Fast neutron damage to HPGe Detector

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An observable decrease in the energy resolution of large HPGe detectors was first seen after the irradiation of 5*10^7 n/cm^2<ref>P. H. Stelson, J. K. Dickens, S. Raman, and R. C. Trammell, “Deterioration of Large Ge(Li) Diodes Caused by Fast Neutrons,” Nuclear Instruments and Methods 98,481 (1972).</ref>. I choose a factor of 50 below that value to be the maximum allowable neutron irradiation.

The maximum neutron flux from a point source will occur exactly at the center of the detector face, where the expression for integral flux over a period [math]\Delta t[/math] is simply: [math]\Delta t\times n_{rate}\times \frac{1}{4\pi d^2}[/math], where [math]n_{rate}[/math] is the neutron rate of the source.

The number of days it would take to reach an integral flux of 10^6 n/cm^2, as a function of the distance from source to HPGe face is shown below. The calculation uses a neutron rate of 19,066 [math]\pm[/math] 300 n/s, which was the neutron rate of the CF-252 source on 01/2017 (see here for discussion of source rates).


  • The formula used in the graph above is (black solid line),

[math]y=\frac{5\times 10^6}{(19066 * 60^2 * 24 * \frac{1}{(4*\pi * x^2)})}[/math]


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