Answer to Question #9287 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
When using a standard "pancake" probe to survey for contamination, many nuclear medicine technologists leave the plastic cover (used by the vendor when shipping to prevent probe damage) on the probe. The reasoning is that 99mTc is a metastable gamma emitter and they are trying to prevent contaminating the probe.
The problem with this logic is found by taking actual measurements. I measured a "drop" of 99mTc on a counter top with the 2-micron-thick pancake probe with the plastic cover off and on. Here are the results
Survey distance—2.54 cm
Virtual point source—a drop
Plastic cover on—16.7 cps
Plastic cover removed—133 cps The difference is an 8:1 ratio.
The question that I have is, if 99mTc is ONLY a metastable gamma emitter, why does the emission "spectra" seem to be like a beta-gamma emitter. Is there a beta component to the emission spectra that could account for this difference?
Your question is a good one, and your observations may appear at first somewhat confusing because, as you have noted, 99mTc, which “decays" through what is referred to as an isomeric transition, is characterized by the major radiation associated with the metastable nuclide deexcitation—namely, the 140 keV gamma ray that you and others in the nuclear medicine field are well familiar with and use for diagnostic studies on a daily basis. What we don’t hear much about are the other radiations that accompany the deexcitation of the metastable 99mTc to its ground state configuration, 99Tc.
These radiations include a number of conversion electrons and some Auger electrons. The conversion electrons originate as an alternative to gamma-ray emission as a means for the excited nucleus to release its excess energy. This is done through a transfer of the excess nuclear energy to an atomic electron that may then get ejected with kinetic energy equal to the difference between the nuclear excess energy and the binding energy of the electron in the atom. Auger electrons are emitted as an alternative to characteristic x-ray emission; this occurs when an excited atom, produced after an electron vacancy has been created in the atom, as might be the case when a conversion electron is ejected from the atom, transfers the excitation energy directly to an electron that gets ejected from the atom. The Auger electrons and many of the conversion electrons produced by deexcitation of 99mTc are too low in energy to penetrate any significant thickness of air or the window of the pancake probe that you mention (I believe the window thickness is somewhat greater than the 1 micron you mention, probably closer to 7 microns if the window is made of mica.)
In about 11 percent of the transitions of 99mTc to its ground state, however, the conversion electrons have energies between about 120 keV and 140 keV. These energies are sufficiently high that when the detector is close to the source the electrons may be detected with quite high efficiency, much higher than the detection efficiency for the 140 keV gamma rays. As a result, even though the energetic conversion electrons have a combined yield that is only about 12 percent of the 140 keV gamma ray yield, they may heavily weight the reading on the bare window Geiger Mueller (GM) probe that you are using. This would explain your observation that the count rate was eight times greater with the bare detector than with the capped detector. The plastic cap is sufficiently thick to prevent the conversion electrons from reaching the detector.
As to which configuration, cap-on or cap-off, is appropriate when making measurements on a day-to-day basis, this depends on how the detector was calibrated and how it is being used. In many instances, the pancake probes are calibrated in exposure or kerma rate units—e.g., nGy s-1. This is often done by an external licensed contractor, frequently using a 137Cs gamma-ray source. The detector is often used by nuclear medicine staff for a variety of measurements. These include external exposure (or kerma) rate measurements to demonstrate that ambient rates in various work areas are consistent with recommended guidelines, and use of the cap to cover the detector is appropriate. In addition, the same probe may be used to do routine area surveys that include possible detection of contamination on various surfaces. It is common again to use the probe with the cap in place. It is true that detection of contamination from 99mTc could be enhanced by removing the probe cap and holding the detector close to potentially contaminated surfaces. In common usage, however, the probe is used to evaluate ambient radiation levels and, in most all cases of contamination involving 99mTc at medical facilities gamma sensitivity is sufficient to detect contamination at levels of significant concern, and the advantages of using the probe cap generally outweigh the advantage of possibly increased sensitivity offered by the bare detector.
If it were desired to use the probe to evaluate the level of surface contamination by99mTc, the use of the bare detector might be justified. In such a case, however, the detector would have to have been calibrated specifically for assessment of surface contamination by 99mTc. This requires the use of different types of calibration sources than are used when calibrations are done in terms of exposure or kerma rates. Most nuclear medicine programs are required to implement routine contamination survey procedures in which they take wipes, usually at least weekly, of various surfaces that might get contaminated by use of the 99mTc. The wipes are then counted, often in a NaI(Tl) well counter, which offers high efficiency for the 140 keV gamma rays from the 99mTc.
Thus, the use of the GM pancake probe with the cap on generally satisfies the needs of the nuclear medicine program for assessing external exposure rates and for alerting staff of any accidental spills. The wipe survey program serves to detect the presence of low-level contamination that might result from ongoing procedures and might not be high enough to yield noticeable readings above background when routine external radiation measurements are made with the GM survey instrument.
Thanks for the question. Good luck in your continuing work in nuclear medicine.
George Chabot, PhD, CHP