Spectroscopy With Superconducting Sensor ... - IEEE Xplore

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 2, APRIL 2013

Spectroscopy With Superconducting Sensor Microcalorimeters K. E. Koehler, D. A. Bennett, E. M. Bond, M. P. Croce, D. E. Dry, R. D. Horansky, V. Kotsubo, W. A. Moody, M. W. Rabin, D. R. Schmidt, J. N. Ullom, and L. R. Vale

Abstract—The total reaction energy of individual nuclear decays was measured using microcalorimeters with transition-edge-sensor (TES) thermometers. For alpha-decaying actinides (e.g., U-235, Pu-239, Np-237, Am-241), is in the 4–6 MeV range. Nearly all of this energy goes into the relatively light alpha particle, and approximately 100 keV is left over for the much heavier, recoiling daughter atom. Alpha-particle energy spectroscopy with TES-microcalorimeters has shown the ability to simultaneously resolve peaks that overlap in conventional alpha spectroscopy, with resolution now less than 1 keV full-width-at-half-maximum (FWHM) at 5.3 MeV. For total reaction energy spectroscopy, we use the same TES design as our alpha detectors, but embed a small radioactive sample (of about 1 Bq) directly inside an absorber designed to capture all the emitted particles (alpha, recoil nucleus, electrons, X-rays) with near 100% efficiency. We have measured -spectra of alpha-decaying isotopes with spectral resolution of 2–3 keV FWHM. For some actinide analytical problems, the -spectrum is simpler than the alpha-spectrum: fewer peaks, further apart, and easier to quantify. We will discuss sensor design, methods for embedding radionuclides, and spectral data. Index Terms— Bolometers, cryogenics, radiation detectors, spectroscopy, superconducting devices.

I. INTRODUCTION

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ICROCALORIMETRY employs cryogenic thermal sensors for ultra-high resolution spectroscopy of various types of radiation such as gamma, x-ray, and alpha. By offering superior energy resolution in a single non-consumptive measurement, microcalorimetry is a potential new tool for nuclear forensics, international safeguards, and treaty monitoring for rapid, simultaneous analysis of multiple actinides. Alpha-particle energy spectroscopy with TES-microcalorimeters has been shown to be a particularly powerful analytical tool, yielding resolutions of less than 1 keV at 5.3 MeV [1] compared with the resolution seen in silicon detectors, which

Manuscript received June 15, 2012; revised September 11, 2012; accepted October 08, 2012. Date of publication January 09, 2013; date of current version April 10, 2013. This work was supported in part by the Department of Energy, Office of Nonproliferation Research. K. E. Koehler was with Los Alamos National Laboratory, Los Alamos, NM 87545. She is now with the Department of Physics, Western Michigan University, Kalamazoo, MI 49008 USA (e-mail: [email protected]). D. A. Bennett, R. D. Horansky, V. Kotsubo, D. R. Schmidt, J. N. Ullom, and L. R. Vale are with National Institute of Standards and Technology, Boulder, CO 80305-3337. E. M. Bond, M. P. Croce, D. E. Dry, W. A. Moody, and M. W. Rabin are with Los Alamos National Laboratory, Los Alamos, NM 87545. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2012.2225639

Fig. 1. Spectroscopy concept diagram. (a) External radioactive source in close proximity to microcalorimeter sensor for alpha spectroscopy, (b) radioacspectroscopy, and tive source embedded completely within the sensor for (c) external and internal radioactive sources in use simultaneously for highly informative combined alpha spectroscopy and spectroscopy experiments. In (c) the external and internal samples do not have to have the same composition.

