G. Drexlin

Current Direct Neutrino Mass Experiments

In this contribution, we review the status and perspectives of direct neutrino mass experiments, which investigate the kinematics of -decays of specific isotopes (3H, 187Re, 163Ho) to derive model-independent information on the averaged electron (anti)neutrino mass. After discussing the kinematics of -decay and the determination of the neutrino mass, we give a brief overview of past neutrino mass measurements (SN1987a-ToF studies, Mainz and Troitsk experiments for 3H, cryobolometers for 187Re). We then describe the Karlsruhe Tritium Neutrino (KATRIN) experiment currently under construction at Karlsruhe Institute of Technology, which will use the MAC-E-Filter principle to push the sensitivity down to a value of 200 meV (90% C.L.). To do so, many technological challenges have to be solved related to source intensity and stability, as well as precision energy analysis and low background rate close to the kinematic endpoint of tritium -decay at 18.6 keV. We then review new approaches such as the MARE, ECHO, and Project8 experiments, which offer the promise to perform an independent measurement of the neutrino mass in the sub-eV region. Altogether, the novel methods developed in direct neutrino mass experiments will provide vital information on the absolute mass scale of neutrinos.

The high resolution neutrino calorimeter KARMEN

Abstract KARMEN is a 56 t scintillation calorimeter designed for beam dump neutrino experiments at the neutron spallation facility ISIS of the Rutherford Appleton Laboratory. The calorimetric properties are demonstrated by cosmic muons and laser calibration. The measured energy resolution of the detector is σ E / E ≈ 11.5%/√ E [MeV], the position resolution σ x = 5 cm and the timing resolution σ t ≈ 350 ps.

Anomaly in the time distribution of neutrinos from a pulsed beam stop source

Abstract Analysis of the charged and neutral current reactions 12 C (ν e , e − ) 12 N and 12 C (ν, ν′) 12 C ∗ induced by neutrinos from π+- and μ+-decays at rest reveals an anomaly in the time distribution after all π+ have decayed: the measured time constant for subsequent events differs substantially from the value of 2.2 μs corresponding to the μ+ lifetime. This anomaly cannot currently be explained by background processes or errors in the experimental set-up. A satisfactory description of the time spectrum is achieved by assuming it has two components, one exponential with a 2.2 μs time constant, the other a Gaussian signal of 83±28 events at 3.6 μs after beam-on-target. A speculative explanation, but one fully consistent with all the data, is that these delayed events originate from the decay of a slowly moving (β∼0.02) massive neutral particle produced in the beam stop. Further measurements to improve statistical significance are necessary.

Monitoring of the operating parameters of the KATRIN Windowless Gaseous Tritium Source

The KArlsruhe TRItium Neutrino (KATRIN) experiment will measure the absolute mass scale of neutrinos with a sensitivity of m??=?200?meV/c2 by high-precision spectroscopy close to the tritium ?-decay endpoint at 18.6?keV. Its Windowless Gaseous Tritium Source (WGTS) is a ?-decay source of high intensity (1011?s?1) and stability, where high-purity molecular tritium at 30?K is circulated in a closed loop with a yearly throughput of 10?kg. To limit systematic effects the column density of the source has to be stabilized at the 10?3 level. This requires extensive sensor instrumentation and dedicated control and monitoring systems for parameters such as the beam tube temperature, injection pressure, gas composition and so on. In this paper, we give an overview of these systems including a dedicated laser-Raman system as well as several ?-decay activity monitors. We also report on the results of the WGTS demonstrator and other large-scale test experiments giving proof-of-principle that all parameters relevant to the systematics can be controlled and monitored on the 10?3 level or better. As a result of these works, the WGTS systematics can be controlled within stringent margins, enabling the KATRIN experiment to explore the neutrino mass scale with the design sensitivity.

Monitoring of all hydrogen isotopologues at tritium laboratory Karlsruhe using Raman spectroscopy

We have recorded Raman spectra for all hydrogen isotopologues, using a CW Nd:YVO4 laser (5 W output power at 532 nm) and a high-throughput (f/1.8) spectrograph coupled to a Peltier-cooled (200 K) CCD-array detector (512 × 2048 pixels). A (static) gas cell was used in all measurements. We investigated (i) “pure” fillings of the homonuclear isotopologues H2, D2, and T2; (ii) equilibrated binary fillings of H2 + D2, H2 + T2, and D2 + T2, thus providing the heteronuclear isotopologues HD, HT, and DT in a controlled manner; and (iii) general mixtures containing all isotopologues at varying concentration levels. Cell fillings within the total pressure range 13–985 mbar were studied, in order to determine the dynamic range of the Raman system and the detection limits for all isotopologues. Spectra were recorded for an accumulation period of 1000 s. The preliminary data evaluation was based on simple peak-height analysis of the ro-vibrational Q1-branches, yielding 3σ measurement sensitivities of 5 × 10−3, 7 × 10−3, and 25 × 10−3 mbar for the tritium-containing isotopologues T2, DT, and HT, respectively. These three isotopologues are the relevant ones for the KATRIN experiment and in the ITER fusion fuel cycle. While the measurement reported here were carried out with static-gas fillings, the cells are also ready for use with flowing-gas samples.

