Physics Letters B 265 ( 1991 ) 53-56 North-Holland PHYSICS LETTERS B A search for neutrinoless double 13decay of 48Ca "~ Ke You a, Yucan Zhu a, Junguang Lu a, Hanseng Sun a, Weihua Tian a, Wenheng Zhao a, Zhipeng Zheng ~.b, Minghan Ye ~.b, Chengrui Ching b.c, Tsohsiu HO ..c, Fengzhu Cui d, Changjiang Yu d and Guojing Jiang d a b c a Institute oftligh Energy Physics, Academia Sinica, P.O. Box 918, Beijing 100039, China China Center of Advanced Science and Technology (Worm Laboratoo'), P.O. Box 8730, Beijing 100080, China Institute o.flheoretical Ph.vsics, Academia Sinica, P.O. Box 2735, Beijing 100080, China Institute of Optics and Fine Mechanics, Academia Sinica, Changchun, China Received 10 December 1990; revised manuscript received 5 June 1991 A search for the neutrinoless double [5decay of 4~Ca is carried out in a coal mine near Beijing. Large scintillation crystals of natural CaF2 were used as both detector and 13source. Results obtained after a total of 7588.5 h of data taking give 9.5 × 102~ yr (76% confidence level ) as the lower limit ofthe half-life of neutrinoless double 13decay of 48Ca. 1. Introduction For a long time the nuclear double 13decay without neutrino emission has attracted considerable attention as a test o f lepton n u m b e r conservation and a crucial experiment for the study o f the properties o f neutrinos. Recently several groups have been carrying out double 13 decay experiments on the nuclei 76Go, 828e, l°°Mo, 136Xe, and ~5°Nd. The decay energy of 4SCa-)48Ti+2e is 4.27 MeV, much higher than that o f other potential double 13decay nuclei. The large phase space factor therefore is favorable for c o m p e n s a t i n g the disadvantage o f the small matrix element. Moreover, the higher decay energy will help to avoid a low energy background. In 1966, Ter Mateosian and G o l d h a b e r used a 48Ca enriched C a F 2 ( E u ) crystal as scintillator which contained 1 1.4 g of4SCa. A lower limit o f 2 × 102o yr for the half-life o f neutrinoless double 13 decay o f 4SCa was obtained [1 ]. The crystals were grown by the Harshaw Chemical C o m p a n y and the reported energy resolution o f the crystal was rather poor [ 2 ]. Recently, in the course o f studying the scintillation properties o f several kinds o f crystals, wc found that Project supported by the National Natural Science Foundation of China. the unactivated CaF2 crystals grown by the Institute o f Optics and Fine Mechanics at Chang Chun have a fairly good energy resolution. When quartz window photomultipliers were used, the full energy photoelectron peak o f the 661 keV "/ray o f 137Cs was clearly seen with an energy resolution o f about 7%/x//E ( MeV ) for small CaF2 crystals [ 3 ]. This suggests the possibility to obtain an i m p r o v e d limit on the halflife for 48Ca neutrinoless double [3 decay by using large natural unactivated CaF2 crystals as both the 13source and the detector. Obviously, in comparison with a "thin source" there is the advantage o f a large quantity o f source nuclei. Though the energy resolution o f CaF2 crystals is much worse than that o f Ge crystals, a larger a m o u n t o f 13 source material may somewhat compensate for this disadvantage. A laboratory in a coal mine near Beijing has been set up. The height o f rock above the laboratory is 512 m, equivalent to about 1300 m water. We have run the experiment for a total of 7588.5 h. 2. Detector Four cylindrical CaF2 crystals are stacked together. Each has a cylindrical part about 12.0 cm long and 17.8 cm in d i a m e t e r and a conical part 3.8 cm long 0370-2693/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved. 53 Volume 265, number 1,2 PHYSICS LETTERS B and 10.0 cm in diameter at the smaller end. The weight of each crystal is.9716.5 g, 8684.3 g, 10140.1 g, 8828.6 respectively. The total weight is 37369.5 g which contains 43.0 g of 48Ca. Each crystal is separately canned in an oxygen-free high conductivity copper can. Specially purified MgO powder (K < 27 ppm, U <0.013 ppm) is used as the reflective layer between crystal and can. To reduce the possible adsorption of natural radioactive gas inside the can, we have sealed the crystals in an atmosphere of pure argon. Photomultipliers XP-2041Q with a quartz window are" used to collect the ultra-violet scintillation light while the natural radioactivity in glass is avoided. Each crystal is coupled to one photomultiplier, respectively. The energy response of the CaF2 crystals had been calibrated with gamma radioactive sources such as 2 2 N a , 137Cs, S4Mn as well as electron beams from a microtron for energies up to 10 MeV [4]. The CaF2 crystals have an identical linear energy response to both electrons and y rays from 0.5 MeV to 10.0 McV. Four our large 4aCa crystals, the energy resolution is better than 10% at an energy E = 4 . 2 7 MeV. Fig. 1 is the schematic drawing of the detector assembly. The crystals are surrounded by a plastic scintillator of NE110 as the anti-coincidence veto. Limited by funds, we had to use existing plastic scintillators from our laboratory. The thickness of the scintillator used for veto is 2.5 cm for the sides and bottom and 5.0 cm for the top, the efficiency for the detection of minimum ionization particles is = 100%. 