PHOTOGRAPHIC RECORDING METHODS IN NUCLEAR PULSE SPECTROMETRY

Applications of photographic techniques as a tool in measuring pulse amplitude (PA) distributions fall into 3 general groups: 1. (1) Recording of individual events as they arrive from the nuclear radiation detector, with subsequent analysis based on visual or photoelectric inspection of the record. 2. (2) Time exposures of a large series of events, the distribution analysis being provided by the exposure-density correspondence in the photographic process. 3. (3) Use for permanent storage of results from counting equipment, after the pulse sorting has been performed electronically. The first and third groups will be reviewed in sections 2 and 5, and some improvements of existing techniques suggested. Quantization of variables is discussed in §§ 2.5 and 2.6 as a simple means for removing ambiguities in automatic scanning of individual event records. For the second group a thorough presentation of design problems and evaluation procedures is attempted (sections 3 and 4). Gray wedge (GW) techniques lead to a straight-forward quantitative interpretation of photographed PA spectra. The simplest version of a fast GW spectrometer consists of a commercial oscilloscope and a special plug-in adaptor unit. The adaptor described in § 3.3 provides double rectangular pulse shaping, various shape corrections, overload protection, and generates an exponential sawtooth voltage derived from the linear oscilloscope sweep. This simple GW spectrometer is particularly useful at high counting rates (up to 105 counts/sec). For a more general use (at both high and low intensities) additional parts such as pulse stretches, gating circuits and various sweep or wedge arrangements are needed (§ 3.4). Stretching and gating requirements will be discussed from a general point of view, and details of circuits used at the ETH will be presented in §§ 3.5 through 3.7. Various methods of producing GW effects are reviewed and the calculated wedge characteristics and light efficiencies compared (§ 3.8). Advantages of the electronic type of GW are the freedom from light losses, simplicity of construction, and versatility in selecting different wedge characteristics. Photographic procedures (§ 3.10) include the use of a printing process to obtain easy-to-read graphs of the PA spectra. Empirical calibration is described in §§ 3.11 and 3.12, and the sources of deviations from the calculated behaviour are discussed. Absolute intensity evaluation from GW pictures is made possible, as shown in section 4, if the spectrometer is equipped with a few electronic counting channels and an automatic channel limit marker to establish the correspondence between the photographic curve and the channel counts. In coincidence measurements both individual dot recording and density recording of two-dimensional distributions are found useful. Again, combining the photographic technique with a relatively small number of counting circuits permits the determination of absolute intensities. The high speed at which information from electronic counting equipment may be accepted for permanent storage on film makes the use of photographic techniques particularly attractive for automatic recording (section 5). Programming of a serial memory spectrometer including dead time correction is described in § 5.2. A scanning mechanism is proposed in § 5.3 to read previous results back into the spectrometer memory, with provision for semi-automatic computation of linear combinations.