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

*

3

4

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