A High Efficiency General Purpose On-Line Silícon Polarimeter System

Polarimetry based on double scattering techniques usually use carbon or helium on polarization analysers. But this limits the polarimeter efficiency when high resolution in energy is sought and vice-versa. The polarimeter system we have developed overcomes these limitations by using a silicon analyser which acts as a ΔE detector at the same time. Basically the polarimeter consists of a lmm thick silicon ΔE detector and two silicon stopping detectors for detecting L and R scattered particles at 28°, and another detector behind the analyser used in anticoincidence to eliminate the unscattered particles as shown in fig. 1. The basic design of the polarimeter has already been described 1). We have devised a system to use the polarimeters (one on each side of the beam) with a PDP-9 computer on-line. Three 400-channel pulse height analysera. each with its memory divided in halves are used to record ΔE1,2, E1L,R and E2L,R pulses respectively. The ADC outputs corresponding to these signals give the energy deposited by the reaction products in ΔE detectors and their total energy respectively. The primary data are stored on-line and written on a magnetic tape as records consisting of 133 events, each event being defined as three channel conversions in coincidence from three ADC’s. The basic logic is to record a new event whenever there is an acceptable signal in any one of the ΔE detectors, ADC1 simultaneously with a signal in any of E detectors, ADC2 or ADC3. This event corresponds to a new channel conversion only in one of the E ADC’s depending on ΔE signal location in the first ADC, the previous channel conversion being repeated for the other E ADC. The separation of these events into those due to protons and to other particles, for each detector telescope,is done setting energy windows on ΔE and E channel conversionsto match with the energy loss in the analyser detectors and the total energy of these particles,as determined from the reaction kinematics. Thus this technique enables one to identify the reaction products and obtain their energy spectra without using analog particle identifying circuits. This type of polarimeter with an on-line particle identification system can play an important role in polarization measurements involving low count rates,as generally is the case in double scattering, triple scattering and in simultaneous measurements of asymmetries when more than one type of particles is produced in a nuclear reaction. The system described here is easily adaptable to handling four polarimeters simultaneously without changes. With larger ADC’s it can handle easily eight to sixteen polarimeters.