Nuclear Instruments and Methods in Physics Research A 386 ( 1997) 439-442

NUCLEAR INSTRUMENTS

L METHODS IN PHYSICS RESEARCH

Section A ELSEVIER

Development of the bolometer for the P+P+decay experiment Yutaka It0 a,*, Makoto Minowaa, Wataru Ootania, Keiji Nishigakia, Yasuhiro Kishimotoa,

Takayuki Watanabe a, Youiti Ootuka b a Depanmenr of physics, School of Science, University of Tokyo, 7-3-l Hongo, Bunkyo-ku, Tokyo 113. Japan

b Cryogenic Cenrez University of Tobo, 2-l l-16 Yayoi, Bunkyo-ku, Tokyo 113. Japan

Received 19 April 1996; revised form received 12 August 1996

Abstract We developed a bolometer with a 0.5 g CdTe absorber using a high-sensitivity NTD germanium thermistor. An energy

resolution of 24 keV (FWHM) is obtained for 0.66 to 1.8 MeV y-rays. It shows a good linearity of the response in the energy range between 0.66 and 5.5 MeV. The gain is found to be stable for a period of 12 days of continuous operation and relatively insensitive to temperature changes of the refrigerator. The temperature change of the refrigerator for this period is within fl mK. Since this material contains double positron emission (p+/?+) nuclides l”Cd, ‘“*Cd and ““Te, a new series of double beta decay experiments may become possible with this new method.

Nuclear double beta decay without neutrino emission is forbidden by the standard electroweak interaction model of the elementary particle physics. An observation of this pro- cess would imply non-zero mass Majorana neutrinos or exis- tence of the right-handed weak charged current. The longest half-life limits explored so far are obtained from double beta decay (p-p-) experiments on ‘%e and ‘%Xe [ 11. How- ever, the interpretation of the measured half-life in terms of the neutrino mass is model-dependent and sensitive to the approximations used to calculate the nuclear matrix ele- ments. Since these matrix elements are not precisely known, it is important to compare mass limits derived from half- life limits for as many candidate nuclides, e.g. ‘*Se, ‘%d, 1°0Mo and l”Nd, as possible,

In this respect, double positron (p+@) decay as well as positron emission/electron capture( p+/EC) and double electron capture (EC/EC) processes are also to be exper- imentally studied despite a kinematical hindrance of the decays compared with the P-P-decay. We hereafter refer these three process generically as to /3’@ decay. There has also been a theoretical interest in these processes re- cently raised by Hirsch et al. [ 21. Their point is that the P+IEC decay without neutrino emission is much more sensitive to the existence of the right-handed current than to the non-zero neutrino mass compared with the ordinary Ou@-p-decay. These processes are energetically allowed for several other nuciides including 5sNi, @Zn, 92Mo, “Ru.‘%d, ‘**Cd, ‘*‘Te, and so on. Existing half-life limits

* Corresponding author. Tel. +81 3 3812 2111 ext. 7622, fax +81 3 3814

8806, e-mail ito@icepp.s.u-tokyo.ac.jp.

for the @P+decay are not so restrictive as those for the P-P-decay because there have been no high-resolution detectors which contain P+@candidate nuclides [ 31.

The composite bolometers consisting of a large mass ab- sorber and a small thermistor are developed and used for dark matter detection [4] and P-P-decay [ 51 experiments taking advantage of a wide range of materials that can be used. A similar detector is also useful for the @P+decay experiment. In this paper, we report on the development of a bolometer with an absorber of CdTe which contains of @@decay nuclides ‘“Cd, losCd and “‘Te.

Collimator’

Fig. I. B&meter with a CdTe crystal.

Ol68-9002/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved

PUSOl68-9002(96)01166-7

440 P Ito et ol./Nucl. Instr. and Meth. in Phys. Res. A 386 (1997) 439-442

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Time ( set ) Pulse height ( volts )

Fig. 2. (a) Pulse shape for a I.1 MeV y-ray of 65Zn. (b) Pulse height distribution for y-rays of 65Zn.

Fig. 1 shows the apparatus. The CdTe crystal with a di- mension of 10 x 10 x 1 mm3 and mass of 0.5 g is mounted on a mixing chamber in a dilution refrigerator. The crystal is supported by nine thin contact probes. The base temper- ature of the mixing chamber is measured to be 10 mK with a cerium-magnesium-nitrate magnetic susceptibility ther- mometer. The whole system is enclosed in a copper cylinder for the thermal radiation shielding.

