Summer Student Practice, 2009, JINR Dubna1 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

2 Motivation - Studying the processes of the interaction of slow neutrons with nuclei ( 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.

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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 Purity [%] Atomic Mass [a.m.u] Density [g/cm 3 ] Size [mm 3 ] Sample x100x0.2 W

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6 The neutron Time of Flight (TOF) spectrum Fig.1 ) The neutron TOF spectrum for sample-in and open beam along with background level of W sample

7 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: N is the atomic density per cm 2 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

8 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

9 Detection system Our detection system for Ɣ rays emitted during neutron capture is liquid scintillation detector consists of six photo multiplier tubes surrounding the sample

10 Scintillation detector

11 Transmission measurements L ∆L∆L Detector Flight path L, m Source Collimator Sample dt ∆E= 2∆t ∆E = 2∆L E t E L ∆E= ∆t(μs) E 3/2 L(μs)

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13 nσ0Г/Δnσ0Г/Δ

14 Breit-Wigner formula for cross section Capture cross section :total resonance width :width of neutron resonance :energy at the center of the resonance capture

15 Application of neutron cross section 1) Finding the neutron flux at certain energy 2) Determine resonance parameter Г Ɣ, Г n

16 Experiment Sample :Ta 181 n = 1.5*10 21 nuclei /cm 2 Time of irradiation:360 min curve

17 Results fluxn(E)ζ Ɣ N∑N∑ Anσ0Г/Δnσ0Г/Δ Δ ev ev ev n(E)ζ/ flux

18 Measured flux

19 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

20 thanks