First observation of the neutral current nuclear excitation 12C(v,v′)12C∗(1+, 1)

Abstract The neutral current nuclear excitation 12 (v,v′) 12 C ∗ (1 + , 1; 15.1 MeV) has been observed for the first time. For v e and v μ from μ + -decay at rest the flux averaged cross section was determined to be NC (v e + v μ ) > = [10.8±5.1 (stat.) ±1.1 (syst.)]×10 −42 cm 2 .

Sensitivity of Next-Generation Tritium Beta-Decay Experiments for keV-Scale Sterile Neutrinos

We investigate the sensitivity of tritium β-decay experiments for keV-scale sterile neutrinos. Relic sterile neutrinos in the keV mass range can contribute both to the cold and warm dark matter content of the universe. This work shows that a large-scale tritium beta-decay experiment, similar to the KATRIN experiment that is under construction, can reach a statistical sensitivity of the active-sterile neutrino mixing of sin2θ ~ 10−8. The effect of uncertainties in the known theoretical corrections to the tritium β-decay spectrum were investigated, and found not to affect the sensitivity significantly. It is demonstrated that controlling uncorrelated systematic effects will be one of the main challenges in such an experiment.

Monitoring of tritium purity during long-term circulation in the KATRIN test experiment LOOPINO using Laser Raman Spectroscopy

Abstract The gas circulation loop LOOPINO has been set up and commissioned at Tritium Laboratory Karlsruhe (TLK) to perform Raman measurements of circulating tritium mixtures under conditions similar to the inner loop system of the neutrino-mass experiment KATRIN, which is currently under construction. A custom-made interface is used to connect the tritium containing measurement cell, located inside a glove box, with the Raman setup standing on the outside. A tritium sample (purity > 95 %, 20 kPa total pressure) was circulated in LOOPINO for more than three weeks with a total throughput of 770 g of tritium. Compositional changes in the sample and the formation of tritiated and deuterated methanes CT4-nXn (X=H,D; n=0,1) were observed. Both effects are caused by hydrogen isotope exchange reactions and gas-wall interactions, due to tritium β decay. A precision of 0.1 % was achieved for the monitoring of the T2 Q1-branch, which fulfils the requirements for the KATRIN experiment and demonstrates the feasibility of high-precision Raman measurements with tritium inside a glove box.

Cross section of the charged current reaction 12C(ve, e−)12Ng.s.

Abstract The charged current nuclear transition 12 C(v e , e − ) 12 N g.s. has been observed in the KARMEN experiment. The flux average cross section for ve from μ+ decay at rest is determined to be 〈σ〉 = [8.1±0.9(stat.)±0.75 (syst.)]×10−42cm2. For the first time also the energy dependence of the cross section has been measured for neutrino energies up to 50 MeV.

Radon induced background processes in the KATRIN pre-spectrometer

The KArlsruhe TRItium Neutrino (KATRIN) experiment is a next generation, model independent, large scale tritium β-decay experiment to determine the effective electron anti-neutrino mass by investigating the kinematics of tritium β-decay with a sensitivity of 200 meV/c2 using the MAC-E filter technique. In order to reach this sensitivity, a low background level of 10−2 counts per second (cps) is required. This paper describes how the decay of radon in a MAC-E filter generates background events, based on measurements performed at the KATRIN pre-spectrometer test setup. Radon (Rn) atoms, which emanate from materials inside the vacuum region of the KATRIN spectrometers, are able to penetrate deep into the magnetic flux tube so that the α-decay of Rn contributes to the background. Of particular importance are electrons emitted in processes accompanying the Rn α-decay, such as shake-off, internal conversion of excited levels in the Rn daughter atoms and Auger electrons. While low-energy electrons (<100 eV) directly contribute to the background in the signal region, higher energy electrons can be stored magnetically inside the volume of the spectrometer. Depending on their initial energy, they are able to create thousands of secondary electrons via subsequent ionization processes with residual gas molecules and, since the detector is not able to distinguish these secondary electrons from the signal electrons, an increased background rate over an extended period of time is generated.