outside– Aluminum foils

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D C B A Z

Assignment of the foils

50

cm

15

20

10

2.5

- 2.5

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Experiment(4)• Activity measurement

γ measurementHP-Ge

β-γ coincidencePla. scin. NaI(Tl)

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Outline

• Introduction

• Methodology

• Experiments

• γmeasurement and results

• β-γcoincidence measurement and results

• Conclusion

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Apparatus

– HV = +3000 V

– Gain : 0.72 x 20

– Shaping time: 6μ s

(1/3)

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(2/3)Setup

Detector

Source

5cm

Lead shielding

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Measurements

• 2 measurements with golden foils:– 2 hours after activation, measured time: 300s– 3 days after activation, measured time: 600s

• 1 measurement with aluminum foils: 20 hours after activation, measured time: 5400s

Reaction Half-time Main gamma-

rays (keV)

Intensity (%)

Isotope abundance

(%)197Au(n,γ)198Au 2.695 d 411.8 96.00 100

27Al(n,a)24Na 14.997 h 2754.01368.6

99.9499.99

100

(3/3)

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Energy Calibration

Energy (keV)Channel of centroid

Error (channel)

121.78 508.91 1.63

244.70 1026.48 1.91

344.28 1445.82 1.95

778.90 3276.20 2.63

964.08 4055.77 2.81

1085.87 4568.60 2.85

1112.07 4679.09 2.93

1408.01 5925.62 3.33

Fitting function:Y = A + B * XA = -3.999 ± 0.105B = 4.211 ± 1E-4

(1/5)

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10 100

Fitting function:

lg log

0.424 0.075

1.073 0.027

Ea b

E

a

b

Energy (keV)

Efficiency Error (%)

411.800.0043 4.64

1368.630.0011 5.04

Efficiency Calibration (2/5)

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(3/5)Result: Thermal Neutron

DistributionA B

C

Z

D1.12E+08

3.27E+06

3.47E+07

3.75E+07

1.95E+07 3.57E+06

1.73E+06

1.71E+07

5.87E+06 7.83E+069.58E+06

3.35E+073.85E+07

Be Target

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• Fast neutron flux density:

• Epithermal neutron flux density:

Position Density flux(cm-2s-2)

Error (%)

A 5cm 1.96E+07 6.92

B 2.5cm 9.97E+06 6.97

C 10 1.07E+06 8.05

Result: Epithermal & Fast Neutron Flux

(4/5)

Position Density flux (cm-2s-2)

Error (%)

A 10cm 2.97E+06 5.41

B 0cm 1.58E+07 5.36

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Position Φfast Φther Φther / Φfast

A 5 1.96E+07 1.12E+08 5.71

B 2.5 9.97E+06 4.37E+07 4.38

C 10 1.07E+06 3.57E+06 3.34

(5/5)

Position Φ epi Φ ther Φther / Φepi

A 10cm 2.97E+06 3.47E+07 11.7

B 0cm 1.58E+07 3.75E+07 2.37

Comparison

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Outline

• Introduction

• Methodology

• Experiments

• γmeasurement and results

• β-γcoincidence measurement and results

• Conclusion

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β-γcoincidence measurement

• Principle

– Principle of coincidence

– Principle of absolute activity

measurement with β-γcoincidence

system

• Experiments and Results

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Principle of coincidence

– β γβ-

β γ

Det.1 Det.2

Coincidence

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Principle of coincidence

Pulse 1

Pulse 2

Coincidence

Pulse

t

t

<t <t >t

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Coincidence technique• True coincidence & accidental coincidence

– True coincidence events: correlation

– Accidental coincidence events: no correlation.

eg.βfrom one source and γfrom another source.

• Resolving time for coincidence system– The shortest time which the system can distinguish

between two signals

)( 2121 ttnnnaccidental – t1:the width of signal 1

– t2:the width of signal 2

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Resolving time measurement

0-td td

Counting rate

2τ τ-electronic resolving time

Delay

Coin. scaler

Dis.

Dis.

Pulse generato

r

Delay

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Resolving time measurement

counting rate

-td 0 td

2τ’ τ’-physical resolving time

Delay

Delay

Det.1

Det.2

Dis.

