Robert T. Johnson

A search for fractional charges in native mercury

Abstract We have searched 2.0 mg of native mercury for fractional charges. Details of the apparatus and measurement procedure are presented. We find no fractional charges, giving a limit on quark concentration comparable to other experiments carried out on refined materials.

X-ray detector using superconducting aluminum tunnel junctions

We report on tests of a prototype detector for 6-keV X-rays, using series arrays of tunnel junctions. Tests with higher-energy particles indicate an energy resolution of 4 keV, at 0.3 K and with a warm pre-amp. At lower temperatures and with a cooled FET, the resolution should approach 100 eV.

A search for neutrons from fusion in a highly deuterated cooled palladium thin film

We have carried out an experiment to search for neutrons released from a thin film of Pd-10% Ir. A film of thickness ≈2000 Å was prepared by sputtering. The film was then cooled to 77 K and was charged with deuterons by low-energy ion implantation. Deuterium was incident on the film as neutral D and D2 with a kinetic energy of 1000 eV. Paraffin was used as a neutron moderator and absorber, with a Nal detector to observe the 2.22-MeV γ-ray expected from neutron capture by hydrogen neuclei. The high local concentration of deuterons in the film could, in some fusion scenarios, have produced a high rate of deuterium fashion. If this happened, this implantation technique could form the basis of a viable fusion power generator. However, we observed no excess neutron production over background. We were also sensitive to 23.8-MeV γ-rays (from fusion producing4He) but observed no peak at this energy.

A search for free quarks in heavy-ion collisions at 60 and 200 GeV/nucleon

Abstract We have carried out a search for quarks produced by the heavy ions at the CERN SPS. A mercury target exposed to the oxygen and sulfur beams was concentrated by distillation. The residue was tested using a modified Millikan apparatus. No fractional charges were observed. We set upper limits on quark production of about 10 −7 quarks per beam-particle interaction, or about 10 −6 quarks per central collision.

Characteristics of high-transmission-probability tunnel junctions for use as particle detectors

Interest in the problem of the galactic dark matter has stimulated development of particle detectors sensitive to very low energies. The authors describe superconducting tunnel junctions with thin barriers which may be suitable for this purpose. They present I-V (current-voltage) characteristics and data on the temperature dependence of the subgap tunneling current. They examine the barrier transmission probability and the resistance of the null junction. They also present some scanning-electron-microscope observations of the thin films of the tunnel junctions, which show that faster deposition produces smaller grains and increasing film thickness results in larger grains from an apparent fusing of smaller grains. >

Measurement of tunnel junction resistance during formation

We have measured the characteristics of aluminum tunnel junctions during and immediately after the formation of the junction. This has permitted us to observe changes in the oxide barrier, in vacuum and in air. By observing the barrier resistance during sputtering, we were able to diagnose and correct problems due to plasma discharges which were damaging the junctions. We report preliminary results from junctions passivated with a silicon nitride cap layer.

A prototype dark matter detector using a series array of aluminum superconducting tunnel junctions

The authors have fabricated an array of series-connected superconducting aluminum tunnel junctions on the surface of a large sapphire crystal, as a prototype of a detector for massive, weakly interacting elementary particles. Such particles might constitute the missing matter of our galaxy. Tests on a 14-g crystal with 6-MeV alpha particles gave an energy resolution of 450 keV full width at half maximum (300 keV due to electronics and microphonics alone). The ultimate energy resolution to be obtained from this type of detector is discussed.<<ETX>>

Search for free quarks produced in ultra-relativistic collisions at BNL and CERN

Abstract A high intensity experiment was performed to search for free quarks at BNL and CERN using ultra-relativisitic beams. The experiment was designed to trap quarks in a Hg target or liquid Ar tank. No free quark candidate was found. Limits from 10 −7 to 10 −10 quarks per incident ion are reported.

Prototype detector for 6-keV x-rays using nine series arrays of tunnel junctions

We have fabricated a device consisting of nine arrays of series- connected superconducting aluminum tunnel junctions on a thin sapphire substrate, as a detector of 6-keV X-rays. Tunnel junctions are of interest as particle detectors because their theoretical minimum excitation energy is on the order of one milli-electron volt, a factor of one thousand lower than conventional semiconductor detectors. We have experimented with a new SiN passivation layer, intended to prevent further oxidation of the tunnel barrier on exposure to air. Preliminary tests with 200 angstrom thick SiN layers indicate improved stability of junction resistance on exposure to air. We will present results from a device with a 400 angstrom thick passivation layer. We will also report on tests of a detector without a SiN layer, where we observed coincident pulses between two of the nine arrays. The energy resolution for 6-keV X-rays is about 1 keV, limited by noise. If the signal size can be increased, pulse height and timing information from the nine separate arrays of this detector should allow simultaneous determination of position and energy.

Particle pulses from superconducting aluminum tunnel junction detectors

Superconducting aluminum tunnel junctions have been developed for use as particle detectors. Results on the static characteristics of these devices and from tests of these detectors with 6-keV X-rays are presented. An extrapolation of the properties of these detectors to one suitable for dark-matter detectors is discussed.