Maintaining Low Detector Backgrounds in Alpha Spectrometry

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Introduction

Alpha spectrometry is an extremely useful and sensitive method for detection of alpha-emitting nuclides in a variety of materials. One of the main reasons the technique is so sensitive is because of its very low background. In order to illustrate the importance of maintaining as low a background as possible, one may consider the following equation modified from Currie (1968) for the case of determining the lower limit of detection (LLD) at the 95% confidence level for an alpha spectrometric analysis:

The factor of 4.66 is derived for a 5% probability of Type I (false positive) and Type II (false negative) errors for paired blank and sample measurements. The constant 2.71 represents the counts for a zero blank case and corresponds to a 5% probability of a false negative. The factor of 2.22 is a conversion from dpm to pCi units.

Examination of these relationships shows that once the sample size, chemical recovery, and counting geometry are optimized for a particular set of analyses, the best way to achieve an improved LLD within a fixed counting interval is to reduce detector background. When new, a typical 450 mm² ion-implanted silicon detector such as a Canberra Series A PIPS (Planar Implanted Passivated Silicon) would have a background count rate on the order of 0.004 cpm for the 3-8 MeV region or less than 0.001 cpm under individual regions of interest, usually about 300 keV wide. Unfortunately, this situation soon changes because of detector contamination dominated by two processes: (i) alpha recoil; and (ii) "volatilization" of polonium. Alpha recoil contamination occurs when an alpha-emitting nuclide on the source plate decays to an alpha-emitting daughter or string of daughters. Since the specific activity is inversely proportional to the half-life for a fixed number of atoms, recoil will produce the most background activity when relatively short-lived daughters are produced. However, if the half-lives in question are very short (say up to a few hours), they will decay away quickly enough to be of little concern in alpha spectrometry. Particularly serious are those cases that involve transfer of recoil daughters with half-lives from days to weeks, short enough that a reasonable amount of parent activity will produce a significant amount of recoil contamination, and long enough that decay back to normal background levels will require an inappropriately long time. In addition, the effect is chronic; i.e., similar recoil-producing samples counted in the same chamber will produce a long-term build-up of detector background which could eventually become serious.

Some common examples of decay-chains that produce recoil contamination include ²²⁸Th (produces a-emitting daughters ²²⁴Ra-²²⁰Rn-²¹⁶Po- ²¹²Po-²¹²Bi), ²²⁹Th (²²⁵Ac-²²¹Fr-²¹⁷At- ²¹³Po), and ²²⁶Ra (²²²Rn-²¹⁸Po-²¹⁴Po). It is important to realize that even b-emitting nuclides ejected by alpha recoil can contribute to alpha background if they subsequently decay to alpha emitters. For example, the direct daughter of ²²⁹Th is ²²⁵Ra which decays by b-emission to the a-producing daughter ²²⁵Ac.

Contamination of detectors by polonium isotopes, such as ²¹⁰Po (t_1/2 = 138.4 days), must occur by some other process than alpha recoil. Note that ²¹⁰Po, the last radioactive member of the ²³⁸U decay series, is the daughter of ²¹⁰Bi, a beta-emitter. The transfer of polonium from a source to a silicon detector has been attributed to "aggregate" recoil and inherent volatilization of polonium at low pressure. Whatever the actual cause, it is clear that polonium activity is indeed transferred to detectors, a very serious problem with long-lived ²¹⁰Po and even worse when working with ²⁰⁹Po (t_1/2 = 102 years) as a yield tracer.

What can be done about this problem? Unfortunately, most recoil contamination cannot be cleaned off a detector because its imbedded into the silicon itself. Although PIPS detectors are cleanable, using high quality acetone and a cotton swab, the cleaning will only remove surface contamination. This type of cleaning may be useful for removing a significant amount of the polonium contamination, however. The best solution to the build-up of alpha recoil on silicon detectors is to prevent it from happening in the first place.

This technical brief will discuss the contamination of silicon detectors by first presenting the physical basis for alpha recoil and then showing the experimental conditions an analyst can use to substantially reduce the effects of recoil and polonium volatilization on detector background. Keeping the background as low as possible by these techniques will ensure that the best possible detection limits can be achieved.

