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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
2
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)
5
Introduction (2)
–Application of neutrons:•Therapy
•Neutron Activation Analysis
•Material structure
•Nuclear reaction …
6
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.
7
Methodology
• Activation method to detect neutron
8
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
14
Experiment(4)• Activity measurement
γ measurementHP-Ge
β-γ coincidencePla. scin. NaI(Tl)
15
Outline
• Introduction
• Methodology
• Experiments
• γmeasurement and results
• β-γcoincidence measurement and results
• Conclusion
16
Apparatus
– HV = +3000 V
– Gain : 0.72 x 20
– Shaping time: 6μ s
(1/3)
17
(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)
20
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
23
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
24
Outline
• Introduction
• Methodology
• Experiments
• γmeasurement and results
• β-γcoincidence measurement and results
• Conclusion
25
β-γcoincidence measurement
• Principle
– Principle of coincidence
– Principle of absolute activity
measurement with β-γcoincidence
system
• Experiments and Results
26
Principle of coincidence
– β γβ-
β γ
Det.1 Det.2
Coincidence
27
Principle of coincidence
Pulse 1
Pulse 2
Coincidence
Pulse
t
t
<t <t >t
28
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
29
Resolving time measurement
0-td td
Counting rate
2τ τ-electronic resolving time
Delay
Coin. scaler
Dis.
Dis.
Pulse generato
r
Delay
30
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.
31
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
32
Absolute activity measurement with β-γcoincidence system
• Corrections for the counting rate
nnbackgroundnnn
backgroundnnn
nbackgroundnnn
2)()(
)()(
)()()(
33
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
34
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
35
β-γcoincidence measurement
• Principle
– Principle of coincidence & some
concepts
– Principle of absolute activity
measurement with β-γcoincidence
system
• Experiments and Results
36
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
37
Experimental process-1
• Check the detectors with oscilloscope and MCA– HV for NaI(Tl): -1850V; HV for Pla.:-2000V
38
Experimental process-2
• Set the threshold of the discriminator– Very important!
Gate Generator
NIM-TTL
Amp
Det.
Dis
Shaping
MCA
input
gate
39
Spectra after setting the threshold
• Threshold(NaI)=-85.8mV
• Threshold(Pla.)=-406.0mV
40
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
41
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
42
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
43
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
44
Outline
• Introduction
• Methodology
• Experiments
• γmeasurement and results
• β-γcoincidence measurement and results
• Conclusion
45
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.
46
Acknowledgement
• JICA• Osaka University• Professors, assistant teachers • RCNP• ……..
Thanks a lot!!
47