are limited to 8–10 keV full-width-at-half-maximum (FWHM) for 5 MeV alpha particles [2]. Microcalorimeters exploit the low noise property of low temperature (around 100 mK) detectors to achieve high resolution, operating on the principle that the energy resolution of , where a simple microcalorimeter is proportional to is the heat is the temperature of the microcalorimeter and capacity [3]. By operating a superconducting Mo/Cu bilayer in the transition between normal and superconducting, the resistance of the film is highly dependent on slight changes in temperature. Voltage-biasing the TES ensures stable sensor operation in which changes in the film resistance correspond to changes in current, which can be detected and read out by superconducting quantum interference devices (SQUIDs). Actinide alpha spectrometry normally requires chemical sample preparation. For conventional sensors, this includes time-consuming separation of actinide from actinide and electrodeposition [4]–[6]. For microcalorimeters, this radiochemical separation is not required, but electrodeposition with low self absorption is needed to ensure ultra high resolution alpha spectra. By embedding a radioactive source completely within the sensor and measuring the total reaction energy of individual nuclear decays using TES-microcalorimeters, we seek to minimize source preparation and increase detection efficiency by increasing the detector solid angle to , while still being able to characterize a complex energy spectrum with 2–3 keV resolution. Fig. 1 is a concept diagram illustrating the geometrical differences between the external source, alpha specspectroscopy configuration and the internal source, troscopy configuration. The end goal of spectroscopy is the rapid and accurate determination of isotopic and elemental spectroscopy sensors based on different ratios. Similar low-temperature thermometers have been recently developed for metrological applications [7]–[10]. spectroscopy has the added advantage of simplifying spectra in certain cases. One such case is shown in Fig. 2. This

U.S. Government work not protected by U.S. copyright.

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SPECTROSCOPY WITH SUPERCONDUCTING TRANSITION-EDGE SENSOR MICROCALORIMETERS

Fig. 2. Simulated alpha and spectrum. The simulated alpha spectrum of a Pu and Am shows five alpha peaks measured with mixed source with microcalorimeter detectors (red). This alpha spectrum has a maximum separation of 13 keV, whereas the simulated spectrum (black) of the same sample shows only two peaks with a separation of 45 keV. In this case, spectroscopy reduces the number of peaks and increases the separation, allowing for simplified isotopic analysis. Simulation does not include effect of escaping photons Am gamma ray). (e.g., 59.5 keV

spectrum was randomly generated using a Bortels function (see Section V.A) for each energy, with relative intensities assigned from literature values and an assumed isotopic composition. In this mixed actinide sample of Pu and Am, the alpha spectrum has five dominant peaks with only a 13 keV separation between the primary peaks from each isotope. The same sample measured using spectroscopy yields a spectrum with only two peaks—one for the value of each isotope—with a 45 keV separation. There is only one peak for each isotope because all the energy of each decay-reaction product is captured and turned into heat. This includes the kinetic energy of the alpha particle, kinetic energy of the daughter atom, and the excited state (nuclear and atomic) energy of the daughter atom, so one value is measured regardless of the alpha decay path. It is possible that not all the energy is thermalized within the absorber, so peaks lower than the value resulting from photons escaping the absorber could complicate spectra. In the case of Am, the 59.5 keV gamma escape results in an escape peak that is 15 keV lower than the Pu peak. Despite adding a secondary peak to the spectrum, the spectrum still has fewer peaks and is better separated than the associated alpha spectrum, but additional escape peaks is an important consideration when comparing spectroscopy to alpha spectroscopy. II. EXPERIMENTAL APPARATUS Following the process from [9], source preparation for spectroscopy is simple and requires minimal chemistry. To embed the radionuclides, a drop of the dissolved radionuclides in HCl is allowed to dry on a piece of 15 or 25 micrometer-thick gold foil. The gold foil is folded over to a final dimension of approximately 1 mm 2 mm. It is diffusion welded in atmosphere for 16 hours, effectively encapsulating the radionuclides. For Po sources, the material is self-deposited (see Section IV) and since Po is highly volatile, the foils are merely folded over and not diffusion welded. The gold absorber is epoxied to a gold pad coupled to a Mo/Cu bilayer transition-edge sensor on a SiN membrane as shown in Fig. 3. The 100 nm thick gold pad (made of two rectangles 3.2 mm 1.3 mm and 2.4 mm 0.3 mm) is deposited by an electron beam evaporation process onto a 500

