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Applied Radiation and Isotopes
journal homepage: www.elsevier.com/locate/apradiso
Conversion electron spectrometry of Pu isotopes with a silicon drift detector
S. Pommé a* J. Paepena, K. Perâjârvib, J. Turunenb, R. Pôllânenb
a European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium b STUK-Radiation and Nuclear Safety Authority, P.O. Box 14, FI-00881 Helsinki, Finland
HIGHLIGHTS
• Built set-up for conversion electron spectrometry with silicon drift detector.
• Conversion electron spectra measured from 238,239,240Pu and 241Am.
• Compared with other SDD set-up and Si(Li) detector.
ARTICLE INFO ABSTRACT
An electron spectrometry set-up was built at IRMM consisting of a vacuum chamber with a moveable source holder and windowless Peltier-cooled silicon drift detector (SDD). The SDD is well suited for measuring low-energy x rays and electrons emitted from thin radioactive sources with low self-absorption. The attainable energy resolution is better than 0.5 keV for electrons of 30 keV. It has been used to measure the conversion electron spectra of three plutonium isotopes, i.e. 238Pu, 239Pu, 240Pu, as well as
241Am (being a decay product of 241Pu). The obtained mixed x-ray and electron spectra are compared with spectra obtained with a close-geometry set-up using another SDD in STUK and spectra measured with a Si(Li) detector at IRMM. The potential of conversion electron spectrometry for isotopic analysis of mixed plutonium samples is investigated. With respect to the 240Pu/239Pu isotopic ratio, the conversion electron peaks of both isotopes are more clearly separated than their largely overlapping peaks in alpha spectra.
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
ELSEVIER
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Article history: Received 24 April 2015 Accepted 20 November 2015 Available online 22 November 2015
Keywords: Plutonium Isotopic ratio Nuclear security Safeguards Silicon drift detector
1. Introduction
Plutonium (Pu) is generally produced in a variety of isotopic mixtures in reactor fuel. The predominant isotope, 239Pu, is produced by neutron capture of 238U and the heavier isotopes by further neutron capture. Spent fuel may contain approximately 50-60% of 239Pu, 25% of 240Pu, 15% of 241Pu, 5% of 242Pu, and 2% of 238Pu. The 238Pu, formed through decay of 242Cm, is responsible for much of the Pu decay heat because of its short half-life (87.74 a). The 239Pu is fissile and can be used as fuel for a chain reaction in a reactor or a nuclear weapon. Weapon makers prefer a high enrichment in 239Pu because of its low rate of heat generation and relatively low spontaneous emission of neutrons and gamma rays, whereas the presence of 240Pu may limit a bomb's potential by causing predetonation due to its neutron flux from spontaneous
* Corresponding author. E-mail address: stefaan.pomme@ec.europa.eu (S. Pommé).
fission. Modern weapon designs - using deuterium-tritium fusion to boost the fission yield, decrease weight and increase safety -can be made insensitive to the isotopic mix, but at a low level of sophistication a high concentration of 240Pu is considered a barrier to nuclear proliferation (Feiveson et al., 2014).
The measurement of isotopic amount ratios is an important topic in safeguards and nuclear forensics applications, and needed to support non-destructive assay (NDA) methods - calorimetry and neutron coincidence counting - in providing the total plutonium content of a sample (Sampson, 1991). For large quantities of material, gamma spectrometry techniques are used for the analysis. The alpha decay of the 236.238.239.240.242.244Pu isotopes and the beta decay of 241Pu primarily populate the ground state and/or the lowest excited states of their daughter nuclides, and the gamma transitions depopulating those low-lying excited states have a high probability of being internally converted (DDEP, 2004-2015). As a result, the intensity of characteristic gamma-ray emission is very low and isotopic analysis by gamma spectrometry is a challenge
http://dx.doi.org/10.1016/j.apradiso.2015.11.052
0969-8043/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
for low activity levels of plutonium. Only the ingrowth of 241Am (and a small fraction of 237U) from the decay of 241Pu leads to significant gamma emission. The most problematic isotope is 242Pu, due to its low abundance, long half-life and its few gamma rays similar in energy and branching ratio to those from 240Pu (Sampson, 1991).
Small amounts of Pu in environmental and biological samples are addressed with more sensitive techniques for isotopic analysis, such as mass spectrometry (Jakopic et al., 2010), alpha-particle spectrometry (Bortels and Collaers, 1987), neutron induced fission track analysis specific for 239Pu (Moorthy et al., 1988) and liquid scintillation counting for 241Pu (Ikaheimonen, 2000). The isotopic ratio measurements can provide important information for identifying a source of contamination, e.g. to distinguish particles from global nuclear weapon fallout and from a nuclear reactor accident (Kudo, 2001; Evrard et al., 2014). The age of plutonium material (i.e. formation of 241Pu by irradiation or last chemical separation from 241Am) can be derived from measurement of the 241Am/241Pu isotope ratio (Varga, 2007) or extrapolation of the 241Am ingrowth over time (Sibbens et al., 2008). Plutonium clocks are used for age dating in nuclear forensics (Wallenius and Mayer, 2000; Sturm et al., 2014; Pommé et al., 2014).