Principles of Alpha Recoil

Alpha decay is observed for elements heavier than lead and for a few nuclei as light as the lanthanide elements. The decay of atom X with Z protons and a mass number A may be represented as follows:

The decay energy can be calculated from the known atomic masses, because the binding energy released corresponds to the disappearance of masses between the products and the reactants (Choppin and Rydberg, 1980). This decay energy, or Q-value, may thus be calculated by the equation:

where,

The factor 931.48 converts from conventional atomic mass units (amu) to units of energy in millions of electron volts (MeV). Thus, when using this formulation, the mass difference must be evaluated in these units. These masses may be obtained from tables in several sources such as the "Table of the Isotopes" (Weast, 1984). As an example, the energy released during the decay of ²²⁶Ra to ²²²Rn and an alpha particle may be calculated as follows:

Note that the mass given for the alpha particle is for the entire helium atom. Although an alpha particle is just the nucleus of the helium atom, the associated electrons still need to be accounted for. Since alpha decay usually results in products formed in their ground states, the total decay energy (Q) is partitioned into just two parts: (i) the kinetic energy of the daughter nucleus (E_D); and (ii) the energy (E_a) imparted to the alpha particle. Knowing that energy and momentum must be conserved, it is possible to calculate the amount of energy imparted to each by simply using mass ratios as follows:

For the example of the ²²⁶Ra decay, the kinetic energy of the alpha particle is 4.78 MeV while the ²²²Rn daughter nucleus carries away only 0.086 MeV. This is a typical result, with the relatively small alpha particle carrying away several MeV while the much more massive recoil nucleus is imparted with £ 100 keV. An important point here is that while the kinetic energy of the recoil daughter is small in relation to the energy of the emitted a-particle, it is very large in comparison to chemical binding energies which are generally < 5 eV. Thus, the recoil daughter easily breaks all chemical bonds by which it is held to adjacent atoms. This is the process by which radioactive daughters may be ejected from sources prepared for alpha spectrometry and contaminate silicon detectors.

Prevention of Contamination

Because of the large difference in size between the alpha particle and recoil nucleus, it should be possible to place an absorber between the source and detector which is thin enough to allow the high-energy alphas through while stopping the more massive daughter nuclides. A report by Chetham-Strode et al. (1961) showed that this general approach will work although not many details were provided on the experimental procedure. In what has now become somewhat of a classic paper, Sill and Olsen (1970) provided the necessary detail to show conclusively that this approach indeed inhibits recoil contamination. The authors achieved an approximate 1000-fold reduction in recoil contamination by leaving enough air in the chamber to produce 12-16 µg/cm² of air between the source and detector and by applying a negative potential of 6 volts to the source plate. Air is the preferred material for an absorber because commercial films are either too thick and degrade the alpha spectra or too flimsy to handle conveniently.

To determine the effect of pressure on recoil contamination, Sill and Olsen used an electrodeposited ²²⁵Ac source to provide recoil of its 4.8-minute a-emitting ²²¹Fr daughter. The recoiling atoms were collected on 2-inch polished stainless steel plates separated from the source by a 1-mm plastic spacer ring and a removable barrier so that recoiling atoms could not strike the collector plate until the collecting period began. Both the source and collector plates were placed inside a pressure-controlled test chamber that was evacuated to the desired pressure, the barrier removed, and the recoiling ²²¹Fr atoms collected for a fixed time. The collector plate was then quickly transferred to an alpha spectrometer for assessment of recoil.

Figure 1.
Decrease in recoil contamination of ²²¹Fr from a ²²⁵Ac source as a function of air thickness. Results replotted from data presented in Sill and Olsen (1970).

