When the water evaporates, the crystals cling to the plate well enough for han- 

 dling. 



Crystals may be split in sections about 3 millimeters thick by setting the 

 crystal on edge, pressing a razor blade against it and then tapping gently with a 

 light hammer. Again sections may be cut from a rock with a rotating wheel and 

 diamond powder abrasive. Sections 1 centimeter square and 3 millimeters thick 

 are satisfactory. 



Above the hot plate and sample is placed a photomultiplier tube, which con- 

 sists of several photoelectric cells in series in a single evacuated tube, arranged 

 to give great sensitivity. Different photomultiplier tubes that are available give 

 maximum sensitivity in different parts of the spectrum. An RCA-5819 tube is 

 satisfactory for general purposes. It may be operated either by 900 volts of 

 radio B-batteries, or by rectifiers and electronic circuits. The heating plate and 

 photomultiplier tube are placed in a light-tight box provided with a door. Inas- 

 much as sensitive photomultiplier tubes are injured for highly sensitive work by 

 bright light, it is advisable to have a push-button switch set in the door so that 

 whenever the door is opened for introducing or removing samples, the photo- 

 multiplier tube is disconnected. 



The sensitivity of the apparatus and the constancy of the measurements are 

 improved by placing a wide cylinder of clear quartz or glass between the sample 

 and the phototube. It acts by internal reflection to funnel most of the light emit- 

 ted by the heated sample directly to the phototube and makes possible a gas-tight 

 separation between the upper and lower parts of the dark box so that the photo- 

 tube does not become over-heated by the heating plate. For the most effective op- 

 eration, at very low light intensities, the photomultiplier tube can be cooled with 

 a stream of cold air to reduce the background current or noise level. 



The current from the photomultiplier tube is proportional to the intensity 

 of light emitted by the thermoluminescent material. It is amplified and recorded 

 on a moving chart such as is manufactured by Leeds and Northrup, Brown- 

 Honeywell, Esterline-Angus, Variac, or others. A single pen recorder may be 

 used, but a double recording of both light intensity and temperature is helpful. 

 An actual recording is shown in Figure 10-1. 



A determination takes about 10 minutes, and when it cannot be continued 

 longer because of the continuous radiation emitted by the hot plate, the heating 

 plate and sample are removed and cooled for the next determination. If a series 

 of determinations is being made, the heater is cooled rapidly by placing it on a 

 cake of dry ice. 



Additional glow-curve peaks can be obtained at low temperatures. The 

 sample and heating plate are cooled with liquid air by conduction along a thick 

 metal rod (Hecklesberg and Daniels, 1957) . The irradiation of the material 

 with X-rays or gamma rays is carried out in the cooled apparatus, and then the 

 plate and sample are heated gradually to room temperature and above. Of course, 



185 



