fulltext

Feasibility of bb decay searches with Ce isotopes

using CeF

scintillators (*)

R. BERNABEI(1), P. BELLI(1), A. INCICCHITTI(2), D. PROSPERI(2)

C. BACCI(3), F. DENOTARISTEFANI(3), G. J. DAVIES(4)(**) and C. J. DAI(5)

(1) Dipartimento di Fisica, Università di Roma “Tor Vergata” - Roma, Italy

INFN, Sezione di Roma II - I-00133 Roma, Italy

(2) Dipartimento di Fisica, Università di Roma “La Sapienza” - Roma, Italy

INFN, Sezione di Roma - I-00185 Roma, Italy

(3) Dipartimento di Fisica, Università di Roma TRE - Roma, Italy

INFN, Sezione di Roma - I-00165 Roma, Italy

(4) INFN, Laboratorio Nazionale del Gran Sasso - 67010 Assergi (AQ), Italy (5) IHEP, Chinese Academy - P.O. Box 918/3, Beijing 100039, PRC

(ricevuto il 5 Febbraio 1997; approvato l’11 Marzo 1997)

Summary. — A feasibility study of bb decay searches with Ce isotopes using CeF3

scintillators has been carried out deep underground at the Gran Sasso National Laboratory (LNGS). The first experimental limits on the bb-0n decay of136_{Ce and} 142_{Ce are presented.}

PACS 23.40 – b decay; double b decay; electron and muon capture. PACS 14.60.Pq – Neutrino mass and mixing.

PACS 14.60.St – Non-standard-model neutrinos, right-handed neutrinos, etc.

1. – Introduction

Scintillators have played a significant role in the last years in many research fields of underground physics, such as Dark Matter searches [1], bb decay searches [2, 3], solar neutrinos [4], studies on neutrino properties [5], etc. However, to use them successfully in underground experiments searching for rare events, large efforts were necessary to improve their original “sea level” features, mainly with regard to the radiopurity. In many cases this goal has been reached successfully; therefore we point out here the interest in considering seriously the possibility of similar efforts for the radiopurification of the new CeF3scintillator recently developed by the Crystal Clear

collaboration [6] for completely different purposes, such as electromagnetic calorime-try at future accelerators.

(*) The authors of this paper have agreed to not receive the proofs for correction. (**) Permanent address: Blackett Laboratory, Imperial College, London, UK.

Three years ago we considered the possibility of using a small CeF3scintillator for a

particle Dark Matter search deep underground, profiting from the very favourable19F spin factor for the WIMP-nucleus spin-dependent interaction [7]. However, the resulting energy threshold (about 20 keV verified by using 109_Cd, 241_{Am and} 57_Co

sources) and the poor radiopurity discouraged us from pursuing this, preferring to use CaF2(Eu) scintillators [8]. More recently we have taken advantage of the availability of

a test shield (empty for short time) and of a large CeF3detector to further verify the

typical intrinsic response of “large” commercial CeF3deep underground.

Moreover, several authors have pointed out the relevance of studying the bb decays of the Ce isotopes [3, 9, 10]. As a by-product of this study, therefore, we have obtained the first experimental limits on the bb-0n decays of 136_{Ce and of} 142_{Ce using the}

source-detector approach originally suggested in [11].

2. – The response of CeF3scintillators deep underground

An extensive study on a large number of CeF3 crystals can be found in [6] where

their scintillation properties are completely described. CeF3 has a high density

(6.16 g/cm3_{), fast response (decay constants: 5 ns the faster and 30 ns the slower one)}

and sufficient light output (55% of BGO; peaks emission: 286, 300, 340 nm).

In fig. 1 we summarise the main features of the response of the small CeF3

scintillator ( 2.2 32.232.5 cm3_{; 0.0745 kg), showing the measured spectrum from a}57_Co

source (122 keV g’s; s/E 422% )

(

)

Fig. 1. – Response of a small CeF3 scintillator—(2.2 3 2.2 3 2.5) cm3—showing a) the measured

spectrum from a57_{Co source (122 keV gammas) and b) the higher energy background measured}

underground during 88.25 hours

(

)

crystal was directly coupled to an EMI9265B46/FL, 29 diameter photomultiplier (PMT) and the shield was similar to the one described later. For this crystal the most relevant contamination peaks were at 46.5 and C200 keV in the lower energy spectrum (not shown here) and around 885 keV in the higher one

(

)

In our more recent measurements deep underground we have used a large commercial 14 3232 cm3 _{crystal, i.e. 0.345 kg of target material. The detector,}

surrounded by 5 cm of low-radioactivity copper, was closed inside a Plexiglas box, continuously flushed with high-purity nitrogen gas (long stored deep underground) and maintained at about 1.1 atm to avoid the presence of residual environmental radon. The Plexiglas box was placed inside a shield made of 10 cm of low radioactivity copper, 15 cm of low radioactivity lead, 1.5 mm of Cd and 4/10 cm polyethylene/paraffin. The special plastic material (Supronyl) that envelops the whole shield assures a first-level radon reduction and allows us to maintain a nitrogen atmosphere inside the whole shield.

