Development of low-background Cs\(_2\)ZrCl\(_6\) detectors to study rare decays in Zr isotopes

In this work, we present the first complex study of Cs\(_2\)ZrCl\(_6\) (CZC) scintillating crystals in terms of their chemical purity, crystal radiopurity, scintillating performance and pulse-shape discrimination ability. An ICP-MS measurement exhibits a high purity of the CZC material (overall impurity level less than 10 \(\mu \)g/g). The CZC crystals show a very low content of radionuclides from U/Th natural decay chains. For \(^{40}\)K and \(^{137}\)Cs only limits at the level few mBq/kg were set after more than 650 h of measurement with a high purity germanium (HPGe) detector. Nevertheless, they contained cosmogenic \(^{134}\)Cs with an activity of about 50 mBq/kg. The measurements with two CZC crystals (11 and 24 g) as scintillating detectors over 456.5 d in the low-background DAMA/CRYS facility, deep underground at LNGS, supported a substantial radiopurity of these crystals leading to a counting rate of 0.17\(_{-0.11}^{+0.13}\) (kg\(\cdot \)keV\(\cdot \)yr)\(^{-1}\) at the \(Q_{2\beta }\) of \(^{96}\)Zr (3.35  MeV). These detectors show a high particle-discrimination ability for \(\beta (\gamma )\) and \(\alpha \) particles. Limits on the half-lives of the double beta decays (DBD) of \(^{96}\)Zr and of \(^{94}\)Zr with and without neutrinos emission were also set at the level \(T_{1/2} \sim 10^{17}\)–\(10^{20}\) yr (90\(\%\) C.L.). The detailed analyses of the internal background components will also be useful for further developments of Cs\(_2\)ZrCl\(_6\) low-background scintillating detectors and for the optimization of the experimental conditions.

This manuscript has no associated data or the data will not be deposited. [Authors’ comment: The data produced in this study are contained in the publication.]

1. This uncertainty is related to the used method of measurement, i.e. semi-quantitative mode, where the calibration is over a wide mass range. Better accuracy can be achieved for a certain element only with a dedicated calibration using a standard (i.e. a certified solution with a known amount of the element of interest). However, such precise calibration is typically not performed when acquiring a wide range of elements.

2. It should be emphasized that in this work, when not differently specified, keV means keV electron equivalent.

3. The \(\alpha /\beta \) ratio is the light yield ratio between an internal \(\alpha \) particle and an electron with the same kinetic energy of the \(\alpha \) particle. Its knowledge allows the \(\alpha \) energy scale determination from gamma energy calibration.

4. It should be stressed, that the value of the \(\alpha /\beta \) ratio also depends on many experimental parameters. Here, to evaluate the \(\alpha /\beta \) ratio, the scintillating signals were recorded and analyzed in a 24 \(\mu \)s long time-window.

5. Only statistical errors coming from the data fluctuations were taken into account in the estimations of the lim S values, and systematic contributions have not been included in the half-life limit values, being negligible.

Authors and Affiliations

1. Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133, Rome, Italy

   P. Belli, R. Bernabei, V. Caracciolo, R. Cerulli, A. Leoncini & V. Merlo

2. INFN, Sezione di Roma “Tor Vergata”, 00133, Rome, Italy

   P. Belli, R. Bernabei, V. Caracciolo, R. Cerulli, A. Leoncini & V. Merlo

3. INFN, Sezione di Roma, 00185, Rome, Italy

   F. Cappella & A. Incicchitti

4. Dipartimento di Fisica, Università di Roma “La Sapienza”, 00185, Rome, Italy

   F. Cappella & A. Incicchitti

5. INFN Laboratori Nazionali del Gran Sasso, 67100, Assergi, AQ, Italy

   M. Laubenstein & S. Nisi

6. Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, ON, K7L 3N6, Canada

   S. S. Nagorny

7. Arthur B. McDonald Canadian Astroparticle Physics Research Institute, Kingston, ON, K7L 3N6, Canada

   S. S. Nagorny

8. Department of Chemistry, Queen’s University, Kingston, ON, K7L 3N6, Canada

   V. V. Nahorna & P. Wang

Corresponding author

Correspondence to R. Bernabei.

Communicated by Robert Janssens.

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