Monthly Archives: August 2015

How Do Geiger Counters Work?

Dear Dr. Zoomie – I’ve got all these Geiger counters but I have no idea how they work. Plus, somebody told me that they can only be used for a few types of measurements. Is it true that I can’t use my Geiger counter to measure radiation dose rate? And how DO they work, anyhow?

Geiger counters are part of a family of radiation detectors called “gas-filled detectors.” These detectors – as suggested by the name – are filled with gas. They have an electrode in the middle of the chamber and are set up so that there’s an electrical voltage between the electrode and the metal wall of the chamber – in a Geiger counter, for example, the voltage difference between these is about 900 volts. When radiation hits the molecules of gas in the tube it strips electrons off of the atoms – this process is called ionization. The electron is attracted to the positive charge of the anode and the rest of the atom (a positively charged ion) rushes towards the wall of the tube. Then the electron travels through the wires that make up the electrical circuit and recombines with the ion – part of this electrical circuit is a device that measures the flow of electrons.

Geiger-Mueller Tube

Geiger-Mueller Tube

Of course, it’s hard to measure a single electron – luckily the instruments don’t have to do so. When the electron and ion are accelerated towards the electrode and chamber walls they gain a LOT of energy because of the high voltage – they bump into other atoms and knock electrons off of them in a process called secondary ionization. Those electrons and ions, in turn, cause even more ionizations and so on – this amplifies the original signal by a huge degree, to a point where it can be measured.

Creation of Discrete Avalanches Proportional Counter

Creation of Discrete Avalanches in a Proportional Counter

In a Geiger counter, the voltage is so high that the entire chamber of gas becomes ionized – this gives the highest sensitivity to incoming radiation. In other detectors (called ion chambers and proportional counters) the voltage is lower and the amount of gas amplification is lower as well; only some of the gas atoms are ionized in these detectors. The amount of amplification from increasing voltage is shown in the graphic.

Geiger-Mueller Region

To make sense of this graph – especially when it comes to understanding some of the limitations of Geiger counters – it helps to understand that alpha particles have a lot more energy than do beta particles. It also helps to understand that radiation dose is a measure of the amount of energy that’s deposited by radiation in an object – more energy means more dose. So looking at this graph, we can see that high-energy radiation hitting a detector leaves a bigger signal than does low-energy radiation. Now look to the right, in the Geiger-Muller region – in this region the high-energy radiation produces exactly the same signal as the low-energy radiation. So we can’t tell the difference between high-energy and low-energy radiation, which means that we can’t necessarily tell how much radiation dose a person was exposed to if we’re just measuring with a Geiger counter. This is one reason that we can’t always use a Geiger counter to measure radiation dose rate accurately. If a Geiger counter, for example, is calibrated to measure radiation dose rate from the radionuclide Cs-137 it will be right on the money as long as you’re trying to measure radiation dose from this nuclide. But what if you’re trying to measure radiation from cobalt-60 (Co-60)? Well, then you’re in a bit of trouble – radiation from Co-60 is twice the energy as radiation from Cs-137 so whatever your detector reads will be only half the actual radiation dose rate. On the other hand, a lot of radionuclides are lower-energy; in this case, your meter is going to read a higher dose-rate than is actually the case. The bottom line is that a Geiger counter will only give an accurate radiation dose-rate reading if it’s measuring the same radioactive material it was calibrated with. This is why Geiger counters aren’t always the best instruments to use to measure radiation dose rates.

It turns out that there is one type of Geiger tube that will give fairly accurate readings from a wide range of radiation energies – they’re called energy-compensated Geiger counters. These are designed to give a fairly constant reading across a wide range of radiation energies, so if you’re using an energy-compensated GM you actually can make accurate dose-rate readings. Otherwise, it’s probably best to make your dose-rate measurements with an ion chamber.