The thermistor of the bolometer is made from neutron transmutation doped (NTD) germanium [ 61 with a dimen- sion of approximately 1.5 x 1 x 0.5 mm3. It is glued with GE varnish on the top face of the crystal. Electrical connection is made with a pair of gold wires of 50 pm diameter welded on 100 nm thick gold pads which are vapour-deposited on heavily doped regions of the top face of the sensor. Another end of the each wire is led into a source follower ampli- fier circuit through a pair of about 40 cm-long manganin wires of 50 pm diameter. The manganin wires run through a superconducting tube which shields against electromag- netic interferences. A bias current of 1 nA is applied to the thermistor and the voltage change across the thermistor is read out through the source follower circuit. The source fol- lower circuit with a junction field effect transistor( J-FET), Hitachi 2SK163, is situated at the 4 K stage, the very upper end of the inner vacuum chamber of the dilution refrigera- tor. Since the J-FET does not function at 4 K, it is warmed up to over 100 K by a heater wound around it. The output of the source follower is lead through a coaxial cable to the outside of the refrigerator and connected to a low-noise am- plifier with voltage gain of 4.4 x 104. The amplified signal is then sent to a digital oscilloscope through a double pole RC low-pass filter with a cut off frequency of 100 Hz and recorded by a personal computer.

To study the detector performance, the absorber is irra- diated with a-rays of 24’Am source, which is installed in- side the refrigerator. Various standard y-ray sources are also

placed outside the refrigerator. Fig. 2a shows a typical pulse shape of a single 1.1 MeV

y-ray event from @Zn The rise and decay time are 2.5 and . 90 ms, respectively. The pulse height distribution is shown in Fig. 2b. The energy resolution of 24 keV (FWHM) is obtained at this energy. It is constant within 24 f 1 keV for y-rays with energies in the range between 0.66 and 1.8 MeV. Energy calibration using these y- and a-rays shown in Fig. 3 indicates a good linearity of the response in the energy range between 0.66 and 5.5 MeV. It is also seen that the response of the detector to the y-rays is identical to that of the a-rays.

Long-term stability of the detector response is also stud- ied with a continuous operation of the detector for 72 days without interruption. Fig. 4 shows the mean pulse heights for the standard y-ray sources in the calibration runs and the temperature of the mixing chamber of the dilution refriger- ator as a function of the time. It is found that the detector gain is stable well within the energy resolution and the tem-

Energy ( keV ) Fig. 3. Linearity of the bolometer.

I: Ito et aL/Nucl. Instr. and Meth. in Phys. Res. A 3X6 (1997) 439-442 44

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Fig. 4. (a) Mean pulse height for standard y-ray sources. (b) Temperature of the mixing chamber as a function of time.

perature keeps constant within fl mK during this period. Although the temperature is stable in itself, the changes are apparently large enough as implied by the stability of the detector gain as the specific heat, hence the detector gain, is proportional to the third power of its temperature. It may be explained by the fact that the bolometer is thermally floating from the mixing chamber with the thin supporting contact probes and that the thermistor on it is a heat source as the bias current is applied. As a result of a balance of the heat flow coming from the thermistor and sinking into the mix- ing chamber, the temperature of the bolometer can be much more stable than that of the mixing chamber. This would result in a stable detector gain. There may be another contri- bution to the gain stability. The thermal conductance of the support and the thermistor leads is a function of the temper- ature, and also play a role to compensate the effect of the temperature change.

0.8 t,...,‘,.1’;,I...,,..;..l~ -

8 10 12 14 16 18 20

Base temp. ( mK )

Fig. 5. Mean pulse height for 0.83 MeV y-rays as measured by varying

the temperature of dx mixing chamber.

We tried proving explicitly the relative insensitivity of the detector gain to the temperature change of the mixing chamber by intentionally varying the temperature with a small electric heater attached to the mixing chamber. Fig. 5 shows the mean pulse height for the 0.83 MeV y ray as a function of the temperature of the mixing chamber. It is seen in the figure that the gain is constant within &OS% even if the temperature is varied from 9 to 12 mK.

Using the data taken for the above mentioned long-term

stability test, a lower limit of <y:‘Ec = I .4 x lO’(’ yr was

obtained for the half-life of the neutrinoless “‘Cd @/EC decay. We estimate that the limit could be improved by more than two orders of magnitude if we make use of an array of ten crystals enriched with “‘Cd. It would surpass the present limit of 5.7 x lOI yr [3]. Use of heavier crystals, 5 g each for example, and reduction of cosmic ray backgrounds in a deep underground laboratory would allow the limit to go still 3-4 orders of magnitude higher.

In summary, we have studied the performance of the CdTe bolometer which is to be used for the @@decay exper- iment. The energy resolution of 24 keV (FWHM) is ob- tained for 1 .I MeV y-rays. It shows a good linearity of the response in the energy range between 0.66 and 5.5 MeV. The gain is stable for a period of 72 days of continuous opera- tion and found to be relatively insensitive to the temperature changes of the mixing chamber. The temperature change of the mixing chamber for this period is + 1 mK.

Acknowledgements

The authors are grateful to Dr. Yu.G. Zdesenko for his advice. This work is supported by the lwatani Naoji Foun- dation’s Research Grant, the Grant-in-Aid in Scientific Re- search (A) and the Grant-in-Aid for COE Research by the Japanese Ministry of Education, Science and Culture, and the Yamada Science Foundation.

442 L Ito er al./Nucl. Instr. and Meth. in Phys. Res. A 386 (1997) 439-442

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