M. Le Gros

Planar array of superheated superconductors: An improved superheated superconducting granule detector

We have invented a new superheated superconducting granule detector which is a prime candidate for sensing dark matter and which could prove very useful for other applications. The sensor element is a planar array of spherical granules for which the spread in transition temperatures is nearly an order of magnitude smaller than that for a colloidal device. The detecting efficiency for low‐energy deposition is thereby significantly increased. Preliminary results are described.

Pulsed nuclear magnetic resonance of oriented nuclei in an insulating magnetic material:54Mn−MnCl2.4H2O

The first observation of pulsed nuclear magnetic resonance of oriented nuclei in an insulating magnetic material is reported. The system studied was the ordered antiferromagnet54Mn−MnCl2.4H2O. The response of the54Mn spins to a single pulse of variable length, the free induction decay, and Hahn spin-echo have all been observed. A discussion of the transverse decoherent relaxation timeT2 in MnCl2.4H2O is given.

First observation of double quantum nuclear magnetic resonance of oriented nuclei

Abstract Double quantum nuclear magnetic resonance of oriented nuclei has been observed for the first time. The systems studied were 54MnMnCl2·4H2O and 54MnMn (COOCH3)2·4H2O and rotation patterns, free induction decays and spin echoes all have been observed. The double quantum signal can be significantly larger than the single quantum signal.

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T1 is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional54Mn−Mn (COOCH3)2·4H2O is discussed with particular reference to the relatively short value of T1 in low applied fields. A preliminary experiment in the 1-dimensional system54Mn−(CH3)4NMnCl3 (TMMC) is also described. Pulsed NMRON measurements in54Mn−MnCl2·4H2O are outlined and the advant-systems with fast relaxation emphasized.

Pulsed NMRON studies of insulating magnetic materials

The apparatus and instrumental techniques developed for pulsed NMRON studies of insulating ordered magnets are described. Experiments have been performed on two salts,54Mn-MnCl2·4H2O and54Mn-Mn(COOCH3)2·4H2O, and rotation patterns, free induction decays and spin echoes have been obtained for both single quantum and double quantum transitions. The pulsed technique can produce signals larger than those obtained in CW NMRON, and is well suited for the measurement of the very short spin-lattice relaxation times that can occur in these systems.

Nuclear spin-lattice relaxation in54Mn-MnCl2.4H2O and lower dimensional magnetic systems

We report on nuclear spin-lattice relaxation studies on54Mn-MnCl2.4H2O and 2-dimensional54Mn-Mn(COOCH3)2.4H2O. In the former crystal, we find that at the lowest temperatures a direct process, normally thought of as forbidden, becomes the mechanism for relaxation. In lower dimensional systems the relaxation might be expected to be relatively fast and indeed in the acetate the relaxation timeT1 is ≈1000 s in an applied field of 0.2 T for the Mn2 site and is much shorter at lower fields. In 1-dimensional54Mn-(CH3)4NMnCl3(TMMC) a significant γ-ray anisotropy is observed on cooling indicating thatT1 is also short in this system. The shortness ofT1 in the lower dimensional systems suggests that they may be suitable hosts for on-line experiments.

NMRON studies of the antiferromagnetic and paramagnetic phases of54Mn-MnCl2 · 4H2O

Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study54Mn-MnCl2 · 4H2O. The54Mn spin-lattice relaxation timeT1 in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the dominant relaxation mechanism is a Raman process with the electronic magnons, but at lower temperatures a direct process takes over. NMRON has been observed for the first time in the paramagnetic phase, and a line width of 300 kHz, with both homogeneous and inhomogeneous contributions, is observed. In the antiferromagnetic phase the line width is 35 kHz, and there are also homogeneous and inhomogeneous contributions. The dependence ofT1 for the54Mn spins on field and temperature was studied in the paramagnetic phase. AT1 minimum centred atB0=2.64 T was observed. The hyperfine parameter <54AS>/h=−513.6(3) MHz in the paramagnetic phase, and comparison with the value in the antiferromagnetic phase gives 0.013(1) for the zero point spin deviation.

The position sensitivity of a planar array of superheated superconductors

Abstract A new technique for manufacturing the superheated superconducting granule detector is reported. The new sensor element has the potential for position sensitive detection of particles and radiation with a resolution of at least 100 μm. It consists of a planar array of spherical granules for which the spread in transition temperatures is an order of magnitude smaller than that of the colloidal device. Positional sensitivity (in one dimension) is achieved by analyzing the amplitudes of the signals from two SQUID pick-up coils and therefore only a few channels of electronics are required. The narrow spread in the size and positional ordering of the granules reduces the distribution of signal amplitudes originating from individual grains thereby allowing the dependence of signal amplitude on the distance from the coils to be utilized as signature of position.

SQUID Detection of Superheated Granules

We report two tests designed to study the practicality of a superheated superconducting colloid detector using a SQUID readout system. In the first test, the individual ‘flips‘ of ten 15μm radius tin grains were observed as the temperature was swept through the superheated superconducting — normal phase transition. In the second test, we were able to observe transitions induced by 90 keV γ-rays in a colloid of 5 μm radius grains in epoxy.

Pulsed NMRON measurements on insulators

Selective single and double quantum excitation pulsed NMRON has been utilized to obtain rotation patterns, free induction decays, and spin echoes in antiferromagnetic54Mn−MnCl2·4H2O and the quasi-2 dimensional ferromagnet54Mn−Mn(COOCH3)2·4H2O. The pulsed technique is well suited to observing fast spin-lattice relaxation and T1 values down to 100 ms have been measured in these systems. These short times may make magnetic insulators viable hosts for on-line experiments. The dependence on field and temperature of the54Mn spin-lattice relaxation time T1 has been investigated and a T1 minimum at high fields observed in both systems. A spin-spin relaxation time T2≈50 μs is measured in54Mn−MnCl2·4H2O. The observation of NMRON in the paramagnetic phase of MnCl2·4H2O allows the resonant frequencies in the antiferromagnetic and paramagnetic phases to be compared, yielding a value for the zero point spin deviation in the former phase.