One more thing about Geiger counters – they are great general-purpose radiation detectors because they can measure alpha, beta, and gamma radiation. If you’re measuring gamma radiation you’re probably most interested in measuring dose-rate (measured in mR/hr or, for really low levels, in µR/hr – microR per hour); if you’re measuring alpha or beta radiation then you’re probably looking for contamination and you should be measuring counts per minute (cpm) or counts per second (cps), depending on the type of meter you’re using.

So let’s put all of this together:

  • Gas-filled detectors (such as Geiger counters and ion chambers) are filled with a gas that has an electrical voltage applied across it.
  • When radiation interacts with the gas it causes ionizations, and this small signal is amplified by the electrical voltage – the amount of amplification depends on the voltage.
  • Ion chambers can measure the difference in radiation energy – for this reason they are ideal for measuring radiation dose-rate.
  • Geiger counters, on the other hand, have full amplification of the signal for any radiation energy that strikes them. For this reason, they don’t always give accurate dose-rate readings, especially if the energy of the radiation is different than what they were calibrated with.
  • Having said that, energy-compensated GM detectors are designed to help accommodate this problem – they give fairly accurate dose-rates across a wide range of radiation energies.
  • And finally, Geiger counters can also measure not only gamma radiation dose (measured in mR/hr or µR/hr), but also alpha and beta contamination (measured in cps or cpm).

I know this is a long and fairly complicated answer – I hope it helps!

Do I Need To Provide Dosimetry For My Rad Workers?

Hi, Dr. Zoomie. Quick question for you – I’ve got a small radiation safety program and I’m wondering if I need to get dosimetry for my rad workers. How do I know what I need to do?

This question is either really simple or fairly complicated depending on how you approach it. The full answer is going to take a little time, but let me start with the easy version.

According to regulatory requirements, you are required to provide dosimetry to any radiation workers who can reasonably be expected to receive a dose of 10% of allowable limits – that’s 500 mrem (or 5 mSv) in a year. So in theory all you have to do is to evaluate the radiation dose rates in the areas where your workers spend their time and multiply that by the amount of time they spend in that area per year. For example, if the highest radiation dose rate is, say, 0.2 mR/hr and a person works in that area full-time then they can receive an annual dose of 400 mrem (40 hours per week times 50 working weeks per year times 0.2 mR/hr = 2000 hours x 0.2 mR/hr = 400 mR annually). So these workers are not required to be issued radiation dosimetry. Having said that, you might choose to give them dosimeters since they’re close to the level where badging is required – that’s up to you.

Example of a Luxel Badge

Example of a Luxel Badge

Another easy answer is to look at the model procedure for a radiation dosimetry program. This is found in the volume of NUREG 1556 that’s applicable to your type of a program, or in corresponding state regulatory guidance documents. For example, many model dosimetry procedures call for badging anyone who handles millicurie quantities of gamma or high-energy beta emitting radionuclides. Using this criterion, a lab worker (for example) who works with more than 1 mCi of P-32 (a high-energy beta emitter) is required to be issued a dosimeter, even if he or she never works with them long enough to receive an appreciable dose during the year. Other types of radiation safety programs – industrial radiography for example – are also required to badge specific categories of workers (the radiographer and radiographer’s assistant, in this case).

A third (and final, as far as I know) easy answer is just to give a dosimeter to everyone, but this only makes sense if your company is small enough that this won’t cost a fortune, and if your workers are worried about working around radiation.

OK – those are the easy answers, now let’s take a look at what happens when things get a little more complicated. Let’s look at a few specific types of radiation safety programs and see whether or not dosimetry might be called for. And please note – these are suggestions! You HAVE to follow the regs, but you’re not required to do anything in excess of what they require.

