We report on the first measurement of the absolute phonon energy and the amount of ionization produced by the recoil of nuclei and electrons in a 60 g germanium cyrstal at a temperature of ≊25 mK. We find good agreement between our results and previous measurements of ionization yield from nuclear recoils in germanium. Our device achieves 10:1 discrimination between neutrons and photons in the few keV energy range, demonstrating the feasibility of this technique for large reductions of background in searches for direct interactions of weakly interacting massive particle dark matter

Barnes, P. D., Jr.

Cummings, Alan

Da Silva, Angela

Ellman, B.

Emes, John

Giraud-Héraud, Y.

Haller, Eugene E.

Lange, Andrew E.

Rich, J.

Ross, Ron R.

Sadoulet, Bernard

Shutt, Tom

Smith, G.

Stockwell, W.

Stubbs, C.

Wang, N.

White, S.

Young, Betty A.

Yvon, D.

English

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Santa Clara University Scholar Commons Physics College of Arts & Sciences 12-14-1992 Measurement of ionization and phonon production by nuclear recoils in a 60 g crystal of germanium at 25 mK Tom Shutt B. Ellman P. D. Barnes Jr. Alan Cummings Angela Da Silva See next page for additional authors Follow this and additional works at: https://scholarcommons.scu.edu/physics  Part of the Physics Commons Recommended Citation Shutt, T., Ellman, B., Barnes, P. D., Cummings, A., Da Silva, A., Emes, J., Giraud-Héraud, Y., Haller, E. E., Lange, A. E., Ross, R. R., Rich, J., Sadoulet, B., Smith, G., Stockwell, W., Stubbs, C., Wang, N., White, S., Young, B. A., & Yvon, D. (1992). Measurement of ionization and phonon production by nuclear recoils in a 60 g crystal of germanium at 25 mK. Physical Review Letters, 69(24), 3425–3427. https://doi.org/10.1103/PhysRevLett.69.3425 Copyright © 1992 American Physical Society. Reprinted with permission. This Article is brought to you for free and open access by the College of Arts & Sciences at Scholar Commons. It has been accepted for inclusion in Physics by an authorized administrator of Scholar Commons. For more information, please contact rscroggin@scu.edu. Authors Tom Shutt, B. Ellman, P. D. Barnes Jr., Alan Cummings, Angela Da Silva, John Emes, Y. Giraud-Héraud, Eugene E. Haller, Andrew E. Lange, Ron R. Ross, J. Rich, Bernard Sadoulet, G. Smith, W. Stockwell, C. Stubbs, N. Wang, S. White, Betty A. Young, and D. Yvon This article is available at Scholar Commons: https://scholarcommons.scu.edu/physics/53 VOLUME 69, NUMBER 24 PH YSICAL REVIEW LETTERS 14 DECEMBER 1992Measurement of Ionization and Phonon Production by Nuclear Recoils in a 60 g Crystalof Germanium at 25 mKT. Shutt ') ( B. Ellman (')' ) P D. Barnes, Jr ' '( A. Cummings (') ) A. Da Silva (' ' 7) JEmes, ' ' Y. Giraud-Heraud, ' E. E. Hailer ' ' ' A. E. Lange, ' R. R. Ross, (' ' ' JRich, ' '(6 B. Sadoulet, ' ' 6 Smith, ' ' " &. Stockwell, ' C. Stubbs, ]),(s) & Wang (&),(6) SWhite, ' B. A. Young, ' and D. Yvon' Center for Particle Astrophysics, University of California at Berkeley, Berkeley, California 94720t iDepartment of Physics, Universt ty of'California at Berkeley, Berkeley, California 94720t iDepartment of Material Science and Mineral Engineering, University of California at Berkeley, Berkeley, California 94720'41Lawrence Berkeley Laboratory, I Cyclotron Road, Berkeley, California 94720t51Department of Physics, University of California at Santa Barbara, Santa Barbara, California 93I06Physique des Particules, Centre d'Etudes Nucleaires de Saclay, 9I19I Gif sur Yv-ette-, CEDEX, FranceDepartment of Physics, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia, Canada V6T IZILaboratoire de Physique Corpusculaire, College de France, 75231 Paris, France(Received 11 September 1992)We report on the first measurement of the absolute phonon energy and the amount of ionization pro-duced by the recoil of nuclei and electrons in a 60 g germanium crystal at a temperature of = 25 mK.We find good agreement between our resu)ts and previous measurements of ionization yield from nuclearrecoils in germanium. Our device achieves 10:1 discrimination between neutrons and photons in the fewkeV energy range, demonstrating the feasibility of this technique for large reductions of background insearches for direct interactions of weakly interacting massive particle dark matter.PACS numbers: 29.40.Wk, 14.80.pb, 61.80.Mk, 63.20.