Answer to Question #10278 Submitted to "Ask the Experts"

I have a Beckman Model LS 6500 liquid scintillation counter (LSC). I need to know how I should set the alpha multichannel analyzer (MCA) and beta MCA, after counting ⁹⁰Sr and ²⁴¹Am to calibrate the instrument, so that I can use the instrument later for gross alpha/beta measurements. Also, is this the same procedure for ²²⁶Ra and ²²⁸Ac?

I have not used the Beckman-Coulter LS 6500 LSC, but I am familiar with some other systems that operate in similar, but not exactly, the same fashion. I looked for an available operations manual for the LS 6500 on the Internet, but could find no free version available. I expect, however, that similar approaches to setting up the LSC instruments from different manufacturers for alpha/beta measurements would apply.

As you likely know, the efficiency of scintillation light emission per unit energy deposited in the liquid scintillation cocktail is considerably higher for beta particles than it is for alpha particles, often on the order of a factor of 10. The unfortunate result is that alpha and beta pulses, considered in terms of their respective pulse heights, will often overlap, even though the energy of every alpha particle may significantly exceed that of every beta particle. Typically, the goal is to keep the overlap, or spillover as it is often called, to an acceptably low value while maintaining a high counting efficiency for both alpha and beta radiation.

To improve the resolution between the two types of pulses, instrumentation has been developed that discriminates between the two pulse types on the basis of pulse shape rather than pulse height. This is successful because the alpha-induced scintillation pulses have considerably longer decay times than do the beta-induced pulses. This allows pulse shape, rather than pulse height, logic to be used to discriminate between the alpha and beta decay events. While this greatly improves the discrimination process, from a practical standpoint there is still some spillover between the two radiations. In many modern systems, if one attempted to reduce the beta contribution to the alpha counting region to zero, one might have to sacrifice roughly 50 percent of the alpha counts.

Modern systems have MCAs as part of their analytical complement that are used in both pulse-height and pulse-shape analysis. When setting up the system to do simultaneous alpha/beta analysis, one must typically decide upon what range of pulses (energies) will be stored where and what discriminator level is best suited to the current task. The use of standards, such as the ²⁴¹Am and ⁹⁰Sr-Y that you mention, is a common requirement. Generally, the range of energies used to include the respective radiation types will extend farther for the beta radiation than for the alpha radiation. For example, with one popular LSC, the Packard TriCarb Model 3170, I. Lopes and colleagues, in an internet-available paper titled "Gross Alpha, Gross Beta and Tritium Activities in Portuguese Drinking Waters," describe its use for doing alpha/beta measurements of environmental radioactivity in water samples. The energy window was set at 0 to 1,000 keV for alpha radiation and 0 to 2,000 keV for beta radiation.  When setting up the instrument, one would have to count standard alpha and beta sources, such as the²⁴¹Am and ⁹⁰Sr-Y that you mention, to evaluate how results changed as a consequence of changing the PDD (pulse decay discriminator) level. The standards should be prepared so as to simulate the actual expected sample preparations. If one is intending to make measurements of activity in aqueous samples, for example, this usually means adding a minimum volume of the standard (perhaps 10–20 microliters) to a volume of distilled water equal to what one expects to use for actual samples and then adding this to the volume and type of scintillation fluid that will be used for samples. In actual samples, some quenching will likely occur and the use of internal standards may be helpful to evaluate the alpha and beta counting efficiencies under the quench conditions.

With these standards used as internal standards in actual environmental samples in the above-mentioned Packard instrument, for the energy windows specified, I. Lopes and colleagues (cited above) used a discriminator setting of between about 125 and 130 to obtain favorable results, with less than 0.5% spillover from either the beta-to-alpha region or the alpha-to-beta region, while preserving counting efficiencies of about 95% for alpha radiation and 92% for beta.

The major difference you will encounter when counting ²²⁶Ra is that, if the radium samples are sealed and aged, as I assume they would be, to allow equilibrium ingrowth of the radium progeny, there will be three additional alpha emissions (from ²²²Rn,²¹⁸Po, and ²¹⁴Po) for every ²²⁶Ra decay; additionally, there will be two beta decays from (²¹⁴Pb and ²¹⁴Bi). The consequence is that the alpha counting efficiency will be more than four times greater than what was observed for the ²⁴¹Am, and this must be accounted for in the quantification of ²²⁶Ra in samples that are counted. Using standard sources of ²²⁶Ra would allow you to make a more exact determination of the proper counting efficiency. You also mention ²²⁸Ac as a beta emitter of interest. I'm not sure why you cite this radionuclide in conjunction with ²²⁶Ra, as it occurs as the daughter of ²²⁸Ra in a different decay series but, at any rate, it emits a number of different energy beta particles with the dominant beta particles having maximum energies between about 1.2 MeV and 2.1 MeV and an average energy of 0.375 MeV. I would expect the ⁹⁰Sr-Y standard, which has a somewhat higher overall average beta energy of 0.565 MeV, to provide an acceptable simulation, perhaps yielding a somewhat optimistic estimate of efficiency. The effect of a somewhat lower energy could be assessed using an alternative beta emitter such as ³⁶Cl, which has an average beta energy of 0.251 MeV or ²⁰⁴Tl with an average beta energy of 0.244 MeV.

Recommendations may vary among manufacturers for different instruments, and the above should serve only as possible guidance. If you have the manufacturer's operating manual for your instrument, it should have more specific recommendations for setting up and using the instrument for alpha/beta measurements. If you still have problems, you should try contacting the manufacturer, Beckman Coulter; the company operate a support group through its website.

George Chabot, PhD, CHP