Mass spectrometry techniques (TIMS, AMS, RIMS, ICP-MS) are the most sensitive and precise methods, except that the determination of 238Pu is more difficult due to isobaric interference from 238U (Jakopic et al., 2010). As an alternative, alpha spectrometry has advantages of simplicity, ability to use a radiotracer, and low cost (LaMont et al., 1998). As activity ratios differ from atom ratios by the decay constants, it relatively enhances the signal from the short-lived isotopes, in particular 238Pu. Determining the 240Pu/239Pu ratio is possible (Bortels and Collaers, 1987; Raab and Parus, 1994; LaMont et al., 1998; Pollanen et al., 2012) but problematic due to the limited energy resolution and the spectral interference of the main 239Pu (5156.6 keV) and 240Pu (5168.1 keV) alpha emission peaks. The main peaks of the other isotopes - 236Pu (5767.5 keV), 238Pu (5499.0 keV), 242Pu (4902.3 keV), and 244Pu (4589 keV) - are well separated, but ingrowth of 241Am (5485.6 keV) interferes with 238Pu (Sibbens et al., 2008). In old archived samples, the 241Pu/239+240Pu activity ratio can be evaluated by a spectrometry through the accrued 241Am (Vukanac et al., 2006; Salminen-Paatero et al., 2014).
For a further analytical separation of 239Pu and 240Pu in environmental samples, the a-particle spectrometry can be complemented with additional measurements of the L x rays emitted from the same sources (Hisamatsu and Sakanoue, 1984; Komura et al., 1984; Arnold and Kolb, 1995; Pollanen et al., 2009), taking into account the different emission probabilities of L x rays for 239Pu and 240Pu (Lépy and Debertin, 1994). The measurement can be done on the same source as for the a-particle spectrometry, possibly in an underground laboratory to reduce the background signals (Arnold, 2006).
Low-energy gamma transitions of actinides being highly converted, internal conversion electron (ICE) spectrometry offers an interesting complement - or even alternative - to alpha spectro-metry. Shiokawa and Suzuki (1986) examined the principle on a 243Cm/244Cm mixture using a cooled silicon surface barrier detector. At approximately 150 K, they reached a resolution of 1.8 keV for electrons of 42 keV (60-L3 line of 241Am) and 2.25 keV at 624 keV (661-K line of137Cs). Shiokawa et al.(1989,1990, 1991) developed a new ICE spectrometer using a windowless Si(Li) detector with improved energy resolution of 0.48 keV and 1.43 keV for 42 keV and 624 keV electrons respectively, and succeeded to determine isotopic ratios for plutonium and curium.
The Si(Li) detector is operated at liquid nitrogen temperature and sample changing requires specific measures to avoid condensation of humidity on the crystal surface. At STUK, Perajarvi
et al. (2014) investigated the performance of Peltier-cooled, win-dowless silicon drift detectors (SDD) for ICE spectrometry. These detectors are more user-friendly than Si(Li) detectors and they reach a resolution of 0.50 keV (FWHM) for electrons of 45 keV at operational temperatures between - 20 °C and -10 °C. Good source quality is required, since the energy resolution shows considerable broadening for 1-^m-thick samples, but remains good (0.70 keV) for thin plutonium samples covered with a 0.5 ^m Mylar absorber foil. This is significantly better than the 2.2 keV resolution of ion-implanted planar silicon detectors used for alpha spectrometry (Ahmad et al., 2015).
This work describes a new SDD set-up built at IRMM. Measured 238,239,240Pu and 241Am ICE spectra are shown and compared with spectra from other devices.
2. Set-up
Fig. 1 shows a schematic drawing of the set-up for the internal conversion electron spectrometer at IRMM. A picture of the detector and source chamber is shown in Fig. 2. The detector is a windowless Canberra X-PIPS® silicon drift detector with an active area of 100 mm2 and thickness of 500 ^m. A circular multilayer diaphragm composed of 200 ^m Ta, 35 ^m Cr, 20 ^m Ti and 75 ^m Al limits the effective area to approximately 80 mm2. The detector and its field-effect transistor are thermo-electrically cooled down to - 20 °C, in order to reduce noise and to ensure stable operation in changing environmental conditions. The detector element is mounted on a finger which is inserted into the vacuum chamber. The housing of the detector contains a reset-type preamplifier, a high-voltage bias supply and a temperature controller for the thermo-electric cooler.