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Fig. 3. Photograph of spectroscopy microcalorimeter. The gold absorber Am on the left is epoxied to a gold pad which is thermally with embedded coupled to a TES on the right.

nm thick Cu pad, which is thermally connected to the TES via a 80 micrometer 80 micrometer Cu bridge. The TESs used for these detectors are scaled up in lateral dimensions (1.2 mm 1.2 mm) from previous versions by a factor of three. Although several different noise-suppressing, normal-metal (Cu) structures were used and are still being evaluated, preliminary results indicate that simply scaling up original dimensions has proven to be most effective in reducing instabilities in the TES transition curve [1]. Each detector operates independently in an 8-channel cryostat, which operates stably at 80 mK using a helium pulse-tube cryostat and adiabatic demagnetization refrigerator (see [1] for more details). Detector signals are read out with a low-impedance, low-noise two-stage SQUID amplifier. Since all or nearly all the deposited energy from the decay is thermalized in the absorber, we measure a signal proportional to the -value rather than the energies of individual alpha particles. As such, there is no energy lost due to self-attenuation and the decay detection efficiency is 100%. A complete digital record of each pulse is saved to disk for processing and pile-up rejection is done prior to optimal filtering. Pulses are processed using a Wiener-type optimal filter, and the optimally filtered maximum is used to generate the final spectra. Time-dependent drift in the data that originates from temperature-dependent gain and offset changes in the room-temperature signal readout electronics can be corrected to a large extent. A two-point energy calibration is done using either two alpha or two peaks (see Section V.D). III. HIGH RESOLUTION

SPECTRA

High resolution spectra were obtained from Am, Pu, Po and Po. Current best resolution values for measurements are 2.58 keV FWHM with 4.2 keV tailing at 5.4 MeV as seen in Fig. 4. Recent measurements using similar type detectors have achieved 4.2 keV FWHM resolution on measurements of Pu and Pu [10]. The low energy structure on the tail seen in Fig. 4 is not well understood and the statistics are too low to draw any firm conclusions on the origin of this structure. It could be a result of not annealing the absorber. Since the radionuclides form a thin layer over the entire surface of the gold foil, it is possible we

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Fig. 4. High resolution spectrum of Po. Spectrum taken with the self-dePo peak at 5410 keV. Fitting this peak posited source shows the 5407 keV with a Bortels function yields a 2.58 keV FWHM on the underlying gaussian. The low energy structure on the tail is not well understood but could be due to energy escape from the edges of the absorber. Energy calibration for this specPo and Po. trum is done using external alpha peaks from

are losing energy from the decays which occur at the edges of the foil. A cursory look at pulses from this population show no significant differences from the primary peak, but without sufficient statistics this remains an avenue of further investigation. The tails in peaks from alpha spectra from microcalorimeters are also poorly understood, so no firm conclusions can be drawn at this point as to the origin of this structure. IV. A HIGH ENERGY STRUCTURE Of the sources measured, spectra from Am and Pu show a high energy shoulder on the peak as seen in Fig. 5. To probe this, an external alpha source was placed near the absorber in the configuration shown in Fig. 1(c), so an energy spectrum from the external alpha source and the spectrum from the embedded source could be measured simultaneously. Peaks from the external alpha source did not show this high energy feature, leading us to conclude the high energy shoulder is not a product of the detector readout. A further result of simultaneously measuring clean peaks from the external source is that these alpha peaks were used for energy calibration on the peaks from the internal source, so energy calibration was not dependent on assigning values to poorly understood peaks from the internal source (see also Section V.D). A possible explanation for the double peak is a secondary thermal path resulting from inadequate annealing of the foil absorber. This explanation is unsatisfactory because thermalization differences would likely manifest themselves as pulse shape differences, and no differences other than height were found in the pulses from the two populations. This explanation is further discredited when we measured the spectra for Po and Po. Since Po is able to self-deposit onto gold (the literature sometimes refers to this as spontaneous deposition or auto-deposition) [11], the Po formed thin deposits onto the gold foil without the addition of energy to the system as is the case with electrodeposition. This results in samples which are uniform and well-characterized compared to the unknown form of the sources prepared from drying a drop of solution on the foil. We folded the foil so the Po was encapsulated, but we did not include an annealing step. With this source, we obtained a clean spectrum without the high energy shoulder for both Po