The CeF3 crystal (produced by Preciosa-Crytur a Czech Company) was coupled

through two 39 diameter by 10 cm long lightguides (TETRASIL-B on one side and non-UV plexiglas on the other) to low background EMI9265B53/FL, 39 diameter PMTs. The materials used to build these PMTs have been pre-selected by EMI and, then, samples have had their radiopurity measured in the low background facility at

LNGS using high-radiopurity germanium detectors. The crystal was wrapped in Teflon diffuser as were the light guides. In the present case, the poor intrinsic radiopurity of the detector was the dominant feature of the measured energy spectrum.

For each event the arrival time, the ADC values and the summed pulse shape from a Lecroy transient digitiser were recorded. The PMT connected through the TETRASIL-B light guide gave the trigger. The detector has been calibrated using

137

Cs and22_{Na sources; the energy resolution was s/E 429% at 662 keV and the lowest} reachable threshold, verified by using57_{Co source, was in this case about 150 keV.}

In fig. 2 the whole energy spectrum collected deep underground with this large CeF3 crystal is shown. Clearly there is a large contamination peak around 885 keV.

Considering the features of the CeF3scintillator and the presence of the same type of

contamination in the small detector described above, we consider that this peak may be due to internal a contamination from U/Th chains. Considering the overall experimental situation, we have focused our attention on the higher energy part of the spectrum to obtain the first experimental limits on the bb-0n decay of136_{Ce and of}142_Ce.

These limits can be obtained, although here with limited sensitivity, by looking for Gaussian peaks on top of the background spectrum.

3. – First experimental limits on the bb-0n decays of Ce isotopes

A complete description of the various bb processes for different Ce isotopes can be found in [3].

3.1. The b1_b1_{0n decay of}136_{Ce. – Here we have focused our attention on the b}1_b1

0n decay of 136_{Ce to} 136_{Ba (ground-state–ground-state transition). The mass difference}

between the parent and the daughter atoms is 2400 keV; unfortunately 136Ce constitutes only 0.19% of natural Ce.

No previous experimental limits were available for this process; the theoretical estimates for the lower limit on the mean lifetime are in the 1028 _years

range [10, 12].

In fig. 3a) the energy spectrum collected during 693.1 hours (T) with the large CeF3

is shown. From these data the limit on the mean lifetime of the process can be evaluated according to t D (NeT)Ok_{counts , where N 42.00310}21 is the number of

136

Ce atoms in the detector and e 40.491 is the detection efficiency—calculated by EGS4 code—in the energy window DE 42.0–3.0 MeV, considered here

(

)

At this point we can also consider the data collected with the small CeF3detector

(

)

Ce atoms is 4.32 3 1020 _{and T is 88.25 hours.}

Taking advantage of the better energy resolution

(

)

In this way, from the data of the small detector alone we obtain t D4.0431017 years.

Finally, putting these two results together gives a lower limit for the b1_b1 _0n

decay of136

Fig. 3. – Experimental high-energy distribution collected a) during 693.1 hours to study the b1_b1

0n decay of136_{Ce and b) during 202.1 hours to study the b}2_b2_{0n decay of}142_Ce.

3.2. The b2_b2_{0n decay of}142_{Ce. – We have also considered the b}2_b2_{0n decay of} 142_{Ce to} 142_{Nd (ground-state–ground-state transition). Here the mass difference}

between the parent and the daughter atoms is 1417.1 keV and142Ce constitutes 11.08 % of natural Ce.

No previous experimental limits are available in this case; the theoretical lower limit estimates are in the 1024 _{years range [9]. From the given energy spectrum}

(

)

Then, considering the data in fig. 1b) collected with the small CeF3 detector, its

number of 142_{Ce atoms, the quoted running time (88.25 hours) and the better energy}

resolution, in an energy window of 61s around the expected peak at 1417.1 keV we obtain k_{counts 48.83. In this way, we calculate from the data of the small detector}

alone t D1.9631019_years.

Finally, putting these two results together gives a lower limit for the b2_b2 _0n

decay of142

Fig. 4. – Monte Carlo evaluation of the expected energy spectrum from the b1_b1_{0n decay of} 136_{Ce: a) large detector, b) small detector. The benefit of the better energy resolution of the small}

detector with respect to the large one is evident.

4. – Conclusions

Although the present limits are lower than the available theoretical estimates, they represent the first experimental determinations for these processes and demonstrate the interest in pursuing the development of higher radiopurity CeF3.

Considering that up to now no effort has been put into material selection and growth protocol, significant improvements can be foreseen. Another relevant parameter to reach high sensitivities in this kind of search is the energy resolution (see fig. 4); therefore, as a first generation experiment an array of “small” detectors seems to be most suitable for high-sensitivity experiments on the bb decay of Ce isotopes.

* * *

One of us, G. J. DAVIES, would like to thank INFN for financial support and his

DaMa colleagues for making his stay in Italy so enjoyable.

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