—eIn experiments such as a search for direct interactionsof hypothetical weakly interacting massive particle(WIMP) dark matter or the direct measurement ofcoherent neutrino scattering, a small event rate of nuclearrecoils in the energy range of = 1-30 keV must somehowbe recognized and measured in the presence of a highbackground rate of recoiling electrons from photons andcharged particles [1]. Because nuclear recoils are lessefficient at producing ionization than are recoiling elec-trons, it has long been recognized that the two classes ofevents could be distinguished by simultaneously measur-ing the amount of ionization and the amount of phononenergy produced by interactions in a semiconductor attemperatures well below 1 K [2].We have constructed a 58.4 g germanium device whichperforms this simultaneous measurement at a tempera-ture of =25 mK. The basic operating principles of thisdevice have been described elsewhere [3]. Here wepresent the first direct measurement of the absolute pho-non energy and the amount of ionization produced byboth nuclear recoils from neutrons and electron recoilsfrom Compton scattered photons at low energies. Our re-sults agree well with measurements made by Chasman[4] in the 1960 s with conventional germanium ionizationdetectors and with kinematically determined nuclearrecoil energies. Finally, we demonstrate discriminationbetween electronic and nuclear recoils at energies as lowas a few keV, making this technique very attractive for asearch for WIMP dark matter. Spooner et al. [5] havereported discrimination between neutrons and photons us-ing the same technique with a 0.25 g silicon device, but atenergies greater than about 50 keV.The detector consists of a disk (4 cm diam and I cmthick) of ultrapure p-type germanium, onto which severalneutron-transmutation-doped germanium phonon sensorshave been bonded. The ionization from events is collect-ed by means of a 0.3 to 0.5 V bias applied between identi-cal implanted p+ Ohmic contacts on both faces of thedisk. The detector is operated at a temperature of =25mK in a dilution refrigerator. The front end of the elec-tronics chain is a junction field-effect transistor mountedin the cryostat about 40 cm from the detector. An acbiasing technique is used to avoid I/f noise, microphon-ics, and 60 Hz pickup for our slow ( = 1 msec rise time,=40 msec fall time) phonon signals. Triggering is per-formed on the faster (= 1 Itsec rise time) ionization sig-nal. The time traces for both signals are digitized andfitted on-line to templates constructed by carefullyaveraging many events. This fit in the time domain isequivalent to the optimal filter because the noise is Aat atthe signal frequencies. A mild cut on the "chi-square" ofthe fit rejects pile up and electronically saturated events.In Fig. 1(a) we show the simultaneous measurement ofphonons and ionization from 59.5 keV photons from a'Am source. The source was mounted about 1 cm fromone face of the detector, and collimated to give a rate of—1 Hz on a spot of = 200 pm diameter. A small pieceof aluminum 1 mm thick was used to stop low-energy xrays and a particles. The signals have been scaled so thatthe energies are defined in terms of equivalent energy for1992 The American Physical SocietyVOLUME 69, NUMBER 24 PH YSICAL REVI EW LETTERS 14 DECEM BER 1992OCL80lI-70-60;I-k50 t40-3020 I-r ~r~ t~ %t0.4o 0.2-g$ND0.100 15(i) -- +()(i-- T30This workLindhard, Ref. 9Chasman, Ref. 4Sattler, Ref. 8I45 60 750 10 20 30 40 50 60ionization Energy (keV)8070 8070-60 -.50P40LLI30-20-~ ~~ ~ t ~ ~~ ~CC .'~' ' ~~ l g'~ \g~f'1g~ .p, ~~ ~: ~ ''4 ~ ' 'a~.