Fig. 5. spectra of Am and Pu. A spectrum of Am shows the primary peak at 5638 keV and the 60 keV gamma escape peak at 5578 keV. The 60 keV gamma escape peak comes from 35.9% of the alpha decays which leave the daughter nucleus in an excited state. The 59.5 keV gamma ray released when the daughter nucleus de-excites is not always captured by the absorber, so the energy is not measured, resulting in a secondary peak at an energy of less the energy of the escape photon. Both peaks show a high energy structure 6–7 keV above the primary peak at about one-third intensity. The spectrum Pu also shows a high energy structure on the 5593 keV peak at about of one-half intensity. Energy calibration for these spectra is done using external Po and Po. alpha peaks from

and Po (see Fig. 4). Since the high energy structure was not present in these spectra despite not annealing the foils, it appears that thermal conduction within the absorber is not a limiting factor. From this information, we are able to draw some tentative conclusions regarding the origin of this secondary structure. Because the Po internal sources did not show a secondary peak, whereas both the internal Am and Pu sources did show secondary peaks, the existence of this structure is unlikely to be due to a different response in the calorimeter to alpha particles as opposed to recoil nuclei. Additionally, explanations depending on escape energy of x-rays or conversion electrons are unlikely, since the secondary peak does not appear in Po internal sources. It appears the explanation for the secondary peak may lie in the source preparation. Since the secondary peak did not appear in the self-deposited sources, which we know from alpha spectroscopy to produce very clean depositions, it is plausible that this effect comes from the residue that is left when drying a drop of radionuclide containing solution. If an alpha particle or recoil nucleus deposits energy in the residue—perhaps through lattice damage—, rather than losing its energy in the gold absorber, the amount of energy thermalized could be different. Future steps to probe this question will include drop-drying Po with and without the annealing step to probe the effects of annealing and drop-drying as opposed to self-deposition. Since most actinides will not self-deposit, samples may be prepared by electrodepositing the solution rather than drop-drying.

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Fig. 7. High resolution alpha spectra of Po. The spectrum on top (a), Po alpha source with a tin absorber, shows a 0.7 taken from an external Po alpha source with the gold keV FWHM resolution. Using an external absorbers, we are able to get a 1.7 keV resolution as shown in the spectrum on bottom (b). Deconvolving the lattice damage contribution from the detector response, we find that lattice damage due to alpha particles is slightly higher in gold absorbers. Fig. 6. Simulation of lattice damage. A Monte Carlo simulation of energy lost due to lattice damage is done using SRIM. A fit to this simulated data is shown in blue. At the top, the lattice damage due to an alpha particle in gold results in a 0.46 keV offset in centroid, a 0.223 keV FWHM, and 0.338 keV tailing. A simulation of lattice damage in a measurement results in a 8.72 keV offset in centroid, 0.77 keV FWHM, and 0.30 keV tailing.

