How to Calibrate A Geiger Counter

So far, the Geiger counter is the best radiation monitor option for personal use. It’s compact, relatively affordable, and widely available device on the market. Not only does it measure the radiation particles around us but also monitors the dose rate (dosimeter).

You can also make your own Gieger counter because the tube is also available to purchase. Simply, you have to buy the Geiger Muller tube. Geiger Muller tubes are available in various models and specs. Make sure you buy at least able to detect Gamma and Beta.

However, the Geiger counter that you buy may be the entry-level one. In this case, you need to check its accuracy. Even, if yours is the advanced level, you still have to check it.

There is some information you have to know about this Geiger counter calibration. Professional calibration should never happen in your house unless it passes a certain accepted standard. So, sending it to an accredited calibration lab is the best practice that we highly recommend. Still, you can perform a small checking in your house. Maybe, it’s better than no checking at all.

Exposing the Geiger Counter to A Known Source of Ionizing Radiation

If you own a Geiger counter, then checking the accuracy of your Geiger counter is a simple process. Geiger’s counter work to count or measure the ionizing radiation coming from a radioactive source.

Check your Geiger counter accuracy by exposing it to a known source of ionizing radiations. This means a radioactive or an ionizing emitting source (whose ability to emit ionizing radiations is already measured or known) is measured using a Geiger counter.

Then the ratio or percentage of ionizing radiation detected by the Geiger counter to the ionizing radiation emitted by the radioactive source gives the accuracy of the Geiger counter.

Government Standards of Calibrating and Checking Accuracy of Geiger counter

Calibrating a Geiger counter on Government Issue Standards individually is near impossible. It’s due to the government policies, international standards on how to calibrate the Geiger counter, and the level of radiation involve in this process.

Based on the manufacturer, different Geiger counters can measure different level, range and types of ionizing radiations. Some counters detect Gamma and Beta only, while others detect the three Alpha, Beta, and Gamma. Even, when they can detect Gamma, it’s not sensitive enough to measure the high energy level range. So each tube needs to be calibrated differently based on the radiation it is detecting.

ANSI-STD N323A is the standard that provides the necessary methods and standard steps for performing the method to calibrate your Geiger counter.

This standard can only be performed by a laboratory that has Nuclear Radiation Commission (NRC) approved licensed as per 10-CFR-34 and 20-CFR-35 regulations.

So an individual can’t perform a Geiger counter calibration process and calibrate or inspect the counter on Government Issued Standards.

The only way to have a professionally calibrated Geiger counter is to purchase a Geiger counter that has passed the ANSI-STD N323A standard from a 10-CFR-34 and 20-CFR-35 standard licensed laboratory.

DIY Geiger Counter Calibration

What Standard to Use?

To calibrate a DIY Geiger counter, a Cesium 137 standard sample can be purchased online.

Cesium 137 is a radioactive material that emits beta and gamma ionizing radiation when it decomposes. Therefore, the ionizing radiation level emitted by cesium 137 is not of the range that can harm humans on direct external contact. Aside from that, Beta particle is easy to block. A piece of paper can block Beta radiation. So, it’s relatively safe while helping you check the accuracy of your counter.

Steps by Step Calibrating the Geiger Counter to Gamma Source

  • First, you have to check your standard and find the information on it about its activity (for example, Cesium 137 sample has one micro cure means that sample will have 37000 decays per second).
  • Now, count the background radiation. Turn on your Geiger counter and lay it on a surface to let it measure the background radiation (atmospheric or surrounding radiations).
  • Note the reading after 10 min and it will give you a count per minute CPM value for the background radiation.
  • The next step is to measure the Gamma radiation. In this case, you have to block the Beta radiation to clear the reading only to measure Gamma. Put a 2 to 3 millimeter thick plastic shield between the source and the Geiger counter to block the Beta radiation.
  • Switch on the Geiger counter and let it take reading for 10 minutes, and that will be your new CPM for Gamma radiation.
  • Subtract the new Gamma radiation reading from the new background radiation reading. It’s the value of the sample standard that your Geiger counter gets.
  • Now, realize how the same they are. If the reading you take is not the same, you have to calculate the error in percentage. For example, if the standard value is 100 and your counter’s reading is 80, then its error is 20%. Whenever you take the reading with that Geiger counter, remember that it has 20% error. For example, if it displays 160 RPM, the possible more accurate reading is 200 RPM.
  • If you don’t like the above error calculation, you can change the reading of your Geiger counter to match the standard value.

Calibrating Geiger Counter with Alpha Particle Source

If you want to buy the alpha standard, check the following page:

https://www.drct.com/dss/sources/alpha.htm

https://www.gammadata.se/products/radiation-detection/radioactive-sources/alpha-standards/

https://www.imagesco.com/geiger/radioactive-sources.html

Conclusion

Geiger counter is one of the instruments that you can rely on for detecting and measuring ionizing radiations. As ionizing radiation can be dangerous, the accuracy of the Geiger counter becomes critical. Misleading reading can turn you into a dangerous dose you don’t realize. Therefore, it needs to be calibrated to find the error and even the reading can be adjusted.

Actually, Geiger counter calibration is not easy. Some factors can disturb the calibration. Geiger counter has a specifically developed standard of calibration which is ANSI-STD N323A. It’s said that it can only be performed by a laboratory that has Nuclear Radiation Commission (NRC) approved licensed as per 10-CFR-34 and 20-CFR-35 regulations.

A DIY Geiger counter can be calibrated using some easy and simple steps with the help of a Cesium 137 radioactive sample. This DIY thing is just for small-scale work. Professional work can only be calibrated by government-issued standards.

Reference

  1. https://www.imagesco.com/geiger/buying-a-geiger-counter-pg5.html

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