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Miniray Dosimeter – Operation Manual

Measuring Procedure

The dosimeter can be switched on by pushing the button on the front panel. The button must be kept pushing while measuring.

In laboratories using open radioactive isotopes as well as in places where contamination is possible it is advisable to put the instrument into a plastic bag which can be replaced easily if contaminated. In this way decontamination time can be saved.

The detector (GM tube) is located parallel to the front panel of the dosimeter above the LCD display. For this reason the front panel must be approached to the place to be measured as close as possible. Self adjusting of the instrument happens within 8 to 10 seconds after pushing the button while it must be kept pushing continuously. After the mentioned time elapsed the measurement result can be read on the display.

For instance, the dose rate on the surface of a container is to be measured. The front side of the dosimeter should touch the surface of the container at the point – or line – where dose rate is intended to be measured.

The display will show figures like:

1041

1025

1063

1028

1032

Results show slight fluctuation because of the statistic character of radiation. More accurate results can be achieved by forming an average:

1041 +1%
1025 -2%
1063 +3%
1028 -0,5%
+1032 -0,2%
5174: 5 = 1035   0%

As it can be seen a mean value of 1035 µSv/h was obtained. This calculation is proposed especially for those cases when fluctuation is extremely high. Considering the present measured values one can conclude that any of them could have been given as the final result because deviations from the mean value are less than 5%.

If measurements with legal consequences are to be made care is needed. The following condition must be taken into consideration:

As the detector of the instrument senses beta radiation, too, in spaces where dose rate is to be measured in mixed radiation space (presence of gamma and beta rays), the following procedure is proposed. The detector should be shielded by an approximately 5 to 10 mm thick plexi or plastic sheet by which the possible beta radiation is filtered.

Background Measurement

If no radiation source is in the vicinity the dosimeter will measure the background radiation when pushing the button. As the background radiation is rather low, very different figures will appear on the display. The reason for that is that the time elapsing between arriving particles and the time constant of the instrument is in the same range. In order to get the appropriate measurement result the above mentioned average formation method should be applied.

Example:

Push the button and keep it pushing e. g. for 100 seconds. Read and record the figures appearing on the display in every 10 seconds.

No. Time [sec] Readout
1. 10. 0,24
2. 20. 0,35
3. 30. 0,22
4. 40. 0,13
5. 50. 0,14
6. 60. 0,24
7. 70. 0,31
8. 80. 0,01
9. 90. 0,09
10. 100. 0,16
Total 1,89

Accordingly, the dose rate caused by the background radiation is 0.19 µSv /h at the given place and during the measurement time.

By comparing the mean value with the individual results very high differences can be seen giving reason for accepting only the mean value as a reliable measurement result.

Battery Replacement

When using alkali-manganese batteries the dosimeter can operate for more than 100 hours. Appearance of the LO BAT signal when the instrument is switched on indicates exhausting of the battery. In this case the displayed value can not be considered as a measurement result. Measurement can only be followed after replacing the battery.

Steps of Replacing the Battery:

a) Lay the dosimeter onto its front panel
b) Unscrew the two bolts in the bottom of the case
c) Remove the cover of battery holder
d) Replace battery according to the appropriate polarity indicated in the holder
e) Screw the cover of the battery holder back
f) Check whether the battery is ready for operation by pushing the button for several seconds.

When a particle of the background radiation is detected a sound signal can be heard and simultaneously a figure other than zero appears on the display

Maintenance

In addition to the battery replacement the dosimeter needs the following maintenance works:

1. GM-tube Replacement

It is necessary every 5 years, regardless of the wearing out, or earlier depending on the rate of wearing out.

PRECAUTION:

The dosimeter may be disassembled only after removing the battery because there is a high voltage inside it when in operation.

2. Cleaning

Wiping with soapy or alcoholic wet rag is possible. Do not wash!

Miniray Dosimeter – Technical Data

Principal Concepts of Radioactivity

Back to Miniray Dosimeter Main Page

Technical Data and Operation Manual (downloadable pdf-file)

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