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Electronic transport and event localization in germanium low-temperature detectors for the EDELWEISS Dark Matter experiment
Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
Electronic transport and event localization in germanium low-temperature detectors for the EDELWEISS Dark Matter experiment A. Broniatowski *, N. Mirabolfathi , L. Dumoulin , A. Juillard , L. BergeH Centre de Spectrome& trie Nucle& aire et de Spectrome& trie de Masse, BaL timents 104-108, 91405 Orsay Campus, France Groupe de Physique des Solides, Universite& de Paris VII, 2 Place Jussieu, 75251 Paris Cedex 05, France
Abstract Carrier time-of-#ight measurements are used to identify near-electrode events in a germanium ionization detector operated at cryogenic temperatures. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The purpose of this study is to improve the background rejection e$ciency in ionization/heat germanium detectors for dark matter experiments, by identifying stray ionization events occurring in the near vicinity of the collection electrodes. Such events are known to exhibit a pulse height defect [1], entailing a possible confusion with nuclear recoils, and therefore must be rejected. Based on carrier time-of-#ight measurements, we shall describe a procedure to localize the events with respect to the electrodes. The method is introduced with large energy events (alpha particles and MeV gammas), and preliminary results will be given for lower energies in the tens of keV range.
* Correspondence address. Centre de SpectromeH trie NucleH aire et de SpectromeH trie de Masse, Ba( timents 104-108, 91405 Orsay Campus, France. Tel.: #33-169154590; fax: #33-169155268. E-mail address: [email protected] (A. Broniatowski)
2. Detector description and carrier drift velocity measurements The detector is made of a high-purity germanium crystal cut in the shape of a cylinder 10 mm in height and 20 mm in diameter, with its axis along a 11 0 02 direction. Niobium electrodes (50 nm thick) are evaporated on the top and bottom surfaces of the crystal. The detector is mounted in a dilution cryostat and the ionization signals are read by means of a low noise charge ampli"er (Amptek A250) operated at room temperature. The rise time of the electronics is about 25 ns, allowing su$cient bandwidth for pulse shape analysis. Based on classical time-of-#ight measurements [2], the electron and the hole drift velocities were measured as a function of the electric "eld between 1 and 50 V/cm. Salient results are as follows: (i) The carrier velocities at 50 mK are found in close agreement with data from the literature at about 8 K [3].
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 3 7 5 - 3
SECTION VI.
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A. Broniatowski et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
(ii) The velocities for both types of carriers show a non-linear dependence on the electric "eld, indicative of hot carrier e!ects even at the lower "eld intensities in our measurements. Given these results, a detailed consideration of the di!erent phonon scattering mechanisms is clearly needed in order to explain the energy dissipation processes in these detectors [4].
3. Pulse shape analysis and event localization Fig. 1 represents typical charge signals for MeV photons from a Co gamma source, and for 5.5 MeV alpha particles from an Am source, respectively (in the latter case, the alphas are shot at the centre of an electrode). The form of the gamma
Fig. 2. (a) Charge collection model; < is the collection voltage and E the electric "eld; other notations are de"ned in the text; (b) variation of the charge collection time t as a function of event location along a "eld line; t goes through a minimum at the point ¸v /(v #v ) where the electrons and the holes have equal transit times to the electrodes.
Fig. 1. Typical forms of the ionization signals for MeV photons, in the case that energy deposition occurs in the core of the detector crystal (a), and near a collection electrode (b); temperature: 50 mK; collection voltage: 1.6 V.
