Double Beta Decay with the 20 TeO₂ crystal array

Università di Milano Bicocca e INFN sezione di Milano

Università dell'Insubria

INFN Laboratori Nazionali del Gran Sasso

THE DETECTOR

The detector is an array of 20 crystals of TeO₂, each crystal - with a mass of 340 g - is a single particle cryogenic detector (bolometer). The array is operated in a dilution refrigerator installed in Hall A of the INFN Gran Sasso National Laboratories (L'Aquila, Italy) at a depth of 3500 m.w.e.

The experiment started in 1998 (OLD configuration), in 1999 was stopped for the first time to replace 4 natural crystals with 4 enriched crystals (two crystals enriched in 130Te and two in 128Te).

At the beginning of 2001 the experiment was stopped again and the array remounted (NEW configuration) after:

• cleaning of the crystal surfaces

• modification of the copper mounting

• etching of the copper surfaces of the mounting

• improvement of the internal lead shield

The module

• TeO₂ crystal : 340 g  (¹³⁰Te isotopic abundance ~34%)

• NTD Ge thermistor: used to read-out the signal 

• Si resistor: is used as a heater to generate a reference pulse in order to control time stability of the detector

The 20 crystal tower
 

• the 20 detectors are arranged in a tower and closed in a copper vessel

• the vessel is surrounded by a lead shield made with ancient roman lead having a radioactive contamination in 210Pb < 5 mBq/kg

• the entire structure is contained in the vacuum chamber of a dilution refrigerator

• in the NEW tower the internal roman lead shield thickness was increased by 5 cm on the top plate and 2 cm on the lateral coverage

THE CRYOGENIC SET-UP

• the refigerator - made with selected low contamination materials - is surrounded by a 20 cm thick lead shield. 

• in June 2001 a borated PET shield was mounted around the external lead shield of the cryostat

• the entire apparatus is installed in the INFN Underground National Laboratories of Gran Sasso.

DETECTOR PERFORMANCES

Calibration

It is preformed periodically (every 2 weeks) using a mixed U and Th source placed outside the cryostat but inside the external lead shield. The similar performances of the detectors allow to sum the single detector spectra mantaining the good energy resolution of the single detector.

U+Th calibration (~ 3 days): sum of the natural crystals.

U+Th calibration : the 27 keV line due to Te X rays, FWHM ~ 1 keV.

- the total spectrum

- the anticoincidence spectrum

Background

The total statistics collected up to now with the 20 crystal array consists in 8 separate data sets.

For each run the 20 single spectra are summed, the energy resolutions obtained in the 8 runs are similar

A coincidence analysis is used to reject background events.

BACKGROUND AND RADIOACTIVITY

Background : "electron/gamma region" 0-3 MeV

Background: comparison between the OLD and NEW total spectra between threshold and 3 MeV

• all peaks are identified as gammas from: ²³²Th, ²³⁸U, ⁴⁰K, ⁶⁰Co, ²⁰⁷Bi, ⁵⁴Mn

• at ~ 100 keV the bump due to nuclear recoils of alpha decaying atoms

• at ~ 46 keV the peak due to ²¹⁰Pb

• in the NEW spectrum between 30 and 300 keV the neutron activation peaks

• the continuum in the NEW spectrum is reduced by a factor of ~ 2

Background : "electron/gamma region" 0-3 MeV

Background: comparison between the OLD and NEW spectra in the alpha region

• all peaks are identified as alphas from: ²³²Th, ²³⁸U chains

• the strong reduction of the alpha peaks in the NEW run proves that most of the contamination measured in the OLD run was due to U and Th contamination on the surfaces (crystals and/or copper frame) and not in the bulk of the crystals

DOUBLE BETA DECAY RESULTS

PUBBLISHED in: Physics Letters B 486 (2000) 13

Data

The results here presented are obtained using the entire statistics, including the previous experiments performed with 1, 4 and 8 crystals of 340 g each. They include the new statistic obtained with the 20 crystal array in the NEW run.

Double Beta Decay Limits

A combined likelihood is used to evaluate double beta decay limits.