INSTRUMENTATION
The Universal Humidity Transducer . . . still eludes us. For years we searched for a humidity transducer as versatile as the thermistor temperature transducer. No luck! So we've designed new "state-of-the-art" into instruments for all three classical humidity measurement methods:
wet bulb depression heated dew point refrigerated dew point
use of a transformer or a specially de signed preamplifier. PbS, PbSe, and P b T e were among the earliest I B photon detectors developed. They consist of evaporated or chemi cally deposited layers sensitized by various techniques involving the use of oxygen in one form or another. Pro duction of these detectors is more of an art than a science. PbS and PbSe are commercially available. PbS is oper ated a t room temperature. I t responds to 3μ, has a time constant of several hundred microseconds and a peak JD* of about 10 11 cm H z ^ W " 1 . When cooled to liquid nitrogen temperature, D* for specially prepared detectors may increase by as much as an order of magnitude, but at the expense of time constants, which are. lengthened considerably. PbSe has a response ex tending to 5μ, when cooled to dry ice temperature, a value of JD* of the order of 101U, and a time constant of about 50/zsec. Liquid nitrogen cooling shifts the response to slightly longer wave lengths and makes possible the use of material with element resistance of the order of 1 AfG instead of 100 MSI of PbSe materials designed for operation in the dry iee temperature range. The spectral curves of the lead salt detec tors are shown in Figure 2. The development, of crystal growing and material preparation techniques has
led to the preparation of InSb and liiAs detectors. These are prepared by wellunderstood techniques from single crys tals of the material. InSb can be pre pared to operate in the photoconductive mode or by the formation of a p-n junction in the photovoltaic mode. These detectors have reached such a state of perfection t h a t Z>* approaches its theoretical limit (10 1 1 at a spectral peak of about. 5μ) closer than any other material. Time constants are of the order of l^sec for photovoltaic detec tors and lOjusec for the photoconductive detectors. InAs, because its re gion of response overlaps that of PbS, but requires cooling to at least dry ice temperature, has not found as many ap plications as PbS. I t is available only in the photovoltaic mode. When cooled to dry ice temperature, it has a detec tivity which may exceed that of uncooled PbS, especially near the spec tral cutoff wavelength, and a time con stant considerably shorter than PbS. Several approaches have been used in the construction of detectors with a response beyond 6μ. No single ele ment or binary compound with charge carrier activation energy less than 0.2 eV has so far been found. One method for producing detectors with a threshold at longer wavelengths has consisted of using germanium with an activation en ergy of about 0.7 eV as the host lattice and adding selected impurities. The materials so prepared have a composite response, one due to excitation of charge carriers from the germanium with a response to about ί.7μ, the other
YSPs advanced thermistor technology is used to maximum advantage, helping in each of these methods to minimize the greatest source of error— the temperature sensing element and its readout. Let us send you complete specifications. And should we succeed with the universal humidity transducer, we'll send data on that, too!
YELLOW SPRINGS INSTRUMENT CO. YELLOW SPRINGS, OHIO 45387
Figure 3. Spectral Response of typical impurity activated germanium detectors. All detectors with the exception of Ge:B have a 60" field of view; Ge:B has a 10° field of view
Circle No. 118 on Readers' Service Card
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ANALYTICAL CHEMISTRY, VOL. 4 1 , NO. 14, DECEMBER 1969
