Neutron Emission from Spontaneous Fission J.W. Br mmer 1 N.A. - - PowerPoint PPT Presentation

Neutron Emission from Spontaneous Fission

J.W. Brümmer1 N.A. Khumalo2

• V. Mudau3
• T. Nompunga4

1. University of Stellenbosch, Stellenbosch, South Africa 2. University of the Western Cape, Bellville, South Africa 3. Intern, SABS (South African Bureau of Standards) 4. University of Fort Hare, Alice, South Africa

Supervisor: A.I. Svirikhin

Experimental Goals

l Calibration of the Silicon detector; l Quantify the different amount of isotopes in the target; l Determine neutron lifetime within the detector; l Determine the efficiency of the detector.

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Introduction: The Basics

l What is Spontaneous Fission l SF is a form of nuclear radiation where a nucleus splits into two fragments and emits neutrons and gamma rays; l In elements heavier than xenon (Z=54), there is a decrease in binding energy per nucleon as atomic number increases; l In this region of nuclear size, electromagnetic repulsive forces are beginning to overcome the strong nuclear force attraction.

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Relevant Theory

l For the reaction to be possible, a very important principle has to be obeyed: l Conservation of energy: Ei= Efrag + En0+Q l The initial energy before spontaneous fission has to be conserved; l The energy after spontaneous fission will be distributed between the fission fragments and the emitted neutrons; l Q>0 for reaction to take place. l Typical Reaction: l Neutron capture:

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Relevant Theory

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Experimental Setup

l VASSILISSA separates ER’s from beam and reaction by-products with 3 electrostatic deflectors; l Each has a specific pre-set potential difference that only allows the wanted isotope to travel through the detector;

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Experimental Setup

l The isotope then travels to a position sensitive detector array where it decays; l The decay products, fission fragments and alpha particles, are detected by the silicon detector. l The transportation time of ER to the focal plane detector is a few microseconds which allows for investigation of very short-lived isotopes.

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Experimental Setup

16 Layer Si detector He-3 Chambers Boron polyethylene shielding 6 Pre-amps from Si detector Vacuum Chamber

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Experimental Setup

Si detector in vacuum chamber He-3 filled tubes Boron polyethylene shield

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Resolution and Calibration

l The FWHM of each peak can be measured to obtain the average resolution of the detector: resolution ≈ 30 keV (0.6%). l Calibration was done by comparing the alpha spectra of the three isotopes used in the experiment: 244Cm, 246Cm and 248Cm.

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Calibration of Si-detector

248Cm⟹98,947% 246Cm ⟹1,052% 244Cm ⟹0.002%

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Neutron lifetime in Detector

l The detector has a 128 µs window to allow for neutron lifetime

• measurement. The tmax value will then be 128 µs.

l The graph shows the exponential nature of the lifetime and the exponential fit which yielded the average neutron lifetime.

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Detector efficiency

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ε = 39.6%

206Pb + 48Ca = 2n + 252No Target: 350μg/cm2 206Pb Beam: 48Ca - E1/2 = 223 MeV Intensity: 0.5 pμA (3x1012 pps) σ = 200 nbarn

= 4.06±0.1 neutr. per SF

Earlier reported values 4.15±0.3 (Lazarev - 1977) 4.43±0.45 (Yeremin – 2004)

ν

The 252No Isotope: Neutron Multiplicity

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Experimental Results

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Experimental Results

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Conclusion

l From our study of the neutron detector we have found the following attributes: l The neutron detector has zero crosstalk; l It has a high energy threshold with low gamma sensitivity & low internal background; l The detector has a relatively high efficiency; l The neutron multiplicity could be measured owing to the absence of crosstalk and the detector’s high energy threshold.

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Acknowledgements

l Our research group would like to thank the JINR and for the opportunity of working at the FLNR; l Thanks to our supervisor Alexander Svirikhin and to our co-supervisor Malyshev Oleg (FLNR); l Thanks to the South African organizers Prof. Lekala,

• Dr. Jacobs and Mr. Vusi Malaza;

l Furthermore we acknowledge the contribution of the NRF and DST South Africa that made this research

• pportunity possible.

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References

l [1] A. I. Svirikhin, A. V. Isaev, A. V. Yeremin, et al., Nucl. Instr. and Meth., 54 (2011) p. 644–648 l [2] A.I. Svirikhin, V.N. Dushin, M.L. Chelnokov, et al., Eur. Phys. J., A44 (2010) p. 393–396 l [3] A.V. Andreev, D.E. Katrasev, A.N. Kuznetsov, et al., Eur. Phys. J., A48 (2012) p. 121-126 l [4] E.A. Sokol, V.I. Smirnov, S.M. Lukyanov, et al., Nucl. Instr. and Meth., A400 (1997) p. 96-100 l [5] D.C. Hoffman, et al., J. Nucl. Phys., A502 (1989) p. 21-39 l [6] M. Gupta, A. V. Eremina, I. N. Izosimova, et al., Phys. of Part. and Nuclei, 9 (2012) p. 24–28. l [7] N.E. Holden, M.S. Zucker, Nucl. Data for Basic & Applied Science, Proc. Of the Int. Conf., Santa Fé, New Mexico, 1985 vol. 2 (p. 1631)

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