One of the diagrams from Sill and Olsen's paper dramatically illustrates the effect of air as an absorber for recoil daughters (Figure 1). When the air thickness is increased to 12 µg/cm², the recoil activity falls by a factor of at least 100 and becomes statistically indistinguishable from the detector background. Because the background was rather high in this experiment due to contamination from previous runs, the authors felt that the actual reduction in recoil contamination was closer to a factor of 1000. When this procedure was attempted in an alpha spectrometer, however, the technique failed with almost as many recoil atoms contaminating the detector as when under full vacuum. The critical difference between the test chamber and the alpha spectrometer was apparently the presence of the charged surface on the detector. It seems that although the recoil daughters are attenuated by the air, they continue to drift around inside the chamber and, because they have a net positive charge, they are attracted to the worst possible place, the detector surface. After Sill and Olsen realized this, they applied a negative potential (as little as 2 volts is apparently sufficient) to the source plate by means of a small battery to attract the daughter nuclei back to where they originated. This procedure brought the spectrometer results in line with those observed in the test chamber. Thus, it is essential to employ this combined approach to prevent recoil buildup on detectors: (i) leave the correct air thickness (~16 µg/cm²) in the chamber; and (ii) provide a small negative bias (2-6 volts) to the sample plate relative to the detector.

In a parallel set of experiments, the same authors also showed that there is virtually no loss of counting efficiency and little significant effect on resolution until the air thickness is above about 30 µg/cm². In fact, at a thickness of 12 µg/cm², a loss in resolution of only 1-2 keV FWHM was observed. Thus, with the proper amount of air left in the chamber, one can significantly reduce recoil while losing very little in terms of performance.

In addition to the experiment reported above and in Figure 1, which was performed with a distance of 1.28 cm between the collector plate and the ²²⁵Ac source, Sill and Olsen also checked other source-to-detector distances to ensure that the general approach was still valid. They adjusted the pressure at each distance until the recoil was statistically zero for the counting time used. They found that the range of recoiling atoms was between 12 and 16 µg/cm² for all distances checked. Thus, to be certain that sufficient air is left in the chamber, one should adjust the air pressure remaining in the alpha spectrometer to give a thickness of close to the upper value, 16 µg/cm².

Since it is more convenient for the analyst to deal with air pressure, it is necessary to describe the relationship between the desired quantity, air thickness, and pressure. Since, thickness (µg/cm²) = density (µg/cm³) x distance (cm), one may calculate the necessary pressure required to achieve a desired air thickness by first establishing the relationship between density and pressure. At 20 ^oC, the following equation closely approximates this relationship for dry air (Weast, 1984):

Thus, one may calculate the required pressure to obtain a desired thickness by substituting the required density into the equation and solving for pressure. As an example, I have calculated the relationship between pressure and detector-to-source distance for both an air thickness of 12 and 16 µg/cm² (Figure 2). The experimental data of Sill and Olsen showing the required pressure to stop recoil at various distances, shows excellent agreement to these curves.

A graphic example of the type of background reduction possible using these techniques was described in Sill (1987). An active ²²⁶Ra standard, precipitated with a 100 µg of BaSO₄, was counted at 20% counting efficiency with and without the recommended recoil protection. Under full vacuum with an initially clean detector, a total of 700000 counts was collected in the ²²⁶Ra region over a 2-day period. A 1000-minute background count immediately afterwards showed 2692 counts in the ²²²Rn region. When the experiment was repeated with the recoil prevention system as described above, 500000 total ²²⁶Ra counts in a 2-day period only resulted in 2 counts in the ²²²Rn region in the follow-up background measurement. This represents a very impressive reduction of recoil contamination by a factor of approximately 1600!

Figure 2.
Relationship between air pressure (mm Hg) and distance (cm) to produce an air thickness of 12 and 16 µg/cm². Curves plotted for the case of dry air at 20^oC by equations shown in the text. Also shown are the experimentally-determined pressures required to stop recoiling ²²¹Fr atoms at various source-to-detector distances (Sill and Olsen, 1970).

To recap, the operational procedure for recoil prevention by the analyst is straightforward. First, a "sample bias" supply of about 2-6 volts is arranged with the negative pole to the source plate. Then, after determining what detector-to-source distance will be used in the alpha spectrometers, the required pressure to achieve 16 µg/cm² air thickness is either calculated from the equations presented above or read off the plot in Figure 2. Finally, some method must be devised to reliably control the air pressure in the vacuum chamber.

