The tSPECT Experiment
Current state-of-the-art methods employ trapping of UCN. The present experimental challenge consists in resolving difficulties associated with losses of UCN during contact with materials on collision with mirror walls. This type of losses can be completely avoided by magnetic trapping of UCN in vacuum. Low-field seeking UCN up to a maximum kinetic energy of 60 neV are reflected by a magnetic field difference of one Tesla. The proposed realisation of the three-dimensional magnetic trap involves the cryo-magnet of the neutron decay spectrometer aSPECT , mounted in horizontal position of adaptable height. It has to be equipped with an additional radial multipolar structure on the inside (Fig.3). UCN from the pulsed source are fed from on end into the aSPECT magnet system (bottom Fig.3). The high-field seeking UCN will cross the magnetic potential barrier whereas low-field seeking UCNs will be reflected. By means of Fast-Adiabatic-Passage (AFP) the spin of the high field-seekers will be flipped, such that they can be stored as low-field seekers in the adjacent three-dimensional magnetic trap with volume V ~ 10 L.
The concept of the aSPECT spectrometer allows for a 4𝜋 detection of the decay protons. From the exponential decrease of the proton count rate, the neutron lifetime 𝜏n can be extracted. In order to control the depolarization (spin flip) of UCN during storage, an UCN detector is installed below the aSPECT magnet. Depolarized UCN (after some collisions with the vertical neutron guide) penetrate through the magnetic barrier and are guided towards the detector. Different settings of the magnetic field strength allow us to measure a possible dependence of tn on the neutron velocity spectrum.
The tSPECT experiment is funded by the PRISMA cluster of excellence.
tSPECT is being set-up at the TRIGA Mainz at present. The commissioning and integration of the major components has starteted with two beam times in 2015.
Major components of tSPECT:
- aSPECT superconducting magnet system
- multipole magnet for radial storage
- AFP spin flipper for the capture of the UCN
- proton detection system to measure the decay curve in situ
In a beam time in July 2015 UCN were stored for the first time in tSPECT using axial magnetic and radial material wall storage. For a short summary see the news section. For details click here. The multipole magnet is scheduled to arrive in September 2015. The proton detection system is going to be tested using UCN stored in tSPECT after adaptation to tSPECT in a beam time in November 2015.
 S. Baeßler, F. Ayala Guardia, M. Borg, F. Glück, W. Heil, G. Konrad, I. Konorov, R. Muñoz Horta, G. Petzoldt, D. Rich, M. Simson, Yu. Sobolev, H.F. Wirth, and O. Zimmer, First measurements with the neutron decay spectrometer aSPECT, Eur.Phys. J A 38, 17-26 (2008)
Fig.3: Sketch of upgraded aSPECT spectrometer with three-dimensional magnetic trap for neutron lifetime measurement:
(1) inside cold bore tube, (2) proton detector, (3) magnetic multipole for radial trapping, (4) trapped low-field seeking UCN, (5) AFP-spin flip, (6) electrodes, (7) mirror electrode for 4𝜋 proton detection, (8) UCN switch, (9) UCN guide, (10) UCN monitor detector to control depolarization, (11) cryo-cooler, (12) isolating vacuum.
Right hand side: magnetic field long the z-axis of the aSPECT spectrometer. The effective UCN storage volume is V ~ 10 L