  • Industrial radiography – the model procedures are almost certainly going to call for badging the radiographer and his or her assistant regardless of the amount of dose they normally receive. You might also consider putting a dosimeter inside the room where the camera is stored to measure radiation dose in this area – this is known as an area monitor.
  • Blood bank irradiator (also research irradiators) – these are normally very well-shielded and dose to those working with them is typically fairly negligible. However, you will probably want to give dosimetry to those who operate the irradiator, just in case the shielding becomes cracked or damaged. It’s also a good idea to put at least one area monitor in the irradiator room and in adjacent spaces.
  • Industrial gauges (tank level, density monitoring, process control, etc.) – these sources are typically relatively low-activity and well-shielded and workers usually receive very low exposures. However, it’s not a bad idea to badge anyone who works directly with the gauges – especially if they do maintenance on them or work near the beam that emanates from them.
  • Soil density and soil moisture content gauges – dose rates here are also very low, but the operator often has the opportunity to expose the source directly to the air, so it usually makes sense to badge those who operate these gauges.
  • Nuclear medicine technologists – even if the administered doses are low, these workers are exposed to patients (and sometimes to syringes or capsules) containing radionuclides and they certainly have the potential to receive higher exposures. Nuclear medicine techs (and radiology techs) should be badged unless they have been removed from duties involving any exposure to radiation or radioactivity.
  • Small x-ray devices (lead paint analyzers, gauges to measure coating thickness, etc. – these, too, emit relatively low levels of radioactivity and are usually well-shielded. If they are used in a fixed location it makes sense to install area dosimeters at the operator’s station (or wherever workers stand during operation). If the devices are hand-held it is reasonable to badge the operators – this might include giving them extremity dosimeters (ring badges) if they have to hold the objects that are being tested.
  • Veterinary clinics, podiatrists, dental offices, and similar places using diagnostic x-ray machines – a lot depends on where the x-ray machines are located. If they are in dedicated x-ray rooms then only the operator needs to have a dosimeter. Oh – if it’s necessary to have a person hold an animal during an x-ray then that person should be badged also if they’re a member of your staff (if the owner holds the animal then dosimetry isn’t needed since they’re only going to hold their pet once or twice a year). If you don’t have a dedicated x-ray room then you should consider having everyone leave the room during x-rays; if staff are going to be in the room routinely during x-rays then they should probably be badged also.

There are a lot of other possibilities – these only scratch the surface, but they should help to give you an idea as to who to consider badging at your facility.

Finally – there’s more to running a dosimetry program than just handing out dosimetry. You have to remember to exchange the badges from time to time (this could be monthly, quarterly, or semi-annually depending on your program), you have to remember to notify your badged personnel of their badge readings at least annually (and preferably after every read), and you have to remember to hang onto your dosimetry reports as long as you have your license. There’s more as well, but if you can attend to these bits then you’ll be off to a good start.

If you’d like to learn more about running a personnel dosimetry course you may want to consider Nevada Technical’s 2-day Personnel Dosimetry course which is given once a year in Las Vegas, NV.

Information Resources for Radiation Safety

Dear Dr. Zoomie – I’m new to this whole radiation safety business and I’ve got a lot of questions about the right way to do things. Can you tell me where I can find the information I need to do my job right? Thanks!

If you’re looking for information there are three categories of resources – people, websites, and documents. Let’s take each of them in turn.

The best way to get an answer to your questions is often to ask someone. This can be a consultant, but it can also be someone who works in radiation safety and is willing to lend a hand. If you want to retain a consultant we can modestly put ourselves forward (although there are other people who do this sort of work as well). But before you call us up, there are some other people you can check.


For example, the Health Physics Society ( is the nation’s premier radiation safety professional societies. It might not make sense for you to join the HPS, but joining your local chapter almost always makes sense – local chapters meet at least once annually (some have monthly or bi-monthly meetings), and chapter meetings are a great way to get to know others in your area who also work with radiation safety – you can find out how to join your local chapter by going to the HPS web page.

You can also try to contact people directly. For example, if you have a large research university nearby (or a big hospital) there’s a good chance that the Radiation Safety Officer is a full-time health physicist. So if you call their radiation safety office you should be able to be put in touch with a radiation safety professional who likely has the time and ability to give you a hand. Even if not, he or she can most likely put you in touch with someone – one of their colleagues – who can help out.