Until recently, it was necessary for the analyst to make all the necessary components to achieve these objectives. In order to maintain a certain air pressure in the vacuum chamber of an alpha spectrometer, one could either pump down to the desired pressure and run the chamber static or install a pressure controller of some type between the vacuum pump and chamber. If the vacuum seals are in good shape, some chambers can be reliably run static for 48 hours or more without a significant increase in pressure at these levels, but it is risky to assume the pressure will remain unchanged. A more reliable method is to use a "Cartesian Diver" (Figure 3) which is commercially available but are difficult to install and tricky to operate. Since they contain mercury, only well-trained experimentalists should use these devices. Fortunately, it is now considerably easier to arrange for all the requirements for recoil prevention by purchase of off-the-shelf items. For example, the Canberra Model 7401VR Alpha Spectrometer comes equipped with a pressure gauge which reads from 0-20 µm Hg (rather than 0-1000 µm Hg on the standard Model 7401) and also includes the 7401-RSB, Reverse Sample Bias option which is provided by four 3-volt batteries mounted on the source shelf. Similar capability is available on the Canberra Model 7404VR Quad Alpha Spectrometer. To complete the recoil prevention system this spectrometer can be used in conjunction with Canberra's Model 7400-14A Valved Manifold with Pressure Control which can maintain the air pressure in 1-6 spectrometers by use of a controlled leak adjusted with a front panel control. If direct measurement of pressure is desired, this manifold may be used together with the Model 7400-16 Pressure Gauge Panel which contains a thermocouple pressure gauge with a range from 0-20 µm Hg. Use of these options only increases the net price by a few percent relative to a system price without the recoil prevention option yet will enable the analyst to maintain low backgrounds over an extended period of time.

Figure 3.
Schematic of a Cartesian Diver used to maintain a desired air pressure in an alpha spectrometer.

With respect to contamination by polonium, Sill and Olsen (1970) present evidence that the contamination is apparently related to the nature of polonium on the source and that the "pseudo-recoil" effect can be substantially reduced by oxidizing the polonium on the source plate. This can be done very conveniently by heating the source plate on a uncovered hot plate for five minutes before counting. Prevention of polonium contamination is very important because of the additive nature of the contamination due to its long half-life. Some additional information on polonium contamination is contained in Sill (1987).

Summary

Alpha spectrometry is a very sensitive technique for measuring low levels of a-emitting radionuclides. To ensure that the lowest possible backgrounds are maintained, it is recommended that steps be taken to prevent the two major causes of background buildup, recoil and polonium volatilization. Recoil contamination may be prevented by leaving an appropriate amount of air in the chamber and application of a small sample bias. Canberra Industries, Inc. now supplies the necessary hardware to make this a relatively simple matter. Polonium volatilization may be substantially reduced by simply heating the source plate on a hot plate to oxidize the polonium to a non-volatile state.

Figure 4.
A Canberra Model 7400-14A Valved Manifold with Pressure Control can easily control the air pressure for up to six alpha spectrometers.

Acknowledgements

The author wishes to thank Claude Sill (EG&G Idaho, Inc.) and J.K. Osmond (Florida State University) who reviewed an earlier version of this paper and offered very helpful remarks.

References

Burnett, W. C., 1992. Advanced Alpha Spectrometry- A Short Course Emphasizing Advanced Techniques in Alpha Spectrometry. Canberra Industries, Inc. Training Manual SP-506-3, Meriden, CT, 124 p.

Chetham-Strode, A., Tarrant, J.R., and Silva, R.J., 1961. The application of silicon detectors to alpha particle spectroscopy. IRE Trans. Nuclear Sci., NS-8:59_63.

Choppin, G.R. and Rydberg, J., 1980. Nuclear Chemistry, Theory and Applications. Pergamon Press, New York, 667 p.

Sill, C.W., 1987. Determination of radium-226 in ores, nuclear wastes and environmental samples by high-resolution alpha spectrometry. Nuclear and Chemical Waste Management, 7:239_254.

Sill, C.W. and Olsen, D.G., 1970. Sources and prevention of recoil contamination of solid-state detectors. Analytical Chemistry, 42:1596_1607.

Weast, R. C., 1984. CRC Handbook of Chemistry and Physics. 64th edition. CRC Press, Boca Raton, Florida. Page 6

Canberra wishes to thank:

Dr. Bill Burnett
Environmental Radioactivity
Measurement Facility,
Department of Oceanography,
Florida State University,
Tallahassee, Florida 32306-3048

For his assistance in the preparation of this technical brief.

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