TY - JOUR
AU1 - Fields, Theodore
AU2 - Clayton, Glenn
AU3 - Kenski, Joseph
AB - Characteristics of a Photocell (CdS) and Scintillation Phosphor (NaI) Radiation Detection System 1 Theodore Fields , Glenn Clayton and Joseph Kenski Excerpt Recent investigations of the single-crystal type of CdS detector for radiation measurements have indicated undesirable characteristics such as temperature dependency and nonreproducibility from crystal to crystal. A layer-type CdS cell, developed by the Hupp Electronics Company, was found more effective. This cell is made from CdS powder combined with small amounts of copper and zinc and evaporated in a coarse grid design in a plastic resin. The characteristics of this photoconductive cell are as follows: (1) It has a response peak in the blue-green band around 5,200 A°. The luminous output of the NaI (thallium-activated crystal) roughly overlaps this range when subjected to gamma radiation from radium 226. (2) The manufacturer's specifications indicate a luminous sensitivity, with 100-volt DC potential, of 4 amperes per lumen. The dark current or noise level is approximately 2 × 10 −10 amperes. A 1 × 1-inch NaI (T1) crystal exposed to a point source of 50 mr/hr. will emit approximately 10 −5 lumens. The gamma radiation level desired to be detected was in the range from 0.1 to 10 mr/hr. for clinical work. This would involve levels between 10 −6 and 10 −7 lumens from the phosphor; 10 −6 lumens correspond to about 5 × 10 −4 foot candles if relative geometries of the cell and phosphor are considered. It is believed that the long-term drift will not interfere with readings of radiation above the level of 2 mr/hr. However, a warm-up time of twenty minutes is required for the CdS cell to reach a stabilized value of conductivity with applied voltage. This could be avoided by applying minimal continuous voltage to the cell. The decay or return to zero was accomplished by superimposing on the CdS DC bias voltage (90 volts) a short-duration saw-toothed waveform of a noncritical frequency of approximately 100 cps and 2.5 volts in amplitude. Since the rise-time of the response of the CdS cell is a function of total incident light, the use of a fluorescent light disk bias of 20 microlamberts was found to provide a good compromise between rise-time and sensitivity. This produces an effective noise current of 12 × 10 −6 amperes. The sensitivity versus light bias characteristics is shown in Figure 1. A two-stage direct-coupled transistor amplifier (Fig. 2) performs the dual purpose of matching the 6.0 megohm input impedance and providing 40 decibel gain. The output (Fig. 3) is indicated on a 20-microampere movement with a separate bias battery for balancing out the noise current. This unit consumes 1.5 milliwatts for the meter-balance circuit, 1.8 milliwatts for the relaxation oscillator, and 42 milliwatts to operate the transistors for a total input of 45.3 milliwatts.
TI - Characteristics of a Photocell (CdS) and Scintillation Phosphor (NaI) Radiation Detection System
JF - Radiology
DO - 10.1148/72.1.105b
DA - 1959-01-01
UR - https://www.deepdyve.com/lp/radiological-society-of-north-america-inc/characteristics-of-a-photocell-cds-and-scintillation-phosphor-nai-6S5SS0dmPt
SP - 105
VL - 72
IS - 1
DP - DeepDyve
ER -