8 August 1991 Steel plates 2.0 cm thick and lead bricks 8-10 cm thick are used as the hard shielding material. CAMAC electronics are used in our on-line data taking system coupled to an IBM P C / X T computer and the data are registered on disk. A LED light source is used for the routine calibration of the detectors. Besides, the background spectrum with the anti-coincidence veto off is recorded at regular time intervals, the 4°K peak is also used for calibration. 3. Experimental results Initial testing of the detector system had been carried out in the above-ground laboratory of the Institute of High Energy Physics, the detector was then moved into the coal mine cave. Fig. 2 shows the background spectra observed inside the cave. Spectrum ( 1 ) is obtained without hard shielding and the anti-coincidence veto, the 4°K and 2°8T1 peaks of natural radioactivity are here seen to be prominent. During the first period, the experiment ran with the first two detectors for 1700.0 h, followed by a run with all four crystals for 5888.5 h. The counts per channel I ". I I I -- O°o ° °% /D r I 10 ~- ooo 103 - A I 2 %,% 6 %eoo ,: 10 2 % ia ". °°%°%2"~J"" % ILl 3 go2_ ~z on° (j°og4 else 10_ ~ - eea 10"2 ~.._ 10-30.0 Fig. 1. Schematic drawing of detector assembly. A: CaF: crystal, B: plastic scintillator, C: steel shielding, D: lead shielding. 54 L L 1.0 2.0 I I 30 4..0 Erie r g),(I,/,ev) I 5.0 6.0 Fig. 2. Background energy spectra underground. 1. spectrum without hard shielding and anti-coincidence veto, 2. spectrum with hard shielding, 3. spectrum with hard shielding and anticoincidence veto. Volume 265, number 1,2 PHYSICS LETTERS B around 4.27 MeV for all runs are shown in fig. 3. No peak is present near 4.27 MeV. The total counts within an energy interval (4.270+0.214) MeV are 365. For a rough estimate, the lower limit of the halflife of the neutrinoless 48Ca double 13decay was extracted by using the "sensitive formula" introduced by Fiorini [ 5 ] T~/z>~AX yr, (1) where A is a constant, A = ( 0 7 6 X l n 2 X 6 . 0 2 3 X 1033Xa)/(mX8760), a=0.187% is the abundance of 4SCa, m = 78 is the molecular weight of CaF2. As we assume that the peak of electrons has a gaussan distribution, the area within the energy resolution should be 0.76. MT is the total weight of all crystals times the time of data taking. E is the FWHM of the energy resolution of the detectors in keV, B is the counting rate in the energy region of interest in counts/h/keV/g. In our experiment we have A=8.68X 10 ~4, B = 3 . 4 0 X 10 . 9 counts/h/keV/g, M T = 2.51 X 108 h g, E=427.0 keV, then Ti/2 >11.14X 1022 yr. (2) However, the same data can be treated in another way. We have fitted the experimental data in a rather wide energy range from 3.5 MeV to 6.0 MeV using an exponential function. The z2/DOF(545/315) was reasonable. From this smooth background shape the 30, , , , , 20 1~ , t 25 > . -q I 8 August 1991 Table 1 All the experimental results concerning the 4SCa 0v2~ decay. Experiment Quantity Duration Detector of'SCa [hl T~/2 [yr] [gl Goldhaber'ql.4 Wu b~ 10.6 present exp. 43.0 "~ Ref. [ l l . 689 1150 7588.5 CaF2 (Eu) 2 Xl02° streamer chamber 2 X 102~ CaF2 9.5 X 10~ h~ Ref. [61. Table 2 The light neutrino mass (my) and the right handed current mixing parameter ~1. Experiment Life-time [yr] (mv)(r/=0) [eV] r/((m,,) = 0 ) Goldhaber Wu this experiment 2 Xl02° 2 ×102~ 9.5X 102~ ~<59.5 ~<18.8 ~< 8.3 ~<5.09× 10 -S ~< 1.61X 10 -~ ~<0.74× 10 -2 expected background counts centered at E = 4 . 2 7 MeV within 10% of the energy resolution are 355. Therefore, the upper limit should be estimated from the background statistical error which is quite large. If 1.18 cr is taken the lower limit of the half-life of 48Ca is obtained to be Tt/2>9.5× 102~ yr (76%CL) . (3) In table 1 all the experimental results so far obtained concerning the 48Ca 0v213decay are summarized. Using the experimental results, the upper limits for the light neutrino mass ( m y ) and the right handed current mixing parameter r/ can be calculated and these are presented in table 2. The theoretical formulae which we used are taken from refs. [ 7,8]. -- • Acknowledgement I . . . . . . . . 3.5 4.5 MEv We are very grateful to the Mentougou Coal Mine for the generous support which is very crucial to the running of the experiment. We would thank Professor J.P. Li, J.M. Wu, S.I~. Liu and others of the Radiation Physics Department of the Institute of High Energy Physics for their valuable help and Professor T.P. Li, M. Wu for useful discussions. Fig. 3. Energy spectrum near 4.27 MeV underground. 55 Volume 265, number 1,2 PHYSICS LETTERS B References [ 1 ] E. Ter Mateosian and M. Goldhaber, Phys. Rev. 146 (1966) 810. [ 2 ] J. Menefee et al., IEEE trans. NS- 13 (1966) 720. [ 31 Y.C. Zhu et al., Mod. Phys. Lett. A 1 ( 1986 ) 231. [41W.H. Tian et al., Mod. Phys. Left. A 4 (1989) 213. 56 8 August 1991 [5] E. Fiorini, Proc. Intern. Symp. on Nuclear 13 decay and neutrino (Osaka) (World Scientific, Singapore, 1986) p. I 1. [6] R.K. Bardin, P.J. Gollon, J.D. Ullman and C.S. Wu, Nucl. Phys. A 158 (1970) 337. [ 7 ] C.R. Ching, T.H. Ho and X.R. Wu, Phys. Rev. C 40 ( 1989 ) 304. [ 8 ] H.F. Wu et al., Phys. Lett. B 162 ( 1985 ) 227.