Current Detectors

1. Study of a Direction Sensitive Photo-Sensor (DSPS)Zach Parsons (University of South Dakota, Vermillion, SD, 57069)Milind Diwan (Brookhaven National Laboratory, Upton, NY, 11973) This study was done to investigate a new kind of photo detector, a direction sensitive photo-sensor, which uses a lens to transfer the incident photon angle to a position on a detecting surface. We have studied a basic wide angle lens design. The lens was altered to make it cost effective, while at the same time efficient. The resolution of the lens was relaxed in exchange for the tightening of several other parameters. The parameters that were studied included lens thickness, image plane area, system aperture, and distance from lens to image plane. Current Detectors One of the most demanding potential applications for the DSPS will be a new very large underground water Cherenkov detector for long baseline neutrino physics. The Problem Current photo detectors don't provide direction information about the incident photon, making event reconstruction more difficult and less accurate. Possible Solution: Direction Sensitive Photo-Sensor The Super Kamiokande (SK) detector in Japan (see right) is currently the largest water Cherenkov detector in the world and has been used to detect neutrinos from the sun as well as the atmosphere. The SK detector has 50 kT of pure water viewed by approximately 11000 photo-multiplier tubes (PMT's). Two possible geometries for detecting the photons focused by the lens being considered: • The entire focal plane is covered by photo-cathode material. The incident photon will cause an electron to be emitted from this surface. A high voltage electric field is shaped in such a way as to accelerate this electron by several thousand volts and transport it to a unique position on a silicon detector. This detector will need to have sufficient position resolution (achieved either by pixels or by charge interpolation) to achieve the needed angular resolution. • Photo-detectors, either photo-multiplier tubes or avalanche photo-diodes, are simply placed at the focal surface in a large array. Our design for a direction sensitive photo-sensor could be useful for a liquid scintillator based calorimetric detector. In such a detector a large volume of organic liquid scintillator is placed in a tank and viewed by photomultiplier tubes on the inner wall. The largest such detector so far is the KAMLAND detector also at Kamioka (see left). The second method will clearly need a very large number of detectors compared to the first one, and the angular resolution will be limited by the size of the photomultipliers • Goals • Aperture size of at least 10 cm. • Angular acceptance of +/- 60 degree or larger. • Angular resolution of at least +/- 5 degree. • Sensitivity for photons in the wavelength range from 300 to 500 nm. • We focus on a design with a single lens and a geometry that will lead to a simple robust package that is relatively simple to construct. • While giving possibilities for the overall design, in this study we limit ourselves to the design of the optics. • Possible Applications • Proton Decay • Neutrino Detector • Supernova Detection • Solar Neutrino Detection Photo-Multiplier Tubes PMT’s (see right) have a photocathode surface which, when hit by a photon, gives of an electron. This electron is then accelerated and multiplied through a series of diodes. The downfall of PMT’s is that they only know light and dark, which leaves much to be desired when using this information to reconstruct events. If these tubes were to be replaced by direction sensitive photo-sensors then we could obtain the direction of each detected photon. Optical Design Lens Design A Knowledge of the location of the DSPS and the photon direction with good resolution will greatly aid the reconstruction of events such as done for p→e+ π (see left). It should also improve particle identification, energy resolution, and reconstruction of events with multiple particles. Several designs were studied for the optical portion of the DSPS. Shown on the right are two of the final designs. Calculations made for the designs included: image surface area, angular resolution, and a ratio of the aperture to the lens (see bottom right). Below on the left is a graph of the lenses’ transmission when the light hits the lenses at zero degrees. Design A has better angular resolution than Lens B, but has a relatively large image surface area. Lens B has angular resolution on the edge of the goal set for this study (5 deg), and has a relatively small image surface area. Cherenkov Radiation Lens Design B When a charged particle travels in a medium with an index of refraction (n) with velocity larger than the speed of light in that medium (c/n), a cone of light is emitted (see right). This effect is known as Cherenkov light after the discoverer of this radiation. The figure on the left shows the calculated spectrum of photons in pure water versus wavelength in nm. The upper curve is the spectrum expected at the inner surface of the tank normalized to unit area. The lower curve shows the spectrum after accounting for the quantum efficiency of a typical bi-alkali photocathode. Significant improvement is possible if a new photo-sensor can be made more efficient in the 300 to 400 nm (near ultraviolet) region.