Answer to Question #10015 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

The following question was answered by an expert in the appropriate field:

Q

Has it happened that the glow curves of TLDs (thermoluminescent dosimeters) have been determined by the method of individual photon counting in which the output pulse amplitude from the PMT (photomultiplier tube) has been used in a pulse height analysis method or not?

A

As you know, the common method of operation for assessing doses using TLDs is to operate in a current mode in which the signal resulting from multiple photon interactions on the photomultiplier tube photocathode is processed and the integral light-induced electronic output is recorded as a function of temperature. The glow curve so obtained shows the preponderance of light emissions as it changes with temperature, high temperature emissions being associated with deeper traps in the phosphor material.

The individual photon counting method is an alternative approach and has been used for many years, especially for measuring low doses and for specialized applications such as archaeological dating. The number of photons emitted at a given temperature or over a given temperature interval provides a sensitive measure of dose so that very low doses may be evaluated. It is possible that someone has made measurements of the type you are interested in but, to my knowledge, the single photon counting method has not been used in conjunction with pulse height analysis (PHA) to attempt to determine the shape of the glow curve.

One consideration that would work against this approach is that the traditional glow curve shows light output versus temperature, but different peaks that occur at different temperatures are not necessarily associated with different wavelength photons. This can be seen from some studies that have been done of the spectra associated with the light emissions. For example, the paper by S.W.S. McKeever (Measurements of emission spectra during thermoluminescence (TL) from LiF(Mg, Cu, P) TL dosimeters. J. Phys. D: Appl. Phys. 24:988; 1991) shows results of making spectral measurements of light emissions from LiF(Mg, Cu, P) using a fast scanning monochromator and S-20 photomultiplier tube assembly. The spectral results show curves of output versus wavelength at various specified temperatures. A notable feature of the family of curves is that the respective major peaks at the various temperatures occur at about the same wavelengths. The implication of this is that it does not seem likely that measurements of individual light photon-induced pulse heights would be associated with specific temperatures.

This is not to say that you should not pursue such measurements; you might find some interesting information, but I don’t know that you will be able to reconstruct the glow curves from the individual photon measurements.

George Chabot, PhD

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