Electronic resources

While there are a ton of websites that include – or are even dedicated to – information on radiation, few of them are really good. You have to beware of the great number of anti-nuclear websites that are out there; the information that they have is usually wrong and is almost always incomplete. If you’re looking for scholarly analysis, practical help, or regulatory information your best bet is to go to one of the regulatory agencies, or a professional organization that serves the radiation safety community. Some of these are:

Health Physics Society (includes a great deal of information for the general public, and even more information for members – also has an “Ask the Experts” feature where you can post questions)

International Radiation Protection Association (IRPA) (more of an international resource, including links to information documents and the Proceedings from some IRPA meetings.

Nuclear Regulatory Commission (includes regulations, fact sheets, and regulatory guidance documents)

Environmental Protection Agency (the home page for EPA’s radiation regulations and information on radiation-related topic)

Centers for Disease Control (CDC) (contains a great deal of information on the health effects of radiation and on responding to radiation emergencies)

National Council on Radiation Protection and Measurements (NCRP) (NCRP has published nearly 200 reports on various aspects of radiation safety, many of which are likely relevant for your work)

International Commission on Radiation Protection (ICRP) (the ICRP is an international body that makes recommendations on various aspects of radiation safety)

United Nations Science Committee on the Effects of Atomic Radiation (UNSCEAR) (UNSCEAR has published a number of definitive reports on the sources of radiation – both natural and man-made – and their health effects)

International Atomic Energy Agency (IAEA) (IAEA is best known for conducting nuclear safeguards inspections, but they also have a large number of documents on various aspects of radiation safety, including model procedures, suggested regulations, and incident reports)

American College of Radiology (ACR) (the ACR is primarily a society for clinicians, but they maintain a great deal of information on the safe use of radiation in medicine

Radiation Event Medical Management (a very useful site with information, downloads, and calculators, mostly aimed at emergency response, but with a lot that can be used everyday)

Rad Pro Calculator (includes calculators for radiation unit conversions, dose, decay, and so forth)

In addition to all of these, there are a number of software packages and smartphone apps that might be useful. But since these come and go so rapidly I’ve decided not to list them here – you should do a search to see what is out now (one of my favorites is The Effects of Nuclear Weapons, but the IAEA isotope browser is also useful).

Printed resources

Those of you who are (like me) still a fan of hardcopy references will also find a great deal to make you happy. In addition to the reports of the NCRP, ICRP, IAEA and UNSCEAR (all of which you can download in PDF or purchase hard copies), here are a few of the very many references that you might find useful.

Health Physics and Radiological Health (Johnson and Birky) – if you are going to have only one professional reference in your library it should be this one – it’s the single most comprehensive one-volume reference out there.

Basic Radiation Protection Technology (Gollnick) – a classic text aimed at the technician; presents material that is complete and easy to understand.

Introduction to Health Physics (Cember and Johnson) – a higher-level text aimed at the college student; unless you’re a professional health physicist you probably don’t need this level of detail

Environmental Radioactivity from Natural, Industrial, and Military Sources (Eisenbud and Gesell) – if you’re working with environmental radiation safety or in industries that generate naturally occurring radioactive materials (NORM) then you should have a copy of this book; it’s the definitive text on radiation in the environment

Radiological Risk Assessment and Environmental Analysis (Till and Grogan) – a somewhat higher-level book that will be of most use if you are working on environmental projects or for a company that produces a great deal of radioactive byproducts (e.g. from mineral processing)

Radiation Protection and Dosimetry (Stabin) – an introductory college-level introduction to the science and profession of health physics

This list just scratches the surface. I have two floor-to-ceiling bookshelves filled with professional references and journals, but the majority of these are very specialized and would probably not be of much interest to you. But between the books listed here and appropriate reports from NCRP, ICRP, IAEA, and UNSCEAR you should be in pretty good shape.