Answer to Question #386 Submitted to "Ask the Experts"

I recently got into a discussion with a friend about alpha, beta, and gamma radiation. He said that you could have alpha and beta present without a detectable level of gamma. I do not believe this to be the case. Who is right? Could you please include a reference? Is there currently a single multifunction detection device capable of showing quantitative levels for all three of these types of radiation? Could you please include the name of the manufacturer?

Thank you for your time and assistance in this matter.

Radiation Emissions—Regarding the radiation emissions, I'm not sure whether you mean (a) alpha without gamma and beta without gamma or (b) alpha and beta without gamma from the same radionuclide.

Alpha without gamma—From inspection of the Chart of the Nuclides and various reports containing diagrams and tables of radionuclide transformations, it is apparent that alpha emitters that do not emit gamma radiation are not very plentiful. Polonium-213, with a half-life of 4.2 microseconds, is transformed to ²⁰⁸Pb by emission of an alpha particle. According to International Commission on Radiological Protection (ICRP) Publication 38, nearly 100 percent of the transformations are to the ground state with emission of a 8.376 MeV alpha particle (no gamma). However, a 7.613 MeV alpha particle is listed as occurring with a yield of 4.0 x 10^-5; presumably this transition is followed by a gamma-ray emission with a yield of no more than 4.0 x 10^-5. In another example, ²³⁸Cm, half-life of 2.4 hours, is listed as transforming with a 10.0 percent yield to the ground state of ²³⁴Pu with the emission of a 6.25 MeV alpha particle and with a 90.0 percent yield to ²³⁸Am by electron capture (an alternative to positron decay). Gamma emissions for ²³⁸Cm have a yield of less than 4.2 x 10^-5. Thus, for all practical purposes the level of gamma radiation for these two alpha-emitting radionuclides is not detectable.

Beta without gamma—There are a number of cases of beta emitters that do not emit gamma radiation—tritium, ¹⁴C, ³²P, and ⁹⁰Sr to name a few familiar ones.

Alpha-beta without gamma—There are a few examples of radionuclides that undergo transformation by either alpha emission or beta emission in the thorium, uranium, and actinium decay series but most also emit gamma radiation. A possible exception is ²¹⁰Bi, half-life of 5.01 days. ICRP-38 lists it as a 100 percent yield beta emitter without gamma. However, the Health Physics and Radiological Health Handbook, pp 8-39, indicates beta emission with a yield of approximately 100 percent and emissions of 4.65 and 4.69 MeV alphas with yields of 7 x 10^-5 and 5 x 10^-5, respectively. No gamma emission is listed, but presumably the 4.65 MeV, 7 x 10^{- 5} percent yield, alpha is followed by some form of deexcitation from the 0.04 MeV state—I leave it to you and your friend to determine whether this qualifies as alpha and beta emission, and, if so, whether it is accompanied by a detectable level of gamma.

According to ICRP-38, ²⁵⁰Cm, half-life of 6,900 years, undergoes alpha emission without gamma to ²⁴⁶Pu with a yield of 25 percent and beta emission without gamma to ²⁵⁰Bk with a yield of 14 percent. However, the balance of the transitions are by spontaneous fission, and both prompt and delayed gammas are associated with the spontaneous fission!

Radiation Detectors—Here I am not sure whether you mean (a) a detection system that indiscriminately responds to all three types of radiation, (b) a detection system that can be switched or configured to select the radiation of choice, or (c) a system that simultaneously makes a quantitative measurement of each of the three types of radiation. Also, I am not sure whether you are interested in a portable, field survey instrument or a laboratory system.

I assume you are familiar with thin-window GM detectors that respond to all three types of radiation. A crude estimate of each can be made by resolving the differences observed with a series of measurements—without an interposed absorber (alpha + beta + gamma), with a thin absorber (beta + gamma), and then with a thicker absorber (gamma only). I also assume you are familiar with alpha-beta counting systems that use either a gas-filled proportional counter or a dual-phosphor scintillation detector and some combination of pulse-height discrimination and pulse shape analysis to simultaneously report the alpha count and the beta count. These detectors also respond to gamma radiation and the gamma signals will not be distinguishable from beta signals. The usual design goal is to optimize the beta signal relative to the gamma signal—such as by making the detector thickness or depth large enough to have a significant probability of a beta particle interaction but small enough to minimize probability of a gamma ray interaction.

Simultaneous, but separate measurement of all three types of radiation—This is a greater challenge. A system to do this can be designed with multiple detectors. A laboratory alpha/beta/gamma counting system could employ the guarded, shielded gas flow proportional detector used in alpha/beta systems and, in addition, a separate NaI(Tl) gamma scintillation detector. One set of electronics with at least two channels would report the alpha and beta activity from the alpha/beta detector; the NaI detector output would be analyzed by a gamma analyzer. For example, several versions of such a system are offered by Gamma Products, Inc. If you are interested in this type of laboratory system, I suggest you contact the various suppliers of radioactivity analysis systems to determine what is currently offered.

For survey instruments, there are a number of dual-phosphor alpha-beta scintillation probes that can be used with instrument packages that have selectable or dual channels for alpha and beta discrimination. These instruments will have more or less response to gamma radiation in the beta channel, depending upon the configuration of the beta phosphor. Some scintillation probes are advertised as being sensitive to alpha, beta, and gamma radiation; I suspect one could separate beta and gamma by making measurements with and without a beta shield. One could put together an instrument system with two scintillation probes—a dual-phosphor alpha-beta probe and a NaI(Tl) gamma probe. While not providing a simultaneous, separate measurement of the three types of radiation, it would provide a package that would permit evaluation of all three radiation types with a pair of measurements. Again, I suggest you contact the various suppliers of survey instruments to determine what is available. Our Health Physics Society affiliates list may help.

Charles E. Roessler, CHP, PhD