The chamber is constructed from commercially available components, including a central cubic piece with matching flanges and a hinged door for easy access to the interior. The source holder is placed on the tip of a rotary actuated linear feedthrough, which enables moving the source up or down, thus reproducibly varying the source-detector distance from millimetres up to 13.5 cm at any time. By means of a turbo-molecular vacuum pump, the air pressure is kept well below 10-4 mbar. To avoid condensation of any residual humidity on the surface of the detector, a cold point has been created in the vacuum chamber by means of a thermoelectric cooler kept at a lower temperature ( - 40 °C) than the detector. The front panel of the set-up houses the controllers of the vacuum gauge, the turbo-molecular vacuum pump, and the power supply and temperature readout of the cold point.
In its standard configuration as an x-ray detector, the SDD has a 50 fF feedback capacitor with a gain of 0.9 mV/keV. This would be a convenient setting for the low-energy signals from the x rays and ICEs, but the impact of 5.5 MeV a particles on the detector would cause output signals of 5 V and regular resets of the pre-amplifier. Therefore, the feedback capacitor was increased to 0.5 pF to reduce the gain, potentially at the expense of some loss of energy resolution. The large signals from a particles cause afterpulses interfering with the low-energy spectrum and peak detection in the analog-to-digital convertor had to be disabled for 40 ^s after detection of a signal.
The actinide sources measured in this work are thin layers of highly enriched material deposited on a glass, quartz or stainless steel substrate, mostly prepared by vacuum evaporation (Sibbens and Altzitzoglou, 2007) for the purpose of high-resolution alpha-particle spectrometry (Pommé and Sibbens, 2008; Sibbens et al., 2008, 2010). Whereas in alpha-particle spectrometry a magnet system (Paepen et al., 2014) can be used to eliminate the signal from conversion electrons, by reversing the poles of one of the magnets, the same system can be used as an electron lens to
enhance the electron contribution relative to the alpha and x-ray signals.
In the course of the measurement campaign, issues with energy resolution rose when the detector crystal appeared to be covered with a semi-transparent substance of unknown origin. That layer moved down to the rim of the crystal when placed in a vacuum oven at 80 °C.
3. Measurements
3.1. ICE spectra of Pu and Am with SDD at IRMM
In Fig. 3 a logarithmic plot is shown of the x-ray and ICE spectra
obtained from 238239240pu and 241Am sources. The conversion electron peaks are well separated, with an energy resolution of better than 0.5 keV. Surprisingly, the energy resolution of the x-ray peaks is similar, but not better than for the electrons. The x rays of the Pu isotopes are aligned at the same energy (13 and 17 keV), whereas the ICE peaks differ in position depending on the energy of the converted gamma transition, minus the binding energy of the electron in its original orbital state. A list of energies and internal conversion coefficients for 238,239,240pu was presented by Perajarvi et al. (2014).
Figs. 4 and 5 show selected parts of the 238,239,240pu spectra in a linear scale, i.e. in the energy regions between 20 and 36 keV and between 35 and 50 keV, respectively. The peaks correspond to conversion electrons from the L shell (Fig. 4) and the M + shells
Fig. 2. Picture of the detector housing and the source chamber.
1000 n 100 n 10
1000 100 10
Energy (keV)
Fig. 3. Logarithmic plot of ICE and x-ray spectra obtained with the IRMM spectrometer for enriched 238Pu, 239Pu, 240Pu, and 241Am sources.
(Fig. 5) for the 43.469 keV (238Pu), 51.624 keV (239Pu) and 45.244 keV (240Pu) gamma transitions. The peak widths correspond to an energy resolution of approximately 0.47 keV for 28 keV (240Pu) and 30/34 keV (239Pu) electrons.
The energy resolution was calculated in the following way: (1) the full-width-at-half-maximum (FWHM) region is defined approximately by visual inspection of the spectrum (lower and upper channels corresponding to half of the counts in the peak maximum); (2) in that region the FWHM is calculated from the standard deviation of the data around the mean (FWHM=2.35s). The continuum spectrum underneath the peaks was not subtracted, which makes that the actual resolution should be slightly better than the calculated value. Hence, the relatively high FWHM values (0.50-0.52 keV) of the x-ray peaks.
Besides the enriched sources, also a mixed 238,239,240Pu source was measured. The spectrum in Fig. 6 shows that the L electron peaks are still discernible but spectral deconvolution is needed to
5000 40003000 200010001000 800
<2 600 "
200 1500 1200 900 600300 0
i 1 ....... 1 "I "l1
Energy (keV)
Fig. 4. Linear plot of the 238,239,240Pu spectra showing the ICE peaks of electrons from the L shell for the 43.469 keV (238Pu), 51.624 keV (239Pu) and 45.244 keV (240Pu) gamma transitions.
