FAQ - Radiation Dosimeters

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Richard Hull
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FAQ - Radiation Dosimeters

Post by Richard Hull » Fri Mar 12, 2004 7:58 pm

The small pen dosimeter with its associated charger can be a useful tool in determining the x-ray exposure around a fusor.

I have attached an image below of a common 0-200mr/hr dosimeter pen and charger.

The best dosimeters are the 0-200 or 0-500mrem pens for the continuous fusors. Pulsed fusors might do better with a much more intense pen registration into the 100 rem range.

I prefer to work in the lab with a personally worn 0-200mrem pen that is repeatable and kept charged. I also keep at least two more pens charged and near the fusor and fusor bench to get an idea of "close in" exposure levels.

I will attempt to give a general text on these useful little quartz fiber electroscopes.

Basically, these devices are hermetically sealed electroscopes with a miniature scale and viewing telescope all in one small clip-on pen. A small gold metallized quartz fiber reacts electrostatically against a flat plate to deflect to a greater or lesser extent based on the charge deposited on it. This fiber is seen in the electroscope as an indicator needle viewed against a calibrated scale of radiation dose.

The pen electroscopes are charged or made ready for use by placing them on an externally associated charger which, by applying a voltage to the quartz fiber swings it out into space. While looking into the eyepiece on the pen and pressing down to make contact, one sets the indicator to zero with a control knob on the charger device. In this condition, the pen is said to be charged. As such, it is now ready for use. As the pen is carried around it is exposed to more or less radiation which ionizes the air in the pen's charge chamber more or less, and, more or less, discharges the pen, each time moving the indicating fiber up scale towards the discharged condition. At the end of a day or series of fusor runs the dosimeter is consulted and a reading taken. In a perfect world or a well operated/shielded lab, it is still on or near zero.

If you charge a dosimeter up and it is fully "soaked in" (key factor) and you fire a pulse from an x-ray source and it indicates 10 rem of exposure, then you can go to the bank with that. The only way it will indicate so is that if the entire internal volume of that little pen suffered such a fearsome blast of radiation as to nearly instantly discharge the stored charge on the fiber to the indicated level. Dosimeters are perfect intergrated dose recorders even if only +/- 10% accurate. They record in the ideal fashion and by the method first established as a radiation standard and that is through a determination of bulk ionization.

There are two ways these little suckers will lie to you. The first is improper or incomplete "dielectric soak in". If they have laid about in the discharged state for a long time, a recharging might appear fine, but in minutes the item may be totally discharged again or indicating a horrific radiation dose when there was none. YOU MUST charge and observe over time, the characteristic discharge rate of the dosimeter. With no radiation present, you should be able to charge a dosimeter and leave it laying about for 24 hours and see NO DEFLECTION OFF ZERO or movement upscale. If you do, then you need to keep recharging until the above non-discharge condition IS ACHIEVED. If you can't do this after more than 10 rechargings, then you probably have a bad dosimeter, (see below)

It is wise to keep important, working dosimeters constantly charged and the above will not be an issue.

The second lie can be told by these dosimeters if their hermtetic seal is borken and outside air and moisture can enter the chamber.... OR..... if the center contact to shell insulation is bad creating abnormal leakage. The above two conditions are heralded by the fact that the dosimeter never seems to stay charged in a zero radiation area regardless of a number of rechargings to achieve "soak in". OR if it seems to have a variable "zero rad field" discharge rate. In either case discussed in this paragraph, the pen is worthless and MUST BE DISCARDED.

Finally, for pulse work, these little electroscopes are the ONLY rapid, instantly readable method of determining total absorbed X-ray/gamma dose as no electronic counter has the speed to register it accurately. Notice I said X-ray/gamma dose. These will not give any indication about alpha, beta or neutron radiation dosages.

The dosimeter's principal drawback is, of course, that you have already had the "rads to the nads" and there is no undoing it. It is sort of "read it and weep time". In day to day activity, however, it can indicate that you have had more or less dosage than normal for the work environment. If so, then a snoop around with a counter will indicate the leak or source of the additional radiation dosage and this can then be ameliorated.

Dosimeters and their related chargers are often found surplus at hamfests or surplus stores and on E-bay. They are passive devices and as such need no batteries or repairs. They are either good or they are bad.

Richard Hull
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