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Counting on the Geiger Counter
Responders are on their own whenarriving at a radiologicalemergency. That makes a goodGeiger counter essential equipment
Volume 23, No. 1

Radiation from nuclear or radioactive materials can only be detected with special equipment. The pager-alarm worn by many first responders is one method. The old fashioned way was with a hand-held Geiger counter. There are plenty still around. Many fire departments rely on them to assess the hazard of an emergency situation. What if one were thrust in your hand and it was the only weapon you were counting on in a potentially hazardous situation? Could you use one? This article will review the operation and interpretation of Geiger Counter readings.?? First, let??s review an important fact about radiation exposure.

An old Seinfeld episode features an exchange between a chain-smoking would-be author and her editor Elaine, played by Julie Louis-Dreyfus. The surly author exclaims that there are no ??degrees of consequence?? ?V either something is a consequence or it is not a consequence!?? There??s nothing in-between!? Well, it was funny, and still is, but it wasn??t quite true. There are degrees of consequence especially when dealing with all forms of radiation. For example, imagine yourself walking along the beach in the sun. If you time it properly you can achieve a healthy looking tan, or if you over do it, you will get a sun burn. If you manage to overexpose your skin every summer of your life, all other competing factors being equal, you can increase your chances of getting skin cancer. Extending each individual exposure so that the cumulative life-time exposure to solar radiation is increased, increases your chances for an acute (sun burn) and long-term detrimental effect (skin cancer).? The situation with nuclear radiation exposure is similar.? We attempt to keep individual exposures to a minimum commensurate with achieving the mission of the first responder. An acute high dose of radiation could result in radiation sickness and an increase in the chance that many years in the future, a cancer of some type may occur.

The One, Two, Threes of Geiger Counter Operation

Geiger counters are good for measuring most forms of nuclear radiation, but not all.? Nuclear radiation comes in four major varieties. Geiger counters can detect three of these varieties with relative degrees of difficulty.? They can detect beta-particle radiation and to an extent, gamma-ray radiation. They have trouble measuring some weak forms of alpha radiation and cannot detect neutron radiation. We think terrorists might use beta and gamma radiation because the sources that emit these types of radiation are more commonly available and exist in industry in significant quantities.

Operating a Geiger counter is relatively easy. Refer to Figure 1 as you read.

1.?Check the batteries. Geiger counters run on ordinary batteries. Most if not all of them have a battery-check setting found on the main on/off switch. The indicator on the dial must fall within the ??battery OK?? arc or else the Geiger counter will not operate properly.

2.?If the Geiger counter has an audio on/off switch, turn it on to hear the clicks associated with detection of radiation. This allows you to keep your eyes on the mission at hand while your ears assess the rate of clicks and therefore, the level of radiation or radioactive contamination.

3.?The meter faceplate will have printed on it ??cpm?? for counts (or clicks) per minute.? In a ??background?? or natural radiation field you will probably hear and read about 0 to 50 cpm. That is normal and is the baseline from which you measure excess radiation. If you get a higher reading while measuring background, it could mean that you are already in a higher radiation field. Move to an area that you know to be radiation-free. If the Geiger counter still reads high, the probe may be contaminated with radioactivity or perhaps the Geiger counter is malfunctioning. To eliminate the possibility of contamination, the Geiger counter probe should be cleaned and background tested before each use.

4.?To make meaningful measurements, the Geiger probe should be pointed directly at surfaces. For example, the debris and shrapnel from a radiological bomb may be contaminated with radioactive substances. This debris may settle on people, street surfaces, vehicles and buildings. To test these surfaces, point the open-end of the Geiger probe at them. Short range radiation, like beta particle radiation, may not be detected unless the probe is within an inches of the suspected surface. A relatively slow scan is advised to allow the Geiger counter to respond.?

