Nuclear Medicine Blog

< back to results

Exposure Filters on a Geiger-Müller (GM) Counters for Alpha Particles

January 23, 2013, 8:30am
Ann Marie Alessi, BS, CNMT, NCT, RT(N), FSNMMI-TS, Product Sales Manager, Nuclear Medicine, Biodex Medical Systems, Inc.

The survey meter is an essential resource a technologist has to determine the presence of radiation contamination in typical surveys such as during routine area surveys, checking incoming packages, testing after spills or other radiation incidents and measurement of waste held for decay-in-storage prior to disposal.  A literature review of incident and overexposure reports indicate that a majority of these events occurred because a radiation worker did not have or did not use a survey meter. With the development of new radiotracers for therapy using alpha and beta emitters, it is important that the proper equipment is used to survey for contamination.

As you may know, a GM pancake probe detector has a significant overresponse at lower energies between approximately 20 to 160 keV seen with alpha particle emitters (see red line on graph below).  Any exposure or dose measurements taken with an unfiltered GM pancake detector would thus have unacceptable errors at these lower energies. Using this filter while surveying for contamination from alpha particles, flattens the energy response to within ± 20% referenced to 137Cs (662 keV) over an energy range of 33 keV to 1.2 MeV.

 

Many survey meters are being offered with options for a GM pancake probe detector to be equipped with an energy compensation filter to flatten the energy response and facilitate measuring exposure. This filter simply snaps on across the face of the GM pancake probe and can be used when surveying for contamination from alpha emitters and is easily removed for other measurements.

Survey techniques for Alpha particles

Keep in mind that moisture or dirt may block alpha particles, making them impossible to be measured. All shielding material must be removed from the suspected source. When surveying for alpha, it is very important to get the window of the detector within 1.9 cm (0.75 in.) of the surface to be surveyed. It is recommended that the instrument response (F/S) switch be placed in the F position during surveys.

Move the detector over the area to be measured as slowly as possible (typically 2.5 to 5.1 cm {1 to 2 in.} per second) keeping in mind that the instrument response time is approximately four seconds when the F/S switch is in the F position.

To Filter or not to Filter

Testing by manufacturers of  the alpha emitter radium-223 approved for use in therapy, have shown that surveys for low levels of contamination using typical rate meters connected to GM pancake probes without any filter can be easily detected, even when only photon emissions are measured.

Samples in syringes were prepared to demonstrate the suitability of typical pancake detectors for monitoring of waste bags held for decay in storage. These measurements represent values due mostly to the photon emissions from radium-223 as the emitted alphas and betas are attenuated by the syringes. These measurements resulted in conservative exposure and count rate values and thus serve as worst case values.

To demonstrate the detectability of radium-223 in the event of a contamination incident such as a spillage, samples with low activities were prepared on sponges. The samples prepared on sponges reflect more realistic exposure and count rate values that would be obtained during routine contamination monitoring because these values include contributions of alpha, beta and gamma radiations from the decay of radium-223 and its progeny.

All measurements were obtained at a distance of 2 cm to simulate a distance commonly used in practice. Exposure rates (mR/h) and count rates (cpm) for the various syringe configurations were first obtained. Efficiencies were determined from the count rate data based on the measured activity on the sponges. An energy response curve provided by the manufacturer of the survey instrument indicates approximately a 25% lower response when measuring radium-223 since the average photon energy for radium-223 in equilibrium is estimated at 0.4 MeV.

The survey meter background exposure and count rate readings were on the order of 0.05 mR/h and 10-20 cpm, respectively, indicating that all radium-223 activity levels resulted in values that were distinguishable from background readings.

Based on this data, one drop of radium-223 spilled on a counter can be detected with a typical pancake detector. For quantitative measurements, rather than just detectability, an energy response correction filter would be necessary.

A zinc sulfide (ZnS(AG)) scintillation probe is sensitive to just alpha radiation is often used in field measurements.

 

References:

1. Ludlum website; Accessed January 13, 2014: http://www.ludlums.com/component/virtuemart/equipment-type-3/medical-117/radiation-safety-183/response-kits-201/model-44-9-with-exposure-filter-461-detail?Itemid=0

2. Radium-223 Dichloride: Bayer Responses to NRC Questions November 8, 2012; Deepika Jalota, Pharm.D., Global Regulatory Strategist, Global Regulatory Affairs, Specialty Medicine, Bayer HealthCare Pharmaceuticals, Inc. NRC website; Accessed January 13, 2014: http://pbadupws.nrc.gov/docs/ML1232/ML12320A450.pdf

Category:
 
View Full Site

This site uses cookies. By continuing to browse the site, you are agreeing to our use of cookies.

Read More