signals reduces to a plain combination of straight segments. This is explained by a simple model (Fig. 2a), assuming a uniform collection "eld. Starting from the impact point of a particle, the electrons and the holes (with respective velocities v and v ) drift along the "eld lines. Neglecting any charge losses by trapping or recombination, the collection current is then given by qN(v #v )/¸, where N is the number of electron}hole pairs generated by the particle, ¸ is the electrode separation and q the elementary charge. Depending on the event location with respect to the electrodes, either the electrons or the holes will be collected "rst. As this happens, the current drops to a value qNv /¸ or
A. Broniatowski et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
qNv /¸, depending on which type of carriers re main. The collected charge is just the integral of the current, hence the form of the signal (Fig. 1a). Particular cases are those where an event occurs at a collection electrode, or at a place in the detector where the electrons and the holes have equal transit times to the electrodes, in which cases the signal reduces to a single segment (Fig. 1b). Compared to gammas, alpha particles exhibit a more complex pulse shape, with a gradually varying slope and a slow trailing edge. This is explained by plasma e!ects, arising from the high densities of electron} hole pairs generated along the particle track [5]. Fig. 2b represents, for a given bias polarity, the variation of the charge collection time t as a func tion of the point of impact x between the electrodes. Inverting the polarity amounts to changing x into ¸!x in the "gure. By e!ecting a cut in the values of t , events occurring in a range of distances from the electrodes can thus be removed from the energy spectrum. Consider, as an example, the case that the crystal is irradiated with MeV photons from a Co source (giving an approximately uniform event distribution) together with alpha particles from an Am source. Given the range of the alphas (about 20 lm in germanium), the latter events are practically localized on the x"0 electrode. Fig. 3a represents in solid line the signal amplitude spectrum obtained with a 3.1 V collection voltage. The 1333 keV cobalt line shows up on top of a broad alpha peak, followed by the 1179 keV line ahead of the Compton edge. The alpha particles thus show an extremely poor charge collection e$ciency. Fig. 3b represents a histogram of the signal durations. Removing those events with t '400 ns (corresponding to surface events on the x"0 electrode) yields the spectrum in dotted line, in which the alpha contribution is almost completely eliminated. The price to be paid for this is a reduction in the total number of gamma counts, proportional to the volume fraction of the detector with a charge collection time longer than 400 ns, about 15% in the present case. A key point in this procedure is the measurement of the charge collection time. For this purpose, each of the signals is "tted against a family of templates. A template has its form de"ned by the event location and the carrier drift velocities. The quality of
329
Fig. 3. (a) In full line, the signal amplitude spectrum of a gamma (Co) and an alpha (Am) source; note the broad contribution from the alpha particle signals superimposed on the 1333 keV gamma peak; in dotted line, the spectrum obtained after eliminating the signals with a collection time longer than 400 ns: the alpha contribution is removed from the spectrum; temperature: 50 mK; collection voltage: 3.1 V; (b) histogram of the charge collection times.
the "t depends on the magnitude of the signal compared to baseline #uctuations, which will constitute a limiting factor in the case of low-energy events. It may also depend on space-charge e!ects associated with carrier trapping. A reset procedure is periodically applied to the detector, so that the importance of these phenomena is much reduced [6]. On account of the di!erence in the electron and hole velocities, events occurring at the positively biased electrode have the longest collection time and therefore are more easily recognized than those at the negative electrode. While these points are currently being investigated, we present the "rst results of an experiment with lower-energy gammas. The detector was exposed to a Co source
SECTION VI.
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A. Broniatowski et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
in view of the geometry of the experiment and the rather large photon absorption coe$cient (1.06 mm\) at this energy.
4. Conclusion
Fig. 4. Spectra of a Co (122 and 136 keV) and an Am (59.5 keV) gamma source (see text); the full line is before, and the dotted line after cutting o! those events with a duration greater than 280 ns; temperature: 50 mK; collection voltage: 6.4 V; the energy resolution (FWHM) is about 3.5 keV.
(122 and 136 keV) and an Am source (59.5 keV) facing the electrodes at x"¸ and x"0, respectively. Fig. 4 represents the pulse amplitude spectrum before (full line) and after (dotted line) eliminating those events with the longer t . The magnitude of the 59.5 keV peak is much reduced compared to the rest of the spectrum, as expected
Even though extremely low collection "elds are used compared to conventional surface barrier detectors, these experiments demonstrate the ability of time-of-#ight measurements to identify nearelectrode ionization events in the MeV range of energy. Preliminary measurements suggest a possible extension into the tens of keV region, with a potential application in dark matter experiments.
References [1] T. Shutt et al., Proceedings of the Seventh International Workshop on Low Temperature Detectors, Munich, Germany, 1997, p. 224. [2] C. Canali et al., J. Phys. Chem. Solids 32 (1971) 1707, and references therein. [3] G. Ottaviani et al., IEEE Trans. Nucl. Sci. NS-22 (1975) 192. [4] P.N. Luke, J. Appl. Phys. 64 (1988) 6858. [5] W. Seibt et al., Nucl. Instr. and Meth. 113 (1973) 317. [6] X.F. Navick, Ph.D. Thesis, University of Paris VII, 1997.