1000 500 0
® 100
Energy (keV)
Fig. 5. Linear plot of the 238,239,240Pu spectra showing the ICE peaks of electrons from the M + shells for the 43.469 keV (238Pu), 51.624 keV (239Pu) and 45.244 keV (240Pu) gamma transitions.
Energy (keV)
Fig. 6. Measured ICE peaks from the L shell from a mixed 238239240pu source (top) and the matching spectra of enriched sources (bottom).
i-r—1-1-1-r
238,239,240,
mixed source
Energy (keV)
Fig. 7. Measured ICE peaks from the M + shells from a mixed 23S 239 240Pu source (top) and the matching spectra of enriched sources (bottom).
quantify the atomic ratios of the Pu mixture. The deconvolution looks more difficult for the M + electrons in Fig. 7. It is likely that the peak shape models used for alpha spectrometry (Pommé and Caro Marroyo, 2015) can be applied directly to the electron peaks. Where the electrons interfere with x rays, different peak shapes and energy calibrations will have to be combined to simultaneously fit both types of signals.
3.2. Measurements with Si(Li) detector at IRMM
The 238,239,240Pu sources were also measured with a Si(Li) detector (Intertechnique, 1990) with 80 mm2 sensitive area and 0.50 cm3 sensitive volume. The resolution of the spectra, shown in Fig. 8, very much depended on the condition of the crystal, as condensation of humidity was building up after each sample change, eventually creating a dead layer and causing a shift and broadening of the electron peaks. The x-ray peaks remained stable and were larger than in the SDD spectra. The calculated energy resolution of the Si(Li) detector was 0.23 keV for x rays at 13/ 17 keV and 0.45 keV for 30/34 keV conversion electron peaks from 239Pu. Sample changing was cumbersome, due to the need for extensive vacuum pumping. The condensation could be removed through reheating of the crystal. This is a long and delicate process
600 500 400 -300 -200 100 0
1000 -800 -
~ 600 CD
300 250 -200 -150 100 50 -0
Si(Li) detector + condensation
Energy (keV)
Fig. 8. Linear plot of ICE and x-ray spectra obtained with the IRMM Si(Li) detector for enriched 238 239240pu and 241Am sources. Unlike the x-ray peaks, the ICE peaks shift and broaden through energy loss in the source, the detector dead layer and condensation on the cooled crystal.
compared to the few minutes needed for cooling or reheating of an SDD.
3.3. Measurements with SDDs at STUK
The 450-^m-thick SDD (Oxford Instruments Analytical Inc) at STUK was described by perajarvi et al. (2014). Measurements of 239,240pu and 241Am sources of low activity were performed in close geometry to the detector. The spectra are shown in Fig. 9. In spite of the unfavourable measurement conditions, the energy resolution is 0.19 keV for 13 keV x rays and 0.75 keV for 28 keV electrons (240pu). The detector has low noise, judging from the spectral quality just above the 0.7 keV threshold.
A new detector system (Oxford Instruments Analytical Inc) was tested in which the detector signal is split into two parallel amplifiers, producing a signal for low gain amplification and another for high gain. Thus the SDD can simultaneously provide an alpha spectrum and an ICE spectrum from the same source, as demonstrated for 240pu in Fig. 10. This opens perspectives to combine the information of both spectra for isotopic ratio measurements. The obtained resolution (FWHM) is 0.19 keV for 13 keV x rays, 0.7 keV for 28 keV electrons and 95 keV for 5.26 MeV a particles. A more complete description of these developments will be published elsewhere.
4. Conclusions
Silicon drift detectors are suitable as internal conversion electron spectrometers with an energy resolution of 0.5 keV. Using split amplification, they can also measure alpha particles in parallel. The combination of ICE and a spectra is potentially a powerful tool for isotope analysis of actinides and therefore could be used in nuclear
■ i ■ ■ ■ ■ i ■ ■ ■ ■ i ■ ■ ■ ■ i ■ ■ ■ ■ i ■ ■ ■ ■ i ■ ■ ■ ■ i ■ ■ ■ ■ i ■ 0 5 10 15 20 25 30 35 40 45 50 55 60
Energy (keV)
Fig. 9. Linear plot of ICE and x-ray spectra obtained with the STUK SDD for enriched 240Pu, 239Pu, and 241Am sources measured in close geometry.
Energy (keV)
Fig. 10. Test spectra by STUK of a particles (top) and x rays/ICEs from the decay of 240Pu, taken in parallel with a new SDD device (Oxford Instruments Analytical Inc) with split amplification.
security, safeguards, environmental and waste management applications. Applied to isotopic ratio measurements of plutonium mixtures, the ICE spectra are of added value in resolving the 239Pu and 240Pu peaks, which is difficult in alpha spectra. As a stand-alone technique, ICE spectrometry requires further research on spectrum deconvolu-tion and reference emission data.
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