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====== 11. Alpha detectors and spectrometry ======

Chapter 11 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

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In an ordinary radiochemical laboratory the alpha-emitting radionuclides studied are those listed in [[textbook:nrctextbook:chapter11#table_111|Table XI.I]]. Of these Po, Ra, [[textbook:nrctextbook:chapter4#thorium|Th]] and [[textbook:nrctextbook:chapter4#uranium|U]] [[textbook:nrctextbook:chapter2#isotope|isotopes]] are naturally occurring radionuclides while [[textbook:nrctextbook:chapter1#plutonium|Pu]] and Am [[textbook:nrctextbook:chapter2#isotope|isotopes]] are [[textbook:nrctextbook:chapter4#artificial_radionuclides|artificial transuranium nuclides]]. The natural alpha-emitting radionuclides belong to the [[textbook:nrctextbook:chapter4#decay_chains|decay series]] beginning from <sup>238</sup>U, <sup>235</sup>U and <sup>232</sup>Th. The sources of the transuranium elements are the the nuclear weapons tests in the 1950' and 1960's and of the Chernobyl accident in 1986 as well as the nuclear waste, especially the spent nuclear fuel. Some of these [[textbook:nrctextbook:chapter4|radionuclides]], such as <sup>235</sup>U, <sup>226</sup>Ra and <sup>241</sup>Am emit [[textbook:nrctextbook:chapter5#gamma|gamma radiation]], which can in some cases be used for their [[textbook:nrctextbook:chapter8|measurement]]. The intensities and/or gamma ray energies are, however, typically so low that the [[textbook:nrctextbook:chapter9|gamma spectrometric measurement]] does not yield accurate results. Moreover, gamma spectrometry does not allow determination of [[textbook:nrctextbook:chapter2#isotope|isotopic composition]]of uranium, which is important information in many studies. Accurate measurements, enabling also determination of isotopic compositions, are obtained either by [[textbook:nrctextbook:chapter11#semiconductor_detectors_alpha|alpha spectrometry]] or by [[textbook:nrctextbook:chapter8#mass_spectrometry|mass spectrometry]]. The former is discussed here in this chapter.
###
{{anchor:table_111}}
{{anchor:alpha_emitters}}

Table XI.I. Most typical alpha-emitting radionuclides studied in radiochemical laboratories.
^Nuclide^Half-life (y)^Alpha energies (MeV) - Intensities (%) in parenthesis^
|<sup>210</sup>Po |0.38 |5.310 (100)|
|<sup>226</sup>Ra |1600 |4.784 (94.4), 4.601 (5.6)|
|<sup>228</sup>Th |1.91 |5.520 (71.1), 5.436 (28.2)|
|<sup>230</sup>Th |75400 |4.770 (76.3), 4.702 (23.4)|
|<sup>232</sup>Th |1.4×10<sup>10</sup> |4.083 (77.9), 4.019 (22.1)|
|<sup>234</sup>U |245000 |4.859 (71.4), 4.796 (28.4)|
|<sup>235</sup>U |7.0×10<sup>8</sup> |4.474 (57.2), 4.441 (18.8), 4.288 (6.0), 4.676 (4.7), 4.635 (3.9) etc.| 
|<sup>238</sup>U |4.5×10<sup>9</sup> |4.270 (79.0), 4.221 (20.9)|
|<sup>238</sup>Pu |88 |5.499 (70.9), 5.456 (29.0)|
|<sup>239</sup>Pu |24100 |5.157 (70.8), 5.144 (15.1), 5.105 (11.5)|
|<sup>240</sup>Pu |6560 |5.168 (72.8), 5.124 (27.1)|
|<sup>241</sup>Am |433 |5.486 (84), 5.443 (13)|

{{anchor:semiconductor_detectors_alpha}}
{{anchor:silicon_detectors}}
===== 11.1.Semiconductor detectors for alpha spectroscopy =====

