Gamma-Ray Scintillation Spectrometer with Logarithmic Pulse-Height Response

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Abstract
Practical use of scintillation spectrometers is unnecessarily complicated by the strong predominance of small pulses encountered especially in measuring complex spectra. The difficulties arising from this general trend can be reduced by deforming the pulse spectra before recording them in a conventional linear pulse‐height analyzer. A discussion of energy dispersion obtained with different types of deformation shows that a logarithmic transformation not only multiplies the counting rates of the original spectrum with a factor proportional to energy, but at the same time increases the energy band which can be resolved with a given number of counting channels. The reduction of counting rate extremes obtained by such a transformation permits attainment of higher total counting speeds with given multichannel equipment (assuming equal scaling factors in all channels); on the other hand, it facilitates accurate intensity measurements using photographic recording techniques. An electronic circuit is described, which accepts input pulses having a sharp rise (<1 μsec) and produces, 30 μsec later, a dc voltage in linear relationship with log [input pulse height]. The 100‐v range of the output voltage is adjustable to cover pulse‐height ratios from 1:2 up to 1:50. Gray‐wedge spectrograms of the γ rays from Ra taken with the usual linear and with the new logarithmic circuit are presented. Theoretical calculations, as well as measurements with various calibration sources show that the photopeak counting efficiency of a NaI spectrometer becomes nearly constant over a wide energy range when the logarithmic energy transformation is introduced.

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