5.?Let??s assume that your meter faceplate reads from 0 to 10K. That means your meter reads 0 to 10,000 cpm when the multiplier switch (center of Figure 1) is set on? ??x1.??? The multiplier in this case means that you must multiply whatever you read on the faceplate by 1. If the multiplier were set on ??x0.1,?? all readings are one-tenth as much as you read from the faceplate. A ??x10?? setting means to multiply all readings by 10. By using the multiplier switch, the Geiger counter is given a wide response range. If you are on the ??x1?? setting and you read 5K on the faceplate, you have detected 5000 cpm. Background is about 50 cpm so clearly you have encountered radioactive contamination.? If the needle indicator ??pegs?? or goes all the way to 10K, accompanied by a crescendo of clicks, don??t panic. Set the multiplier to the next highest setting? - in our example, ??x10?? - and rescan the area. You must now remember to multiply your meter reading by 10. We were seeing 10K+ cpm when the needle pegged. Now, after the multiplier was increased, it may read only 2K cpm. That really means 2K x 10 or 20,000 cpm. All readings on this setting are 10 times higher than on the ??x1?? setting. Remember though, that if you are seeing 0 on the ??x10?? setting, it may be reading something between 0 and 10K on the ??x1?? setting. This is also true of the ??x0.1?? setting. So, switch back down to the lowest multiplier setting if you want the greatest sensitivity as you proceed with your investigation.? Remember, you may be measuring a field of intense gamma-ray radiation (long range radiation) or high levels of radioactive contamination separately or simultaneously. The gamma-radiation component may cause a high rate of clicking at a relatively far distance from its emission point. In the case of a radiological bomb, there may be many emission points scattered from the epicenter of the blast.??

CPMs vs. Rems: How Do We Determine the Hazard?

The reason you may have been issued radiation pagers (or ??chirpers??) is because it is relatively hard to interpret a Geiger counter reading if you are not a subject matter expert like a radiation safety officer or health physicist.? The ??cpm?? is not easily translated into an assessment of human hazard i.e., a radiation exposure rate. Even if you know the exposure rate, you still need to determine if it??s low enough to keep working in the area.?? It is also true that the more cpm the more radiation and/or radioactive contamination is present, but at what cpm does the situation become truly hazardous?? This can be difficult to decide.

The ??rem?? unit describes how much radiation exposure a person can safely receive from a source of radiation. To determine the hazard from radiation exposure, it is far easier for radiation specialists to use the rem unit than to use the cpm unit.? Fortunately, many Geiger counters can read out in both cpm and milli-rem per hour (1000th of a rem per hour abbreviated mrem/hr).

However, unless Geiger counters are calibrated with the radiation source that they are intended to measure in the field, the mrem/hr readings will probably not be accurate. Most Geiger counters are calibrated with a radioactive source called Cesium-137 (Cs-137). This is because Cs-137 has a long radioactive half-life (about 30 years). Therefore, the calibration service companies do not have to purchase a new source (and dispose of the depleted old one) very frequently. Cs-137 emits gamma-rays when shielded for instrument calibration purposes. It is one of the sources that dirty bombs are thought to be composed of.

The fact that mrem/hr readings from a Geiger counter could be inaccurate is not as bad as it seems because the inaccuracy is an over-response to the detected radiation. The Geiger counter indicates that more gamma radiation is present than there actually is. This is better than an under-response that could mislead a first responder into thinking that he or she is safer than is really the case.? The over-response is caused by the detection of gamma radiation at lower energies than that emitted from Cs-137. At these lower gamma energies, the Geiger counter response relative to the response that it yields for the same radiation exposure rate from Cs-137 is higher. Most manufacturers of Geiger counters know how large this over response is and can provide such information.? It should be pointed out that the response of the Geiger counter is accurate over a large range of gamma-energies that should make the over-response problem a rare event.

It is not possible to calibrate the Geiger counter for multiple types of radiation sources so that low and high energies are accounted for. It can be calibrated once and then ??correction factors?? can be supplied for other radiation energies (sources). The reading from the Geiger counter would be multiplied by these factors but, this is impractical for the first responder. Under most conditions, a responder would not know what radiation source or sources were involved in a particular incident making the correction factors useless and very difficult to apply in the chaos of a response.?? One must simply know the limitations of the Geiger counter while using it.

Other limitations of the Geiger counter include:

?X?It cannot perform air-sampling measurements alone. A pump to draw contaminated air onto a filter is needed. The Geiger counter, under special conditions, can be used to measure the filter.

?X?It cannot detect weak beta or gamma-emitting isotopes. Some isotopes such as tritium and iodine-125 emit weak radiation that the Geiger counter cannot.

?X?It cannot be used to identify a radiation source i.e., it cannot tell you if the source is Cs-137 or Cobalt-60.

?X?It may fail to respond in extremely high radiation fields unless it is equipped with special ??anti-saturation?? circuitry. Such circuitry can allow it to respond at radiation levels 100x its maximum reading.