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Semiconductor detectors were discussed already in chapter IX where [[textbook:nrctextbook:chapter9|gamma spectrometry]] was described. In gamma spectrometry the detector material is [[textbook:nrctextbook:chapter9#germanium_detectors|germanium]] whereas in alpha spectrometry the material is silicon. The principal idea in both is the same. They are both diodes composing of an n-type Si/Ge, having an electron donor additive, such as phosphorus, P(V), and a p-type Si/Ge, having an electron acceptor additive, such as boron, B(III). When these are attached to each other a depletion zone is developed around the interface due to combination of electrons and holes on the interface. When a reverse bias voltage is applied across the crystal this depletion zone widens. These are transferred close to electrodes due to the voltage applied. The thickness of the zone is dependent on the voltage applied being typically only 40-60 V in silicon alpha detectors. For the detection of gamma rays the depletion zone needs to be thick, several centimeters, in order to absorb the readily penetrating gamma rays. This is accomplished by using a larger crystal made of very pure germanium and by using a high voltage up to 5000 V. In the case of alpha detection with Si-detectors the depletion zone should be very thin due to the short range on alpha particles in 
silicon, only 30 μm. Typically the depletion zone in silicon detectors used in alpha spectrometry is 100-200 μm.  There are two types of silicon detectors in production (Figure XI.1): surface barrier detectors (SBB) and passivated ion-implanted detectors (PIPS) the latter being a more modern construction mode.
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{{:textbook:nrctextbook:production_of_silicon_detectors_for_alpha_spectr_fig_10_1.png?200 |}}
Figure XI.1. Production of silicon detectors for alpha spectrometry. Left: surface barrier detectors. Right: Passivated ion-implanted detector. (http://www.ortec-online.com/Products-Solutions/RadiationDetectors/silicon-charged-particle-detectors.aspx).

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To produce a detector, the edges of a silicon wafer, with thickness less than 500 µm, are first insulated from each other to prevent continuous current across the wafer. In SBB detectors the insulation is done with epoxy resin and a ring mounted around the wafer. In PIPS detectors the surface of the wafer is first passivated by heating which results in the formation of about 50 nm thick non-conducting SiO<sub>2</sub> layer. This layer is removed from the middle of both sides of the wafer. To transform the silicon wafer into a diode the other side is treated with acceptor atoms producing 
p-type layer and the other side with donor atoms to produce n-type layer. In the SBB detectors this is accomplished by forming a thin, 100-200 nm, layer of Au on the other side (n-type) and a layer of Al on the other (p-type). In PIPS detectors this is done by ion-implantation technique by bombarding high energy atoms on the sides. The PIPS detectors have several advantages over SSB detectors:
###

  *  The surface layer is mechanically and chemically more resistant and can be cleaned with alcohol, for example. In SBB detectors the gold surface is very sensitive and cannot be touched at all.
  * The “window”, the passive layer on the surface is somewhat thinner resulting in a better energy resolution.

###
The detectors are rather small in size (Figure XI.2.) Their diameters are only 2 to 4 centimeters and thickness less than 500 µm (about 1 cm including the metallic cover). Table XI.II. shows properties of alpha detectors available from Canberra. As seen from the table the [[textbook:nrctextbook:chapter8#energy_resolution|resolution]] is very good, 20-40 keV. Resolution is here determined for <sup>241</sup>Am 5.486 MeV [[textbook:nrctextbook:chapter5#alpha_particle|alpha particles]]. Thus the relative resolution is 4-8%. The resolution is better for the smallest detectors, being about two-times better for the smallest detector in Table XI.II compared to the largest. The background in alpha detectors is very low, only 4-16 counts per day being directly proportional to the surface area of the detector. The background is almost solely caused by [[textbook:nrctextbook:chapter4#cosmic_radiation|cosmic radiation]]. It is evident that the [[textbook:nrctextbook:chapter8#counting_efficiency|counting efficiency]] is better for the larger crystals. Thus one needs to make a compromise with respect to [[textbook:nrctextbook:chapter8#energy_resolution|resolution]] on the one hand and to counting efficiency on the other when selecting a detector. The 
selection depends naturally on what is needed, high resolution or high efficiency. When measuring [[textbook:nrctextbook:chapter5#5.2._alpha_decay|alpha]] [[textbook:nrctextbook:chapter6#activity|activities]] in environmental and biological samples the activity levels are typically very low requiring very long counting times. In this case a larger detector would be desirable. On the other hand, the background pulses increase with detector size and the overall performance is also dependent on the sample size in comparison with the detector size. In some cases highest possible resolution is a priority. For example, typically <sup>239</sup>Pu and <sup>240</sup>Pu activities cannot be measured individually from an [[textbook:nrctextbook:chapter11#alpha_spectrum|alpha spectrum]] due to overlap of their alpha peaks. Using a high-resolution alpha detector one may distinguish these two [[textbook:nrctextbook:chapter2#nuclide|nuclides]] by deconvolution technique separating overlapping peaks with a mathematical fitting process.
###

{{:textbook:nrctextbook:alpha_detector_canberra_fig_11_2.png?200 |}}
Figure XI.2. Canberra alpha detectors (http://www.canberra.com/products/detectors/pdf/ 
passivated_pips_C39313a.pdf).\\ 
\\ 

Table XI.II. Properties of Canberra silicon detectors for alpha spectrometry (http://www.canberra.com/products/detectors/pdf/passivated_pips_C39313a.pdf).

