GALEN W. EWlNG Seton Hall University
South Orange. New Jersey 07079
CHOICE OF LIGHT-MEASURING SYSTEM
CI. Thermoluminescence Part 11. Instrumentation Emanuel P. Manche Deparlment of Natural Sciences, York College of the City University of New York, Jamaica, NY 1145 1
INSTRUMENTAL CONSIDERATIONS
measure the integrated amplifier is used light output. A shielded photomultiplier tube is located facing a properly prepared sample in a light-tight enclosure. The sample holder is heated usually by the passage of current through heating elements in intimate contact with it using a servo heating control system. The temperature is programmed linearly a t high heating rates, typically 1-20% s-'. The final temperature is approximately 400°C. Much slower heating rates are sometimes used on samples that have been artificially jrradiated with high doses. Between the photomultiplier tube and the sample, there may he ashutter and a color filter. Thesample is nearly always flushed with a purified inert gas, usually nitrogen. Two thermocou-
ples are located as close as possible to the sample. One couple leads to the temperature controller for feedback control while the other for the purpose of recording the sample temperature, goes to the X-axis of the X-Y recorder. A single thermocouple may be used in certain equipment designsas shown in the figure. The light emitted during the heating cycle is detected by a photomultiplier tube, the sienal from which is amdified and then fed toihe Y-axis of the recoider. A regulated voltage power source supplies the needed voltage for the photomt~ltipliertube. If simultaneous knowledge of temperature linearity is required, a two-pen strip chart recorder or an X-Y-Yrecorder is essential. Here, the x-axis represents time while the signals from the thermocouple and the photumultiplier go to each of the twoamplifiers of the recorder t o yield a graphic display of the latter two signals. Of course, analog-todigital eogverters can be used to obtain the data in dig~talform for further processing.
X-Y RECORDER
REGULATED H V POWER
AMPLIFIER
J
TCMP.
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used, and in particular an the operating characteristics of the PM tube and its associated electronic read-out and power supply circuits. Far thermoluminescence, as for all low-level measurement of radiant energy, consideration must be eiven to measurine
the instrument or itsmodules is of particular concern, and we distinguish thesignal, taken as the analytical information in the output, from the noise also present. All extraneous and essentially random fluctuations added to a signal are termed noise. Such random variations have their ultimate origin in the particulate or discrete nature of matter, charge, and energy (21). In thermoluminescence investigations several strategies exist for improving the measurement of the law-level radiant power. Certainly, one should enhance the signal by all possible means, for example by the useof a sample with a large area exposed to the detector or by increasing the rate of its temperature rise. A second strategy is to reduce noise. This may be accomplished by using a low-noise photomultiplier and cooling it to low temperatures, as well as using a preamplifier that has a noise figure as close tounity as possible. Further, the thermal radiation contribution from thesample may he reduced by means of suitable filters or the selection of a PM tube to match the spectrum of the thermoluminescence signal. A third is tooptimize the SIN ratio by techniques that discriminate between signal and noise. Two techniques that lend themselves well to thermoluminescence measurements are laek-in amplification and photon counting.
LOCK-IN AMPLIFIER
SERVO HEATING CONTROL SYSTEM Flgule 3 Block dmgram of DC analog apparatus for thetmol~mmescencemeasurcmenls P M is a photcmuit p l w t ~ b eTL represenlothe nat~ralInermoLmmescence curve, un Is BOrepresentsIhe oacr b w y radiation during a second heating of the sample, i.e.. aner removal of the natural thermoluminescence. An ootical filter is often used and is located in front of the PMtube.
The loek-in amolifier. known also as the phase w n s i t w ~PIP
