M. Loidl

Purification of molybdenum, growth and characterization of medium volume ZnMoO4 crystals for the LUMINEU program

The LUMINEU program aims at performing a pilot experiment on neutrinoless double beta decay of 100Mo using radiopure ZnMoO4 crystals operated as cryogenic scintillating bolometers. Growth of high quality radiopure crystals is a complex task, since there are no commercial molybdenum compounds available with the required level of purity and radioactive contamination. This paper discusses approaches to purify molybdenum and synthesize compounds for high quality radiopure ZnMoO4 crystal growth. A combination of a double sublimation (with addition of zinc molybdate) with subsequent recrystallization in aqueous solutions (using zinc molybdate as a collector) was used. Zinc molybdate crystals up to 1.5 kg were grown by the low-thermal-gradient Czochralski technique; their optical, luminescent, diamagnetic, thermal and bolometric properties were tested.

First measurement of the beta spectrum of 241Pu with a cryogenic detector.

The LNE-LNHB is developing metallic magnetic calorimeters, a specific type of cryogenic detectors, for beta spectrometry. The aim is the determination of the shape factors of beta spectra. Our latest detector has been designed to measure the spectrum of (241)Pu, a pure beta emitter with an endpoint energy of 20.8 keV. In this paper, the detection principle of metallic magnetic calorimeters is explained and a detailed description is given of the realization of the detector enclosing a (241)Pu source inside the detector absorber. A spectrum resulting from our first measurement is shown and compared with a theoretical spectrum.

Beta spectrometry with metallic magnetic calorimeters

Metallic magnetic calorimeters are a specific type of cryogenic detectors that have been shown to enable precise measurement of the shape of low energy beta spectra. The aim of their use at LNHB is the determination of the shape factors of beta spectra. The beta source is enclosed in the detector absorber, allowing for very high detection efficiency. It has turned out that the type of source is of crucial importance for the correctness of the measured spectrum. Spectra of (63)Ni measured with several sources prepared by drying a NiCl2 solution differ from one another and from theory, whereas spectra measured with electroplated sources are reproducible and agree with theory. With these latter measurements we could confirm the atomic exchange effect down to very low energy (200 eV).

Validation study of a new technique for absolute activity measurement with 4π solid angle metallic magnetic calorimeters

In this paper we present a prototype of a new class of detectors, metallic magnetic calorimeters operating at cryogenic temperatures, which we are developing for absolute activity measurement of low-energy-emitting radionuclides. We explain the detection principle and give a detailed description of the realisation of the prototype, containing an (55)Fe source inside the detector absorber. The analysis of first data taken with this detector is presented and the result of activity measurement compared with liquid scintillation counting. Some ways for reducing the uncertainty that can be achieved with this new technique are proposed.

A new absolute method for the standardization of radionuclides emitting low-energy radiation.

Microcalorimeters (or bolometers) operated at temperatures below 100 mK allow individual counting of photons and electrons with a very low energy detection threshold. The physics is based on the pulse temperature increase of the target (or absorber) of the detector due to the complete absorption of both electrons and photons. Since this target can be constructed with a perfect 4-pi geometry, a bolometer offers potentially a new method for absolute activity measurements of radionuclides emitting low-energy radiation. In this paper we present our first results of a feasibility study of activity standardization of a 55Fe solution with a prototype 4-pi bolometer.

Determination of absolute photon emission intensities of (210)Pb.

Photon emission intensities of (210)Pb have been determined using sources prepared from a standard solution, whose activity was measured by liquid scintillation counting. The absolute γ-ray and X-ray emission intensity was measured by conventional γ-ray spectrometry. Complementary measurements of the L X-ray spectrum were performed using a cryogenic detector, characterized by very high energy resolution and constant detection efficiency. As a result, precise emission intensities of the individual X-ray lines were obtained taking into account the presence of satellite lines.

Status of LUMINEU program to search for neutrinoless double beta decay of 100Mo with cryogenic ZnMoO4 scintillating bolometers

The LUMTNEU program aims at performing a pilot experiment on 0ν2β decay of 100Mo using radiopure ZnMoO4 crystals enriched in 100Mo operated as cryogenic scintillating bolometers. Large volume ZnMoO4 crystal scintillators (∼ 0.3 kg) were developed and tested showing high performance in terms of radiopurity, energy resolution and α/β particle discrimination capability. Zinc molybdate crystal scintillators enriched in 100Mo were grown for the first time by the low-thermal-gradient Czochralski technique with a high crystal yield and an acceptable level of enriched molybdenum irrecoverable losses. A background level of ∼ 0.5 counts/(yr keV ton) in the region of interest can be reached in a large detector array thanks to the excellent detectors radiopurity and particle discrimination capability, suppression of randomly coinciding events by pulse-shape analysis, and anticoincidence cut. These results pave the way to future sensitive searches based on the LUMTNEU technology, capable of approachingand exploring th...

Measurement of the electron capture probabilities of

The ratios of the electron capture probabilities PK, PL and PM of 55Fe have been measured with a metallic magnetic calorimeter, a specific type of cryogenic detector. The 55Fe source was enclosed in the detector absorber, whose dimensions were chosen such that the detection efficiency for Mn K X-rays was larger than 99.99%. Since all electrons and photons emitted by the source are absorbed in the detector, the detection efficiency is virtually 100% for K, L and M captures. The energy threshold was low enough to allow for clear separation of the M captures (~ 80eV) from noise. The capture probability ratios were translated to capture probabilities using the recommended value for the probability of the undetected N captures. The resulting values are in agreement both with the recommended values of PK, PL and PM and with the experimental data of Pengra and coworkers.