Electronic transport and event localization in germanium low-temperature detectors for the EDELWEISS Dark Matter experiment A. Broniatowski *, N. Mirabolfathi , L. Dumoulin , A. Juillard , L. BergeH Centre de Spectrome& trie Nucle& aire et de Spectrome& trie de Masse, BaL timents 104-108, 91405 Orsay Campus, France Groupe de Physique des Solides, Universite& de Paris VII, 2 Place Jussieu, 75251 Paris Cedex 05, France
Abstract Carrier time-of-#ight measurements are used to identify near-electrode events in a germanium ionization detector operated at cryogenic temperatures. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The purpose of this study is to improve the background rejection e$ciency in ionization/heat germanium detectors for dark matter experiments, by identifying stray ionization events occurring in the near vicinity of the collection electrodes. Such events are known to exhibit a pulse height defect [1], entailing a possible confusion with nuclear recoils, and therefore must be rejected. Based on carrier time-of-#ight measurements, we shall describe a procedure to localize the events with respect to the electrodes. The method is introduced with large energy events (alpha particles and MeV gammas), and preliminary results will be given for lower energies in the tens of keV range.
* Correspondence address. Centre de SpectromeH trie NucleH aire et de SpectromeH trie de Masse, Ba( timents 104-108, 91405 Orsay Campus, France. Tel.: #33-169154590; fax: #33-169155268. E-mail address: [email protected] (A. Broniatowski)
2. Detector description and carrier drift velocity measurements The detector is made of a high-purity germanium crystal cut in the shape of a cylinder 10 mm in height and 20 mm in diameter, with its axis along a 11 0 02 direction. Niobium electrodes (50 nm thick) are evaporated on the top and bottom surfaces of the crystal. The detector is mounted in a dilution cryostat and the ionization signals are read by means of a low noise charge ampli"er (Amptek A250) operated at room temperature. The rise time of the electronics is about 25 ns, allowing su$cient bandwidth for pulse shape analysis. Based on classical time-of-#ight measurements [2], the electron and the hole drift velocities were measured as a function of the electric "eld between 1 and 50 V/cm. Salient results are as follows: (i) The carrier velocities at 50 mK are found in close agreement with data from the literature at about 8 K [3].
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 3 7 5 - 3
SECTION VI.
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A. Broniatowski et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
(ii) The velocities for both types of carriers show a non-linear dependence on the electric "eld, indicative of hot carrier e!ects even at the lower "eld intensities in our measurements. Given these results, a detailed consideration of the di!erent phonon scattering mechanisms is clearly needed in order to explain the energy dissipation processes in these detectors [4].
3. Pulse shape analysis and event localization Fig. 1 represents typical charge signals for MeV photons from a Co gamma source, and for 5.5 MeV alpha particles from an Am source, respectively (in the latter case, the alphas are shot at the centre of an electrode). The form of the gamma
Fig. 2. (a) Charge collection model; < is the collection voltage and E the electric "eld; other notations are de"ned in the text; (b) variation of the charge collection time t as a function of event location along a "eld line; t goes through a minimum at the point ¸v /(v #v ) where the electrons and the holes have equal transit times to the electrodes.
Fig. 1. Typical forms of the ionization signals for MeV photons, in the case that energy deposition occurs in the core of the detector crystal (a), and near a collection electrode (b); temperature: 50 mK; collection voltage: 1.6 V.
