Performance of the radphi detector and trigger in a high rate tagged photon beam

We describe the design and operation of a detector system for measuring all-photon decays of mesons photoproduced in a tagged photon beam with energies between 4.3 and 5.4 GeV and a flux of 5×10⁷ tagged photons per second. Photons from meson decays were detected with a lead-glass calorimeter with an energy resolution of 11% at 1 GeV. Various veto and trigger components were also present. Final states with as many as six photons were successfully detected and reconstructed.     

Quantitative characterization of the x-ray imaging capability of rotating modulation collimators with laser light

We developed a method for making quantitative characterizations of bi-grid rotating modulation collimators (RMC's) that are used in a Fourier transform x-ray imager. With appropriate choices of the collimator spacings, this technique can be implemented with a beam-expanded He-Ne laser to simulate the plane wave produced by a point source at infinity even though the RMC's are diffraction limited at the He-Ne wavelength of 632.8 nm. The expanded beam passes through the grid pairs at a small angle with respect to their axis of rotation, and the modulated transmission through the grids as the RMC's rotate is detected with a photomultiplier tube. In addition to providing a quantitative characterization of the RMC's, the method also produces a measured point response function and provides an end-to-end check of the imaging system. We applied our method to the RMC's on the high-energy imaging device (HEIDI) balloon payload in its preflight configuration. We computed the harmonic ratios of the modulation time profile from the laser measurements and compared them with theoretical calculations, including the diffraction effects on irregular grids. Our results indicate the 25-in. (64-cm) x-ray imaging optics on HEIDI are capable of achieving images near the theoretical limit and are not seriously compromised by imperfections in the grids.     

Development of Spark Chambers for Use in Measurements of the Charge Spectrum of High Energy Cosmic Rays

The High Energy Cosmic Ray Experiment, flown on balloons by a group at Goddard Space Flight Center, is designed to measure the charge, the energy, and the arrival direction of high-energy cosmic rays. A wiregrid spark chamber is employed to determine the trajectory of each incident primary particle which satisfies the experiment trigger criteria. Optimum trajectory-determination efficiency depends on parameters, the values of which vary with the charge of the incident particle. The functional dependence on the voltage applied to the wire grids and on the delay in application of the high voltage trigger for several different values of the DC clearing field was measured for fully ionized nitrogen nuclei and for singly charged particles. The voltage was found to be the most suitable parameter to vary in order to maximize the spark formation efficiencies and to minimize spurious spark formation. Because of the low fluxes of high-Z, high-energy cosmic rays, long experiment exposure times such as those obtained in satellites are desirable. To adapt the spark chambers for use with experiments with a duration of more than one year and total accumulated events in excess of 3 × 108, the useful life times of both the high-voltage triggering circuit and the spark-chamber gas needed to be improved. Krytron high-voltage triggering devices have been developed which will perform satisfactorily for more than 3 × 108 simulated events at a rate of 10 events/ sec, an improvement of approximately a factor of 10.     

Response of TlCl(I,Be) Crystals to Energetic Ionizing Radiation

Studies of scintillation TlCl(I,Be) crystals have been conducted using 8-GeV negative pions and 50- to 130-MeV positrons. Pulse shapes and resolutions of these crystals have been measured. Pulses from TlCl(I,Be) are observed to have a fast rise time (?2 nsec) and a complicated decay scheme consisting of at least three separate components. Attempts were also made to observe the Cerenkov radiation from these crystals due to highly relativistic particles. The resolution of the pulse height spectrum for a 2.9-cm diameter by 3.8-cm length TlCl(I,Be) scintillator was measured for 8-GeV negative pions. For comparison, the resolution of the same size CsI(Tl) scintillator was measured using the same experimental configuration. The resolution of CsI(Tl) is only slightly greater than theoretical prediction. The resolution of TlCl(I,Be) is greater, being limited by photon statistics even for 35 MeV of deposited energy. The resolution of a 7.6-cm diameter by 7.6-cm length TlCl(I,Be) crystal used as a Total Absorption Shower Counter for 50-130 MeV positrons has also been measured. This resolution is in reasonable agreement with previously reported data.