Pushing the Limits of Radiation Exposure

Despite the limitations of Geiger counters and pagers, there is no other rapid means of radiological assessment under emergency conditions. Right or wrong, exposure rate measurements will be used to deploy or pull back personnel involved in rescue and assessment operations. What guidelines can be used by first response organizations to determine how to manage the safety of their personnel in a crisis?

The U.S. Environmental Protection Agency has issued Protective Action Guidelines (PAGs) for first responders of a radiological incident (refer to Table 1). A responder would be expected to cease rescue operations after attaining 5000 mrem under most conditions. Life saving operations that a first responder performs would allow a radiation dose up to 25,000 mrem. From this information, we can conclude that radiation doses from 1 to 5000 mrem are considered safe. Higher doses to 25,000 mrem are even possible if the benefit, such as saving a life, is great enough.??

Radiation doses of sufficient magnitude will induce acute radiation sickness (ARS). Such doses are approximately in the range of 100,000 mrem and above.? It is known that on average, a dose of about 400,000 mrem will be lethal about 50% of the time without medical intervention. The higher the total dose, the worse are the effects of ARS. It should be noted that medical intervention for severe acute radiation sickness is basically palliative though efforts to stem the effects of ARS will be made, especially in lower dose cases.? These interventions include bone marrow transplants and the administration of drugs to support the immune response system.

Radiation exposures, even deliberate medical exposures, carry a small risk that could contribute to the development of cancer. This risk has been minimized in industries and practices that use radioactivity and radiation, by maintaining exposures as low as reasonably achievable. The annual occupational dose limit in the United States is 5000 mrem/year. Again, this is for people with potential doses every day of their adult working lives. It was designed for a very different working environment than the one-time only exposure situation a first responder may encounter.

Help Is On The Way

The Geiger counter cannot tell you the difference between Cs-137 or any other radioactive source. Sophisticated, alarm-capable belt-type devices are now available that can detect all four forms of nuclear radiation to some degree and even identify the source of the radiation. These ??spectrometers?? combine other measurement detectors with a Geiger detector. Again, they are not flawless. Their operation is dependent on proper calibration and the skill of the operator who may also have to be familiar with computer operations.

The federal government agencies such as the Department of Energy (DOE), the specialized radiological responders of the military services, and the hazardous materials services of many municipalities carry radiological detection equipment that can identify radiation sources.? Government organizations like the Radiation Emergency Assistance Center/Training Site (REAC/TS) in Oak Ridge, Tennessee provides back-up medical assistance and advice for radiological emergencies. The DOE radiological response assets are organized into the Federal Radiological Monitoring and Assessment Center (FRMAC). This response plan includes aerial and ground-based radiological measurement teams.? Universities and medical centers scattered throughout the nation that maintain nuclear technology or radiological science teaching programs, have laboratories and expertise to analyze various types of samples. Some even maintain devices called ??whole body counters?? that measure certain forms of nuclear radiation emanating from the body (New York University has two of these). And of course, air measurements, which take both time to collect at the scene and then later to analyze are absolute necessities for not only identifying radioactive sources, but for calculating radiation exposure from potentially inhaled material.

All this equipment and scientific expertise is ??after the fact.?? First responders, at least at the beginning of a crisis, are on their own. This is all the more reason that they should learn to use, maintain, and properly interpret the information provided by basic radiation detection equipment such as the Geiger counter.

Table 1

Protective Action Guides (PAGs) for Responders Involved in an Event Involving a

?Radiological Dispersion Device of Improvised Nuclear Device

U.S. Environmental Protection Agency

Dose Limit (mrem)?Emergency Activity?Condition?Remarks

5,000?All ?Reasonable efforts have been made to maintain dose as low as possible?See footnote.

10,000?Protecting important property e.g., powerplants?Unavoidable to maintain dose < 5,000 mrem. Dose monitoring of emergency workers should be implemented?See footnote.

25,000 and greater?Lifesaving operations or protection of large populations?Unavoidable to maintain dose < 5,000 mrem. Dose monitoring of emergency workers should be implemented?For doses that may exceed 25,000 mrem, the responders are recruited on a voluntary basis and made aware of the radiological risks involved.

See footnote.

Dose to eye limited to 3x dose limit of first column. Limit to any other organ is 10x listed value of column 1.

From the Manual of Protective Action Guides and Protective Actions for Nuclear Incidents, Washington DC: USEPA 400-R-92-001, May 1992 and the Federal Register, January 3, 2006 (Volume 71, Number 1).

Also see http://epa.gov/radiation/rert/pags.html

 
 

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