V. LATTICE DAMAGE A. Lattice Damage in

Spectroscopy

The energy measured by the TES is only the energy thermalized in the absorber, rather than the entire value. Energy can escape as is the case with the 60 keV gamma from Am, but energy can also be lost to lattice damage. By lattice damage we mean that the kinetic energy of a particle can be converted to potential energy by dislocating atoms in the crystal lattice of the absorber. This is a random process and the energy loss from lattice damage contributes to a mean energy loss, peak broadening, and tailing in the peak shape [12]. Although the peaks from spectra are not well understood, it appears the peak shapes in some of our data are well-described by a one-tailed Bortels function, which is a convolution of a gaussian (with centroid and FWHM of ) and exponential function (with tailing parameter ) [13]. In our analysis of alpha spectra measured using similar TES detectors, we usually use a two-tailed Bortels, with the assumption that one tailing parameter is due to source straggle and one is due to lattice damage from the alpha particle [12]. Since there should be no source straggle in a -type detector, all tailing is assumed to be due to lattice damage. B. Lattice Damage Simulations Lattice damage has been shown to be a limiting factor for energy resolution in alpha spectrometry [12]. Preliminary Monte Carlo SRIM calculations shown in Fig. 6 have estimated the lattice damage due to a 5.5 MeV alpha particle in gold, assuming

the Frenkel storage energy of gold is 4 eV and the displacement energy is 43 eV [14]. The calculations show an offset of 0.46 keV in the energy centroid of the distribution from the mono-energetic alpha source energy of 5485.56 keV (the highest alpha energy from Am). Measured energy offsets will include not only the component due to lattice damage, but also an offset due to energy straggle in the source for external alpha sources. The peak is broadened to a FWHM of 0.223 keV. Current best measurements on external sources on the gold foil detectors are 1.7 keV FWHM (see Fig. 7(b)), indicating room for improvement in detector performance. In a SRIM calculation of lattice damage due to both the alpha and the recoil nucleus, the offset from the accepted value of 5637.82 keV for Am is substantially more at 8.72 keV. This indicates that the lattice damage due to the recoil nucleus substantially lowers the centroid of the distribution. The FWHM of the distribution increases to 0.77 keV, another result of including lattice damage from both the alpha particle and the much heavier recoil daughter nucleus. C. Peak Resolution With the configuration shown in Fig. 1(a), we have achieved better than 1 keV resolution measuring alpha particles from Po using tin absorbers (See Fig. 7(a) and [1]). Using gold absorbers in the configuration from Fig. 1(c), we are able to get 1.7 keV FWHM resolution on an external alpha source (See Fig. 7(b)). Taking the results from the Wiener optimal filter, we are able to characterize the expected resolution of the detectors from the signal to noise ratio information using the process outlined in [12]. For the alpha particle interacting with a gold absorber, the signal to noise ratio response yields a peak width component of 1.4 keV. By assuming that the measured

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peak FWHM is from the detector response component added in quadrature with the lattice damage component, we are able to determine that the lattice damage contribution to the FWHM due to a 5304 keV alpha particle must be less than or equal to 0.96 keV. Similar calculations with tin absorbers yield a lattice damage contribution in tin from a 5304 keV alpha particle of about 0.7 keV, indicating gold may be a less desirable material for future absorbers. This is consistent with an increased displacement energy for Au (43 eV) as opposed to Sn (22 eV) as quoted in [14]. As evidenced by the signal to noise ratio calculations, there is still room for improvement in detector resolution, since we are not yet in a regime that is limited by lattice damage. To explore the contributions of the daughter nucleus to lattice damage, we placed an external source by the detector in the configuration shown in Fig. 1(c). Resolution degradation in the alpha peaks would be primarily lattice damage from the alpha particles. Therefore, additional broadening in the -value peaks is likely due to lattice damage from the recoil nucleus, since other sources of peak broadening from detector response is the same for both alpha and peaks. SRIM calculations have placed bounds on broadening from lattice damage due to a 95 keV recoil nuclei between 1 and 2 keV. In all type sources, peak widths were larger than those of the alpha peaks measured simultaneously. D. Peak Offset From Lattice Damage The question of which peaks to use for calibration is highly pertinent when the spectra contain both alpha and peaks. Because measurements involve thermalization of both alpha particles and the recoil nucleus, it is likely that lattice damage will be different than alpha measurements. By calibrating the energy spectrum using the alpha lines in a simultaneous measurement of alpha energies and values from Po and Po, we are able to see what the difference is in energy thermalized between alpha and measurements (see Fig. 8). If measurements have the same amount of energy lost to lattice damage, a linear calibration with the alpha lines would result in the peak being calibrated to the correct energy. A deviation in this peak from the book value could indicate a difference in energy thermalization. From the spectrum in Fig. 8, we were able to do a two-point calibration using the prominent alpha peak from Po and the alpha peak from Po to determine that the Po peak was 1.4 keV below the expected value. This energy difference is thought to be caused by lattice damage due to the recoil nucleus. As a secondary test, we were able to calibrate the spectrum using the two peaks and determine that the Po alpha peak was 2.7 keV higher than expected. Part of this energy discrepancy is because the detectors are less linear at higher energies, but the energy offset was in the right direction, indicating more energy is thermalized in alpha measurements. The detectors are nonlinear in such a way that higher energies have less voltage response, so a measurement of a keV difference in the centroid is a lower bound on the offset due to lattice damage. While we know the detectors to be relatively linear, we hope to get more accurate measurements on this energy thermalization


