SrCl 2 crystal for EC/  + search Presented by J.H. So (KNU)

Electron Capture/  + It is a kind of double  + decay.  + decay has three decay mode, such as EC/EC for less than 1 MeV Q-value, EC/  + for 1 MeV to 2 MeV Q-value,  +/  + for more than 2 MeV Q-value. The Q-value of 84 Sr is MeV, so EC/  + is possible.

List of candidates NuclideDecay channel Q ββ, keV Natural abundance Decay modeLimits of Half-life (yr) 50 CrΕβ v+2v1.8* NiΕβ + Εβ +,2E v+2v 7* * SrΕβ v7.3* Ru2β +,Eβ v+2v 3.1* * Cd2β +, Eβ v+2v 0v 2.4* * SnΕβ v6.1* Xe2β +,kβ v4.2* * Ce2β +,kβ v6.9* * Kr2β +,kβ v+2v 0v 2* * MoΕβ + Εβ +,2E v+2v 1.9* *10 18 EC/EC, EC/  +,  +/  + had never been searched until today. We will search at 765 keV region for EC/  +.

Test setup for SrCl 2 Pure SrCl 2 single crystal grown by CZ- method. (Jungin Kim) Cut and polished in the dimensions of 20 mm diameter and 15 mm height. Wrapped in teflon tape & mounted on 2 inch PMT with bialkali photocathode with the help of optical grease. Whole assembly is encased in a light tight covering. (details in Gulrooh’s talk) SrCl 2 PMT 400 MHz FADC High Voltage Computer SrCl g (Sr 9 g)

Relative light yield The light output of SrCl 2 single crystal is 77% of the light output of the CsI(Tl).

Pulse height spectra of different radioisotopes

Energy resolution Energy (keV) Resolution (FWHM) (%)

α/β α/β light ratio

Fluorescence decay time Fast decay time component is 91 ns (12%) and slow decay time component is 907 ns(88%)

Pulse shape discrimination Good discrimination between α- and γ-pulses, for pulse height corresponding to 1MeV energy range

Experimental Technique  Putting source(EC/ β + ) inside 4π CsI(Tl) crystals.  Annihilation of β + gives the below possible signals.  Using coincidence technique to tag the signals.  Signal in figure 3 is the choice. Also we will use longitudinal back-to-back requirement with position measurement by attenuation length. No Yes Fig:1 Fig:2 Fig:3 511keV

Schematic Preamp400MHz FADC 64MHz FADC ROOT High Voltage ☞ -670 V for SrCl 2 ☞ ~ V for 4 π CsI Threshold : 25 Trigger pulse width : 200 ns For SrCl 2 For 4 π CsI(Tl) 10 X 100 X

Calibration ( 22 Na) We used 511 keV gamma ray for calibration and calculation of preliminary limit.

Signals from SrCl 2 and 4 π CsI(Tl) + 3 days data

SrCl 2 signals without 4 π CsI(Tl) signals 3 days data We want to search here.

Expected SrCl 2 limits Q-value of 84 Sr is MeV & having natural abundance of 0.56%. The interesting energy is 765 keV with 511 keV back to back gamma coincidence. Preliminary limit : 1.84 X years (Previous limit was 7.3 X years.) This preliminary limit is calculated by CsI 0-hit data. After back to back coincidence analysis, it will be 10 times better than now. T 0n 1/2 = (ln2/1.64)  6x10 23    (a/A)   (T/B R) 90% C.L. B = background index in cts/(keV kg year) ⇒ 42,000 R = FWHM energy resolution at Q bb in keV ⇒ 16 % at 511 keV ( 22 Na) A = mass number ⇒ 87.62, e = detection efficiency at Q bb ⇒ 100% a = EC/  + isotope fraction ⇒ 0.05 g T = measured time in year ⇒ 1 year

Pure SrCl 2 single crystal developed for the first time of the neutrinoless EC/β + search. The scintillation properties of SrCl 2 was good for EC/  + search. The preliminary limit is 1.84 X years. (Previous limit was 7.3 X years.) We will take data for 3 months. Further background reduction is expected with triple coincidence between  + signal and back to back  coincidence. We will investigate the contamination of 90 Sr.(  decay Q value = 2.27 MeV) from nucleon fall out. Summary and Future plan