Variation of Flux and Energy of 14 Mev Neutrons With Variation of Matrix Around A Sample

Introduction

The 14 MeV neutron generator¹ used for the present work generates neutrons of energy 14.77 MeV and has a flux of 10⁸ n/cm²/sec. When a sample is irradiated then the induced activity ² in the sample is

A=N σ F

Where A, N, σ and F denote activity induced in the irradiated sample, the neutron flux, cross-section of the reaction through which activity is induced and flux of the neutrons respectively.

Now, if two samples 1 and 2 are irradiated simultaneously then the activity induced will be

A₁=N₁σ₁F  and  A₂=N₂σ₂F

F being same for both because of simultaneous irradiation. Taking the ratio of the two activities, we get 

A₁/A₂=Kσ₁/σ₂

Where K=N₁/N₂ is constant for constant weights of sample 1 and 2. A₁/A₂ is constant when σ₁/σ₂ is constant. And σ₁/σ₂ is constant when the energy of the incident neutrons is constant. As with the variation of energy both σ₁ and σ₂ vary, so does the ratio σ₁/σ₂. So knowing this ratio the value of the energy E can be found as σ₁/σ₂ has a constant value for a particular value of E. The actual neutron source is surrounded by an iron water cooling jacket. The results can help to understand the attenuation of the neutron energy and flux and corresponding corrections can be made in the cross-section values obtained from the neutron activation analysis method using these neutrons.

Material and Methods

For observing the variation of flux and energy of neutrons, experiment was done by irradiating two samples simultaneously and varying the matrix around it and noting the variation of activity.

The sample and matrix used along with the concerned data is given in Table 1 and 2.

Table 1: Samples used

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 Table 2: Matrices used

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The excitation curves^{3, 4} are shown in figures 1 and 2. The samples taken together, Copper in powder form and Aluminium as foil within a polythene bag were irradiated with different matrices around it one by one. The time of irradiation, cooling and counting were 10, 2 and 10 minutes respectively. These two samples were taken because of the following reasons:

Easily available

Comparable half life⁵ of the products allowed to take same irradiation time, that is, around 10 minutes.

Excitation curve shows that in the energy region concerned (12-15 MeV), the slope is different (for copper it is increasing and for aluminium it is decreasing) and so σ₁/σ₂ varies with energy or we can say d(σ₁/σ₂) /dE is high and easily noticeable even for small energy change.

Figure 1: Excitation curve for the reaction Cu63(n,2n)Cu62

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Figure 2: Excitation curve for the reaction Al27(n,p)Mg27

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Results and Discussion

The ratio K of number of atoms of Copper to number of atoms of aluminium, that is, N_Al/N_Cu has a value of 0.34. And as such the cross-section ratio is given as

σ_Cu/σ_{A l} =  (0. 34)*(A_Cu / A_Al)

Where A_Cu / A_Al is the activity ratio of copper to aluminium.The value of s_Cu/s_{A l} gives the value of energy of the neutrons.The variation of flux and energy of the neutrons with variation of matrix around the sample as obtained are shown in Table 3.

Table 3: Variation of neutron energy with matrices

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The maximum variation of neutron flux and energy is for carbon matrix. And the least variation is for iron powder.

Acknowledgements

I am thankful to the Council of Scientific and Industrial Research and Department of Science and Technology for their financial support.

References

1. V.N. Bhoraskar, Ind. J. Pure and Appl. Phys.,27: 648, (1989).
2. L.F. Curtiss, An introduction to neutron physics, D. Van Nostrand Company, Inc., Princeton, New Jersey, (1959) .
3. Technical report series no.273, Handbook on nuclear activation data by international atomic energy agency.
4. EXFOR Nuclear Reaction Data, http:// www.nds.iaea.org/exfor
5. C.M. Lederer, J.M. Hollander, and I. Perlman, Table of Isotopes, John Wiley and. Sons, New York (1967).