neutron spectroscopy by time of flight method and determination of neutron beam
DESCRIPTION
Neutron spectroscopy by time of flight method and determination of neutron beam. Prepared by : Sameh Hassan , Yomna Abd El-Moaty Supervisors: L Pikelner, V.Shvetsov FLNP, Dubna. Motivation. - PowerPoint PPT PresentationTRANSCRIPT
Summer Student Practice, 2009, JINR Dubna 1
Neutron spectroscopy by time of flight method and determination of neutron
beam
Prepared by: Sameh Hassan , Yomna Abd El-MoatySupervisors: L Pikelner, V.Shvetsov FLNP, Dubna
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Motivation
- Studying the processes of the interaction of slow neutrons with nuclei ( 1- 105 ev)
-Radioactive capture with gamma emission is the most common
type of reaction at certain energies for slow neutrons. This (n,) reaction often results in product nuclei which are
radioactive. For example:
So it is a method of studying dependence of neutron cross-section on its energy.
Co2760
Co2760
n01
Co2759
*
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Targets and methods
For example to study total neutron cross-sections of tungsten (W)
The time-of-flight (TOF) method is used to measure the transmission of the sample
SampleSize [mm3]
Density [g/cm3]
Atomic Mass [a.m.u]
Purity[%]
W 100x100x0.219.3183.8599.98
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The neutron Time of Flight (TOF) spectrum
0 50 100 150 200 250 300 350 4000
2000
4000
6000
800046.26[eV] 183W
27.03[eV] 183W
18.8[eV] 186W
Coun
ts
TOF Channels (2 s/ch)
Sample (W) Beam Open Beam Background
7.6[eV] 183W4.15[eV] 182W
Fig.1 ) The neutron TOF spectrum for sample-in and open beam along with background level of W sample
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Spectrum processing
The total neutron cross-section is determined by measuring the transmission of neutrons through the samples.
Thus the neutron total cross-section is related to the neutron transmission rate T(E) as follows: )(ln1)( ET
NE
]N0[][)(
BGOBGSNET
N is the atomic density per cm2 in the sample.
N and N0 are the foreground counts for the sample in (sample beam) and out (open beam),
BGS and BGO are the background counts for sample in and out respectively.
The atomic density N in the sample can be calculated from the formula ][//)( 2cmANtN A
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By inserting the values of N and transmissions from the fig.1 we measured the total cross-sections of the entire samples depending on neutron energies.
The measured cross-sections are compared with the evaluated ones from ENDF/B-VI and some other published data
0.1 1 10 1000.01
0.1
1
10
100
1000
10000
100000
To
tal C
ross
-Sec
tion
in b
arn
Neutron Energy in eV
natural W (n,tot) Present data R.E.SCHMUNK R.E.CHRIEN J.A.HARVEY W.SELOVE ENDF/B-VI
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Detection systemOur detection system for Ɣ rays emitted
during neutron capture is liquid scintillation detector consists of six photo multiplier tubes surrounding the sample
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Scintillation detector
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Transmission measurements
L ∆L
2310227.5
stmLeVEn
Detector
Flight path L, mSource
Collimator
Sample
dt
∆E= 2∆t ∆E = 2∆LE t E L
∆E= 2.77. 10 -2 ∆t(μs) E3/2
L(μs)
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13nσ0Г/Δ
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Breit-Wigner formula for cross section
cmevE
EE
ngc
)(
910.86.2
2/4
220
2
c Capture cross section
:total resonance width
:width of neutron resonance
:energy at the center of the resonance0En
0.1 1 10 1000.01
0.1
1
10
100
1000
10000
100000
Tota
l Cro
ss-S
ectio
n in
bar
n
Neutron Energy in eV
natural W (n,tot) Present data R.E.SCHMUNK R.E.CHRIEN J.A.HARVEY W.SELOVE ENDF/B-VI
capt
ure
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Application of neutron cross section
1 (Finding the neutron flux at certain energy2 (Determine resonance parameter ГƔ,Гn M
evE )(1.0
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Experiment Sample :Ta181 n = 1.5*1021 nuclei /cm2
Time of irradiation:360 min
curve
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ResultsΔnσ0Г/
ΔAN∑n(E)ζƔflux
4.3ev0.048330.655275584.8.1047.5
10.3ev0.07548.50.414106402.7.1043.3
13.95ev
0.08781.7760.175634852.1.1042.6
n(E)ζ/ flux
0.64
0.82
0.81
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Measured flux
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Conclusion This method is good in measuring flux as we get almost
the same fraction in our three resonances
The relation between the energy and flux is inversely proportional
Resonances are corresponding to the energy states in our sample
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thanks