~ ~ Qphotons: The photon peak is at 59.5 keV in both the ion-ization and phonon measurements. In this conventionphoton and charged particle events lie along th 1' fe ual hqua p onon and ionization energies (for these interac-tions the partitioning into phonons and ionization is ap-proximately independent of the incident energy. ) A num-ber of events can in fact be seen along this line. Frommeasurements with a NaI counter, we ascribe most oft ese background events to Compton scattered 1.45 MeVy rays from K which is present in the walls of the room.e pea is typically lThe rms resolution of the 59.5 k V k llkeV in phonons and 0.7 keV in ionization.Figure I b) shows the response of the detector to neu-trons and photons from a 50 pCi source of Cf dd-tiion to photons from the Am source. This Cf isotopespontaneously fissions, giving a broad spectrum of neu-trons with mean energy near 2 MeV and a higher ratebroad spectrum of photons of comparable energ . W thother conditions the same as for Fig. 1(a), we placed it10-~ V00 10 20 30 40 50 60 70 60Ionization Energy (keV)FIG. 1. Phonon anfor (a) 59.5 eV hionization equivalent energies measure dphoton s, b Tp otons and Compton scatters of b k dac grounp . ( ) he same measurement as (a) with the additionof neutrons and photons from a Cf source.Recoil Energy (keV)FIG. 2. The ratio of photon equivalent ionization energy to~ ~ ~ ~recoil energy for nuclear recoils, given as a function of recoil en-ergy. Also shown are measurements from Refs. [4] and [8], anthe calculation of Lindhard, with his parameter k set to 0.15.Error bars on our data points are statistical only; systematic er-rors are discussed in the text.without collimation outside our cryostat and beneath 28cmof Pb tthroo o give similar rates of photons and tneu ronsroughout the detector. The neutrons are clearly dis-tinguished in Fig. 1(b) as a second line with a greaterslope, i.e., less ionization per unit phonon energy, than thep otons. Their energy spectrum roughly agrees with avery simple Monte Carlo estimate. (Note that a I MeVneutron can deposit at most 53 keV in germanium. )The fractions of the total recoil energy that appear asionization and as phonons can be obtained from the dataas follows. Energetic electrons and holes created in theinitial interaction eventually relax to the band edges viap onon emission. These N charge pairs carr NE fe original recoil energy Eg, where Es,z is the band-gapenergy. The remaining energy appears as phonons, ex-cept in the case of nuclear recoils, which may lose a smallraction of their energy to the formation of crystal de-ects. As the charge carriers are collected they emit theenergy they acquire from the drift field as phonons. Inseparate tests [6] we have concluded that when the elec-trons reach the implanted regions in this (p+- - +) de-vice theey relax to the Fermi level and release their band-gap energy as phonons. The measured phonon enerthen, isE=~E +—f Ir eNV, where f describes any energy lostnon gy,to defects (r =f 1 for no loss), e is the magnitude of theelectron's charge, and V is the charge collection potential=0.5 V for this data set). In Ref. [61 we describe theabsolute calibration of both the phonon energy E and thec arge signal N The "apparent" .recoil energy fE& issimply the total phonon energy minus the drift heat term.The equivalent ionization energy is N times 3.0 eV theamount of energy per charge pair for photon interactionsin germanium at low temperatures [7].In Fig. 2 we plot the ratio of the photon equivalent ion-ization energy to the total recoil energy for nuclear3426VOLUME 69, NUMBER 24 P H YSICA L R EVI EW LETTERS 14 DECEM BER 1992recoils as a function of the recoil energy, along with mea-surements by Chasman [4] and Sattler [8] and the calcu-lation of Lindhard [9]. The data have been binned byrecoil energy, and we have made the tacit assumptionthat f=1. The error bars indicate statistical uncertaintyonly. To first order, errors in the charge calibration scaleout of the ratio, but there is a small effect arising fromthe subtraction of the drift heat term. We estimate thiserror to be less than 2%. The statistical uncertainty onthe recoil energy is also very small, but errors in the cali-bration may amount to several percent.This is the first low-energy measurement of the