signals reduces to a plain combination of straight segments. This is explained by a simple model (Fig. 2a), assuming a uniform collection "eld. Starting from the impact point of a particle, the electrons and the holes (with respective velocities v and v ) drift along the "eld lines. Neglecting any charge losses by trapping or recombination, the collection current is then given by qN(v #v )/¸, where N is the number of electron}hole pairs generated by the particle, ¸ is the electrode separation and q the elementary charge. Depending on the event location with respect to the electrodes, either the electrons or the holes will be collected "rst. As this happens, the current drops to a value qNv /¸ or
A. Broniatowski et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
qNv /¸, depending on which type of carriers re main. The collected charge is just the integral of the current, hence the form of the signal (Fig. 1a). Particular cases are those where an event occurs at a collection electrode, or at a place in the detector where the electrons and the holes have equal transit times to the electrodes, in which cases the signal reduces to a single segment (Fig. 1b). Compared to gammas, alpha particles exhibit a more complex pulse shape, with a gradually varying slope and a slow trailing edge. This is explained by plasma e!ects, arising from the high densities of electron} hole pairs generated along the particle track [5]. Fig. 2b represents, for a given bias polarity, the variation of the charge collection time t as a func tion of the point of impact x between the electrodes. Inverting the polarity amounts to changing x into ¸!x in the "gure. By e!ecting a cut in the values of t , events occurring in a range of distances from the electrodes can thus be removed from the energy spectrum. Consider, as an example, the case that the crystal is irradiated with MeV photons from a Co source (giving an approximately uniform event distribution) together with alpha particles from an Am source. Given the range of the alphas (about 20 lm in germanium), the latter events are practically localized on the x"0 electrode. Fig. 3a represents in solid line the signal amplitude spectrum obtained with a 3.1 V collection voltage. The 1333 keV cobalt line shows up on top of a broad alpha peak, followed by the 1179 keV line ahead of the Compton edge. The alpha particles thus show an extremely poor charge collection e$ciency. Fig. 3b represents a histogram of the signal durations. Removing those events with t '400 ns (corresponding to surface events on the x"0 electrode) yields the spectrum in dotted line, in which the alpha contribution is almost completely eliminated. The price to be paid for this is a reduction in the total number of gamma counts, proportional to the volume fraction of the detector with a charge collection time longer than 400 ns, about 15% in the present case. A key point in this procedure is the measurement of the charge collection time. For this purpose, each of the signals is "tted against a family of templates. A template has its form de"ned by the event location and the carrier drift velocities. The quality of
329
Fig. 3. (a) In full line, the signal amplitude spectrum of a gamma (Co) and an alpha (Am) source; note the broad contribution from the alpha particle signals superimposed on the 1333 keV gamma peak; in dotted line, the spectrum obtained after eliminating the signals with a collection time longer than 400 ns: the alpha contribution is removed from the spectrum; temperature: 50 mK; collection voltage: 3.1 V; (b) histogram of the charge collection times.
the "t depends on the magnitude of the signal compared to baseline #uctuations, which will constitute a limiting factor in the case of low-energy events. It may also depend on space-charge e!ects associated with carrier trapping. A reset procedure is periodically applied to the detector, so that the importance of these phenomena is much reduced [6]. On account of the di!erence in the electron and hole velocities, events occurring at the positively biased electrode have the longest collection time and therefore are more easily recognized than those at the negative electrode. While these points are currently being investigated, we present the "rst results of an experiment with lower-energy gammas. The detector was exposed to a Co source
SECTION VI.
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A. Broniatowski et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 327}330
in view of the geometry of the experiment and the rather large photon absorption coe$cient (1.06 mm\) at this energy.
4. Conclusion
Fig. 4. Spectra of a Co (122 and 136 keV) and an Am (59.5 keV) gamma source (see text); the full line is before, and the dotted line after cutting o! those events with a duration greater than 280 ns; temperature: 50 mK; collection voltage: 6.4 V; the energy resolution (FWHM) is about 3.5 keV.
(122 and 136 keV) and an Am source (59.5 keV) facing the electrodes at x"¸ and x"0, respectively. Fig. 4 represents the pulse amplitude spectrum before (full line) and after (dotted line) eliminating those events with the longer t . The magnitude of the 59.5 keV peak is much reduced compared to the rest of the spectrum, as expected
Even though extremely low collection "elds are used compared to conventional surface barrier detectors, these experiments demonstrate the ability of time-of-#ight measurements to identify nearelectrode ionization events in the MeV range of energy. Preliminary measurements suggest a possible extension into the tens of keV region, with a potential application in dark matter experiments.
References [1] T. Shutt et al., Proceedings of the Seventh International Workshop on Low Temperature Detectors, Munich, Germany, 1997, p. 224. [2] C. Canali et al., J. Phys. Chem. Solids 32 (1971) 1707, and references therein. [3] G. Ottaviani et al., IEEE Trans. Nucl. Sci. NS-22 (1975) 192. [4] P.N. Luke, J. Appl. Phys. 64 (1988) 6858. [5] W. Seibt et al., Nucl. Instr. and Meth. 113 (1973) 317. [6] X.F. Navick, Ph.D. Thesis, University of Paris VII, 1997.