COBRA
Cadmium-Telluride O-neutrino double-Beta Research Apparatus

 

 

The Idea

COBRA will use CdZnTe semiconductor detectors to search for rare double beta-decay processes. The largest CdZnTe detectors which can be built by current crystal growth technology have a size of about 1 cm³ which corresponds to roughly 6 grams. As much larger source masses are required, the crystals will be arranged in a large 3 dimensional array which can easily be extended as required.

A single 1cm³ CdZnTe detector. For more information see the official COBRA homepage.

Double Beta Decay

Double beta decay. 2 neutrino mode (left) and 0 neutrino mode (right).
Double Beta Decay is a rare process in which two nuclei in the same nucleus simultaneously beta decay, releasing two electrons and two anti-neutrinos:

This rare decay has a half-life of about 10²⁰ years but is allowed as a second order process in the Standard Model of particle physics for a handful of nuclei that cannot decay via single beta-decay. In fact, there are only 35 known nuclei that can undergo this process. Although it has not yet been observed, there is far more interest in another process in which no neutrinos are released - neutrinoless double beta decay (0&nußß):

Observation of these decays would have a significant impact on our understanding of neutrino physics as this process is not allowed for standard "Dirac" neutrinos. However, it can occur if neutrinos are massive "Majorana" particles, as preferred by a number of theoretical models. Furthermore, the half-life of this process is directly related to the effective neutrino mass. Other mechanisms that would realise zero neutrino double beta decay include right handed weak currents and R-parity violating super symmetry. Right handed currents play a more important role in ß⁺ß⁺-decays where two positrons are emitted from the same nuclei:

Measurement of these other decay modes may help to disentangel the underlying physics mechanism. However, the rate for ß⁺ß⁺-decay is rather small and the process is only energetically possible for seven nuclides, one of them being ¹⁰⁶Cd. ß⁺ß⁺ is always accompanied by double electron capture (EC/EC) or ß⁺/EC-decay for which shorter half-lives are predicted, but due to their low-energy signature, they are a lot harder to detect.

Experimental Status

A number of R&D stages are required to prove the validity of the COBRA technique for ßß measurements.

Proto-type setup

In the current phase of the experiment, four 1cm³ crystals are being operated in the Gran Sasso laboratory. The detector array is shielded by clean copper and lead. Additionally, a boron-loaded polyethylene and paraffin neutron shield is installed outside to prevent neutrons entering the array. This avoids background events from neutron capture on ¹¹³Cd which has a very large cross-section.

Copper and Lead shielding under construction (left) and neutron shielding (right).

64 Crystal setup

In Autumn 2005, the next stage of the experiment will be installed in the Gran Sasso laboratory. Sixty four crystals will be mounted in a 4*4*4 array within the existing background shield. With this array all the basic principles of signal detection, data acquisition, and background reduction will be explored including the use of coincidences between crystals as a method of background rejection.

Artist's impression of the 64 crystal array. (courtesy of D. Dobbos)

A Very Large Array

The ultimate aim of the COBRA experiment is to detect extremely rare 0&nußß decays which requires a much larger mass of candidate isotopes. Therefore, after the validity of CdZnTe as a detector material has been confirmed, a very large array of crystals (approximately 100kg) is proposed.

Modules for a large array of CdZnTe detectors.

Advantages of CdZnTe

There are a number of reasons why CdZnTe is a suitable candidate for 0?ßß searches:

Source = Detector.
Using the same material as both the source of ßß-decay isotopes and the detector of the emitted electrons reduces the amount of material in the experiment and thus minimises the amount of background from naturally occurring radioactive isotopes in the experiment. The electrons emitted in ßß-decays can be detected with a high detection efficiency in the semiconductor.

Candidate Isotopes.
CdZnTe contains 9 ßß-decay candidates, all of them can be studied in just one experiment. The most promising candidate is ¹¹⁶Cd. With a Q-value of 2.8 MeV the experimental signature of this decay lies above the highest naturally occurring gamma-line from ²⁰⁸Tl decay at 2.614 MeV. ¹³⁰Te is also a promising candidate with a natural abundance of 34.5 % and a high Q-value of 2.53 MeV. ß⁺ß⁺ decays of ¹⁰⁶Cd can also be searched for in this material.

Good Energy Resolution.
3.5 % FWHM at 662 keV are commonly achieved with 1 cm³ CPG-detectors commercially available. 2.2 % FWHM at 2.614 MeV has been measured in the COBRA test facility. We are also researching the possibility of improving the energy resolution by cooling the crystals by 10-20 degrees.

Room Temperature Operation.
Unlike other semi-conductors (eg. Germanium) CdZnTe can be operated safely and efficiently at room temperature so no cryostats are required.

Detector Technology.
Industrial support is available thanks to great interest in medical applications of CdZnTe.