H. J. de Bruin and B. A. Bridger Flinders University of South Australia Bedford Pork, south Australia 5042
Audible Pocket Monitors for Radiochemistry Laboratory Classes
Personal monitoring of radioactive emination and contamination in undergraduate labdratories is quite hothersome. The amount of radiation is generally too small and the number of students too large to justify the expense and administration of a film badge service. Pencil electrometers are not sufficiently sensitive and like the film badge they do not provide a warning a t the time of exposure but merely an integrated dose over a period of time.
Dosimeters are psychologically like a hangover: their influence during the actual event is negligible. Their delayed revelation sometimes provides an uncomfortable experience that can never reverse the cause of the damage. To the uninformed dosimeters may induce a false sense of security, which is aggravated by the fact that some radiation can he highly directional as in faulty X-ray equipment. Although radiation levels are usually negligible in undergraduate chemical laboratories, an appreciation of the dangers of radiation and contamination is of important educational value.
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G.M. detector
Arnplltler
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Speaker
B Figure 1. A, Radiation monitor. 8, Charging station with five monitors.
Figure 2. A, Radiation monitor, block diagram. 8, Battery current wave
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Volume 50,
Number 5, May 1973 / 373
We have developed a cheap pocket monitor which provides audible warning for p, y, and X-radiation with a sensitivity limited only by the window thickness of the detector, and which can he operated continuously for up to 10 hr. A small station allows five such monitors to he recharged overnight. Figure 1A shows a monitor, which fits comfortably in the breast pocket of shirts or laboratory coats. A safety chain, worn around the neck, prevents accidental dropping. In Figure 1B a recharging station with five such monitors is shown. The device is useful, not only for establishing the presence of radiation fields, hut also for detecting contamination either on parts of the body, workbenches, or clothing. By using these monitors, students have been found to he more aware of the reality of radiation and more confident in the execution of radiochemical experiments. In its present form the device is only semiquantitative and does not replace, but rather supplements integrated dosimetry. In circumstances where dosimetry is not required, such as undergraduate teaching laboratories, the cost of parts and labor to supply several batteries of monitors (approximately $25 per monitor) easily outweighs that of expensive commercial detection devices and-personal dosimeters of one kind or another. Figure 2A is a block diagram of the m0nitor.l A dc to ac conv&er operates from 1.25-V nickel-cadmium cell.
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374 1 Journal of Chemical Education
This ac supply powers a high voltage generator for the operation of the G-M tube and a low voltage generator for the amplifier. The converter is designed to provide a 3.5 kHz oscillation. A radiation induced pulse in the halogen quenched G-M. tube (Philips 18504) is expanded in a diode pump circuit to approximately 0.5 sec. This pulse then triggers a silicon controlled rectifier switch (S.C.R.) which allows the 3.5 kHz oscillation to appear across the speaker (a 300 ohm Sony microphone insert) as a squeak. The charging station simply consists of a transformer, rectifier, and five outlets in parallel each containing a resistance to reduce the charging current to an optimum value. Each monitor contains a blocking diode to prevent discharge into the charging station. R hr-l alThe sensitivity of two counts sec-I per lows the detection of minor contamination during radiochemical manipulation. Although the peak current drawn from the nickel-cadmium cell is 150 mA, the mean value of the current is approximately 45 mA, as shown in Figure 2B. The smallest available rechargeable battery with suitable capacity was a 1.25- V, 450 mA-hr cell which permits continuous operation for at least 8-10 hr. 'The authors will gladly make detailed circuit diagrams available on request.