Coin.HV Scalerβ

γ

Dis.

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Absolute activity measurement with β-γcoincidence system

HVβ

γ

Pla. Dis.

NaI(Tl) Dis.

Delay

nβγ(βγ)

ScalerDelay

Coin. Scaler

Scaler nγ(β)

nβ(γ)

– no delay for Pla. in our experiment

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Absolute activity measurement with β-γcoincidence system

• Corrections for the counting rate

nnbackgroundnnn

backgroundnnn

nbackgroundnnn

2)()(

)()(

)()()(

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Absolute activity measurement with β-γcoincidence system

DFAn

DFAn

)4/(

)4/(

0

0

DFP 4/

0/ AnnPnn

nnnA /0

)( Solid angle

)( FFCorrection factors of

scattering and absorption

)( DDDiscrimination coefficient

of the discriminator

)( Efficiency of the detector

P Probability of detectingγrays while one βsignal being detected

0A Source activity

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Absolute activity measurement with β-γcoincidence system

• Advantages– The results have no relationship with the

efficiency of the detector, data analysis is simple.

• Limits

– To make sure that

There should be

02/1)2/()(/ Annnaccidentnn

1)(/ accidentnn

2/10A

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β-γcoincidence measurement

• Principle

– Principle of coincidence & some

concepts

– Principle of absolute activity

measurement with β-γcoincidence

system

• Experiments and Results

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Experimental setup

– Gold foil’s position in water-drum: (41.32, 0, 5)cm,

the center of the front surface as (0,0,0)

– Distance from source to Plas.:3cm

– Distance from source to NaI(Tl):2cm

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Experimental process-1

• Check the detectors with oscilloscope and MCA– HV for NaI(Tl): -1850V; HV for Pla.:-2000V

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Experimental process-2

• Set the threshold of the discriminator– Very important!

Gate Generator

NIM-TTL

Amp

Det.

Dis

Shaping

MCA

input

gate

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Spectra after setting the threshold

• Threshold(NaI)=-85.8mV

• Threshold(Pla.)=-406.0mV

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Experimental process-3

• Resolving time measurement

-400 -300 -200 -100 0 100 200 300 400

0

10

20

30

40

50

60

coun

ting

rate

/s-1

dt\ns

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

ns5.2812

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Experimental process-4– With 198Au source, Al foils(0.31mm) before

NaI detector.

– With 198Au source, Al foils(0.31mm) before Pla. Detector

– Without source

)(),(),( nnn

)(),()( nnbackgroundn

n

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Results

　　

nβ(β)[s-1] nβ(b.g.)+nβ(γ )[s-1]

2541.66 76.14

nβ [s-1] 2465.52

　　

nγ(γ)[s-1] nγ(b.g) [s-1]

1059.36 0.98

nγ [s-1] 1058.38

　　

nβγ(βγ) [s-1] nβγ(b.g.) [s-1] nβγ(accidental) [s-1]

37.73 1.01 0.73

nβγ [s-1] 35.98

– Counting rates of βsignals,γsignals andβγcoincidence signals

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Results

A[Bq] NAu197(A5cm) σ[b] Φ[cm-2s-1]

1.60E+07 1.81E+21 98.65 8.95E+07

source of uncertainty uncertainty total uncertainty

Astatistical 1.87%

2.74%system 2.00%

NAu197 0.01%

σ 0.14%

• Uncertainty estimation

• Neutron fluence rate at (41.32, 0, 5)cm

• Comparision with HPGE’s result: 1.12E8 ±5.99E6

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Outline

• Introduction

• Methodology

• Experiments

• γmeasurement and results

• β-γcoincidence measurement and results

• Conclusion

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Conclusion• Neutrons were produced by Be(p,n) reaction with 53MeV proton beams from

the cyclotron accelerator, and then were thermalized in the water-drum.

• The space distribution of the neutron fluence rate in the water-drum was measured with activation methods,and the results showed that the distribution is isotropic.

• The activities of the gold foils were measured both with HPGE detector and β-γcoincidence system, and the results were compared with each other.

• Energy spectrum of neutron may need more measurements or calculation.

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Acknowledgement

• JICA• Osaka University• Professors, assistant teachers • RCNP• ……..

Thanks a lot!!

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