ioniza-tion yield of a recoiling nucleus by a direct measurementof phonon energy. Previous measurements with conven-tional ionization detectors have used a monoenergeticneutron beam and either kinematics or the decay ofknown nuclear excited states to determine the energy ofthe recoiling germanium nuclei. In principle, this oldermethod determines the relationship between recoil energyand ionization production. However, the amount of pho-non energy generated and thus the amount of energy lostto defects is not determined. In our method the chargeand phonon relationship is measured, but the energy lostto defects, and hence the full recoil energy, is not. Thegood agreement between our result and that of Chasmanindicates that this energy loss is small.Finally, we note that in Fig. 1(b) we can discriminateneutrons and photons at energies as low as the 2 keVequivalent ionization energy trigger threshold. The levelof this discrimination, however, is currently limited byevents, apparent in Figs. 1(a) and 1(b), which lie in theregion of lower ionization efficiency than the photon line.There are two distinct populations of these events, both ofwhich we attribute to incomplete charge collection. Thefirst is due to 59.5 keV photons which land in a "deadlayer" a few tens of microns thick beneath the implantedcontact region. The other population limits the discrim-ination ratio to about 10:I and is associated with thebackground photons which are (presumably) uniformlydistributed in the detector. We believe that these eventssuffer from incomplete charge collection near the perime-ter of the detector disk, presumably due to build up ofcharges on the surface of the germanium. This issue canbe resolved by tests with collimated sources in various lo-cations, and a guard ring or field shaping structure shouldsolve the problem if our explanation is correct. If the re-jection ratio can be improved by a factor of only 10, thenfive such detectors operating in a state-of-the-art low ra-dioactive background environment for one year would besensitive enough to detect neutralino dark rnatter predict-ed by several models [10].This work was supported by the Center for Particle As-trophysics, a National Science Foundation Science andTechnology Center operated by the University of Califor-nia, Berkeley, under Cooperative Agreement No. ADT-88909616, and by the Department of Energy under Con-tract No. DE-AC03-76SF00098. B.A.Y. is a PresidentialFellow of the University of California.[1) M. W. Goodman and E. Witten, Phys. Rev. D 31, 3059(1985); J. R. Primack, D. Seckel, and B. Sadoulet, Annu.Rev. Nucl. Part. Sci. 3$, 751 (1988); P. F. Smith and J.D. Lewin, Phys. Rep. 1$'7, 203 (1990).[2] For an early discussion of this idea, see B. Sadoulet, inProceedings of the Thirteenth Texas Symposium on Relativistic Astrophysics, Chicago, 1986, edited by M. L.Ulmer (World Scientific, Singapore, 1987), p. 260, andreferences therein.[3) A. Cummings et al. , IEEE Trans. Nucl. Sci. 3$, 226(1991); T. Shutt et al. , this issue, Phys. Rev. Lett. 69,3531 (1992).[4] C. Chasman, K. W. Jones, H. W. Kraner, and W. Brandt,Phys. Rev. Lett. 21, 11 430 (1968), and referencestherein. For comparison, two recent measurements usingconventional silicon ionization counters are G. Gerbier etal , Phys. Rev.. D 42, 3211 (1990); B. L. Dougherty, Phys.Rev. A 45, 2104 (1992).[5) N. J. C. Spooner, A. Bewick, G. J. Homer, P. F. Smith,and J. D. Lewin, Phys. Lett. B 273, 333 (1991).[6) Shutt et al. (Ref. [3]).[7) C. A. Klein, IEEE Trans. Nucl. Sci. 15, 214 (1968).[8] A. R. Sattler, F. L. Vook, and J. M. Palms, Phys. Rev.143, 588 (1966).[9] J. Lindhard, V. Nielsen, M. Scharff, and P. V. Thomsen,Mat. Fys. Medd. K. Dan. Vidensk. Selsk. 33, 10 (1963).[10] K. Griest (private communication).3427

Measurement of ionization and phonon production by nuclear recoils in a 60 g crystal of germanium at 25 mK

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Measurement of ionization and phonon production by nuclear recoils in a 60 g crystal of germanium at 25 mK

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