^Active area (mm<sup>2</sup>)^ Diameter (mm)^ Alpha resolution (keV)^ Typical background (counts per day) ^
|300|20|17|4|
|450|24|18|18|
|600|28|22|22|
|900|34|25|25|
|1200|39|32|32|

{{anchor:alpha_spectrometry}}
===== 11.2. Alpha spectrometry =====

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Figure XI.3 shows the components of an alpha spectrometer. The planar sample is placed in front of the detector and close to it. Both detector and sample are placed in a vacuum chamber to prevent [[textbook:nrctextbook:chapter7#absorption_of_alpha_radiation|absorption of alpha particles]] in air. The voltage (40-60 V) across the detector is supplied by the bias supply. Pulses created in the detector are amplified first in a preamplifier and then in a linear amplifier. The pulses are transformed into digital form in analog-to-digital-converter (ADC) and directed into [[textbook:nrctextbook:chapter8#multichannel_analyzer|multichannel analyzer]] (MCA) for counting pulses and determining their heights.
###

{{:textbook:nrctextbook:electronics_in_alpha_spectrometry_fig_10_3.png?400|}}

Figure XI.3. Electronics in alpha spectrometry (http://www.ortec-online.com/Products-Solutions/RadiationDetectors/silicon-charged-particle-detectors.aspx).

{{anchor:sample_pretreatment_alpha_spectrometry}}
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Prior to counting the [[textbook:nrctextbook:chapter5#5.2._alpha_decay|alpha-emitting radionuclides]], they need to be separated from sample matrix for two reasons. First, [[textbook:nrctextbook:chapter5#alpha_particle|alpha particles]] readily [[textbook:nrctextbook:chapter7#absorption_of_alpha_radiation|absorp]] on sample matrices, solid or liquid, and cannot be directly determined from them. In addition, separation is needed from other alpha-emitting radionuclides due to overlapping alpha peaks (see Table XI.I). In nuclear waste samples a typical set of alpha-emitting radionuclides is the [[textbook:nrctextbook:chapter2#isotope|isotopes]] of [[textbook:nrctextbook:chapter4#uranium|uranium]], [[textbook:nrctextbook:chapter1#plutonium|plutonium]] and [[textbook:nrctextbook:chapter1# transuraniums_first |americium]]. In environmental samples, such as surface soil and surface waters, this set includes also isotopes of [[textbook:nrctextbook:chapter4#thorium|thorium]], <sup>226</sup>Ra and 2<sup>10</sup>Po. In geological samples, not affected by radioactive fallouts and nuclear waste, the typical combination includes isotopes of uranium and thorium and <sup>226</sup>Ra and <sup>210</sup>Po. There are also some other minor components, such as <sup>237</sup>Np, but these typically do not interfere with the measurement of the major components. Radiochemical separations used to separate the alpha-emitting radionuclides as pure components are not discussed in this book. A comprehensive presentation of them can be found from another book of the author of this book, J.Lehto and X.Hou, Chemistry and Analysis of Radionuclides, Wiley-VCH, 2010, 400 pages. In the radiochemical separations the chemical separation methods used comprise of precipitation, ion exchange, solvent extraction and extraction chromatography.