Fig. 8. Alpha and spectrum for Po and Po. The combined alpha/ spectrum on top can be used to probe lattice damage differences in alpha and measurements. The alpha peaks from Po and Po were used as calibraPo peak. The measured centroid of tion points to probe the offset in the the peak after calibration, seen in the histogram at bottom, was 4977.8 keV, lower than the book value by 1.4 keV. This indicates that less of the energy is thermalized with measurements, likely due to lattice damage from the recoil nucleus.

difference by calculating the linearity of the detector more effectively using an external source with more alpha lines. Other factors we will have to investigate more carefully are drift correction and effects of the optimal filtering algorithm. VI. CONCLUSION spectroscopy works well with Po sources and we have achieved record energy resolution, with consistently less than 3 keV FWHM on measurements of Po and Po. In future work, we will continue to investigate whether source preparation plays a role in the peak splitting observed from embedded actinide sources. Continued analysis of lattice damage due to alpha particles and recoil nuclei in both gold absorbers and other absorbers will also be a part of future work in spectroscopy. REFERENCES [1] M. P. Croce et al., “Superconducting transition-edge sensor microcalorimeters for ultra-high resolution alpha-particle spectrometry,” IEEE Trans. Nuclear Sci., accepted for publication. [2] E. Steinbauer et al., “Energy resolution of silicon detectors: Approaching the physical limit,” Nucl. Instr. Meth. Phys. Res. B, vol. 85, pp. 642–649, 1994. [3] S. H. Moseley, J. C. Mather, and D. McCammon, “Thermal detectors as x-ray spectrometers,” J. Appl. Phys., vol. 56, no. 5, pp. 1257–1262, 1984. [4] “A Procedure for the Rapid Determination of Pu Isotopes and Am-241 in Soil and Sediment Samples by Alpha Spectrometry” 2009, IAEA Analytical Quality in Nuclear Applications No. IAEA/AQ/11, IAEA. [5] J. L. Parus and W. Raab, “Determination of plutonium in nuclear materials with the combination of alpha and gamma spectrometry,” Nucl. Instr. Meth. Phys. Res. A, vol. 369, pp. 588–592, 1996. [6] S. P. LaMont, S. E. Glover, and R. H. Filby, “Determination of plutonium-240/239 ratios in low activity samples using high resolution alpha-spectrometry,” J. Rad. Nucl. Chem., vol. 234, no. 1–2, pp. 195–199, 1998.

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[12] R. D. Horansky et al., “Measurement of ion cascade energies through resolution degradation of alpha particle microcalorimeters,” J. Appl. Phys., vol. 107, no. 4, p. 044512, 2010. [13] G. Bortels and P. Collaers, “Analytical function for fitting peaks in alpha-particle spectra from Si detectors,” Appl. Rad. Isotopes, vol. 38, no. 10, pp. 831–837, 1987. [14] C. Broeders and A. Konobeyev, “Defect production efficiency in metals under neutron irradiation,” J. Nucl. Mat., vol. 328, pp. 197–214, 2004.