###
{{anchor:sample_preparation_alpha_spectrometry}}
===== 11.3. Sample preparation for alpha spectrometry =====

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At the end of a radiochemical separation procedure a counting source for alpha spectrometry is prepared. This is done either by electrodeposition of the target element on a steel plate or by microcoprecipitation. The purpose of both methods is to produce a very thin counting source to prevent [[textbook:nrctextbook:chapter7#absorption_of_alpha_radiation|absorption of alpha radiation]] in the source. With this respect the electrodeposition method yields a better, thinner, source but in most cases the performance of microcoprecipitation is also satisfactory. In electrodeposition, the solution observed at the end of radiochemical separation and containing the target [[textbook:nrctextbook:chapter2#nuclide|nuclide]] is poured into an electrodeposition vessel and mixed with ammonium, sulphate, chloride, oxalate, hydroxide or formate as electrolyte and the solution is made slightly acidic. A metal disk - usually made of polished steel or sometimes platinum - is tightly mounted to the lower part of the electrodeposition vessel. A platinum wire is put in the vessel and a constant current (10-150 mA/cm<sup>2</sup>) is set up between the platinum wire and the metal disk so that the platinum wire operates as anode and the metal disk as cathode (Figure XI.4). The current causes a reduction of the metals in the solution and their deposition in metallic form or as hydroxides on the surface of the steel plate. <sup>210</sup>Po is spontaneously deposited on a silver disc and in its sample preparation no electric current is needed.
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{{:textbook:nrctextbook:electrodeposition_equipments_fig_11_4.png?200 |}} Figure XI.4. Electrodeposition equipment (Holm, E., Source preparations for alpha and beta 
measurements, Report NKS-40, 2001).

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Another way to prepare counting sources is microcoprecipitation typically used for actinides. The coprecipitation is carried out with lanthanide fluorides: 10–50 μg La, Ce or Nd is added to the solution and the fluoride (LaF<sub>3</sub>, CeF<sub>3</sub>, NdF<sub>3</sub>) is precipitated through the addition of HF. Since actinides will only coprecipitate with lanthanide fluoride if they are at their lower oxidation states of 
+III and +IV the higher oxidation states must be reduced prior to coprecipitation. After precipitation, the precipitate is collected by filtration on a membrane filter, dried and mounted on the measurement plate with glue for alpha counting. The sample to be measured for radium can be prepared by microcoprecipitation with barium sulphate. Because the mass, and so the self-absorption of the sample, is larger, the resolution obtained after microcoprecipitation will be somewhat poorer than after electrodeposition. Microcoprecipitation is a distinctly more rapid technique, however, and the resolution is usually adequate.
###

{{anchor:tracer_alpha_spectrometry}}
###
The [[textbook:nrctextbook:chapter6#activity|activity]] of an [[textbook:nrctextbook:chapter11#alpha_emitters|alpha-emitting radionuclide]] in an alpha spectrometric measurement following a radiochemical separation is typically determined by adding a tracer to the sample prior to the radiochemical separation. The tracer is another alpha-emitting radionuclide of the target radionuclide element. For all radionuclides given in [[textbook:nrctextbook:chapter11#alpha_emitters|Table XI.I]] except <sup>226</sup>Ra there is a suitable [[textbook:nrctextbook:chapter4#artificial_radionuclides|artificial]] alpha-emitting tracer. For example, when [[textbook:nrctextbook:chapter6#activity|activity]] of the [[textbook:nrctextbook:chapter6#long_lived_radionuclides|naturally occurring radionuclide]] <sup>210</sup>Po is determined in an environmental sample a known [[textbook:nrctextbook:chapter6#activity|activity]] amount of <sup>209</sup>Po (or <sup>208</sup>Po) is added to the sample at the start of the radiochemical separation procedure. <sup>209</sup>Po is an artificial radionuclide produced from <sup>209</sup>Bi in a [[textbook:nrctextbook:chapter11#cyclotrons|cyclotron]]. Both [[textbook:nrctextbook:chapter2#isotope|isotopes]] of polonium behave identically in 
the course of the separation procedure and the same fraction of both isotopes is recovered in the counting source. Due to their different alpha energies the two isotopes can be distinguished from alpha spectrum ([[textbook:nrctextbook:chapter11#alpha_spectrum|Figure XI.5]]). The initial [[textbook:nrctextbook:chapter6#activity|activity]] of <sup>210</sup>Po in the sample can now be simply calculated from the added activity of <sup>209</sup>Po and the number of counts, the peak areas. If, for example, 1.0 Bq of <sup>209</sup>Po was added and the peak areas were 7000 counts for <sup>210</sup>Po and 5000 counts for <sup>209</sup>Po the activity of <sup>210</sup>Po in the sample was $1 \, \text{Bq} \times \left( \frac{7000}{5000} \right) = 1.4 \, \text{Bq}$.
###

{{anchor:alpha_spectrum}}
{{ :textbook:nrctextbook:alpha_spectrum_of_naturally_ocurring_po_fig_11_5.png?200|}} Figure XI.5. Alpha spectrum of naturally occurring <sup>210</sup>Po and <sup>209</sup>Po tracer.
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