Abstract & Overview
BACKGROUND RADIATIONground-based measurements in the region above 3 cm-.This leaves the field to balloon, rocket or spacecraft observations, or to the use of indirect techniques such as measurements of the distribution of rotational states of interstellar molecules by absorption spectra of the interstellar medium.10-14 Since 1967 several groups have carried out direct measurements in this region using rocket and balloon-borne instruments.This report describes a balloon experiment designed to make a direct measurement -1 of the background radiation in the 1-20 cm-region and presents results of two balloon flights made on June 5, 1971 and September 29, 1971, from the balloon facility of the National Center for Atmospheric Research (NCAR), Palestine, Texas. ApparatusThe design of a balloon-borne radiometer to make an absolute measurement of the isotropic background spectrum in the far infrared is constrained in several ways.First, radiation from the optical components of the radiometer should not greatly exceed the incoming radiation.In practice this means that all optical components must be held at liquid-helium temperatures.In fact in this experiment the optical apparatus is immersed in liquid helium.A second constraint is imposed by the poor detectors that are available for the far infrared region.For example, a detector with an area of ~0. 1 cm2 is typically able to detect a minimum of ~1010 photons in one second.The optics must therefore have as large a solid angle-area product as is practical; in the apparatus described in this report it is approximately 0. 3 cmn2 sr.At the same time the radiometer beam must be narrow enough in angle to permit measurements of atmospheric radiation by zenith angle scanning and also small enough in cross section to enter the liquid-helium dewar without being intercepted by surfaces at ambient temperature.The opening in the dewar cannot be made very large without incurring prohibitive losses of liquid helium.A typical flight may last one-half day during which time the instrument must remain immersed in liquid helium.This is ensured by enclosing it in a sealed copper can surrounded by a reservoir of liquid helium which is allowed to evaporate into the atmosphere, thereby providing refrigeration.The liquid helium in the sealed can is used only as a thermal conductor.Because liquid helium shrinks by a remarkable 15% between 4. 2'K and 1. 5 0 K, the entrance window to the radiometer can is recessed so that it will always be in contact with the liquid.Finally, some provision must be made to separate the atmosphere at low altitudes from the liquid helium, since otherwise the radiometer would be covered with air frost and water frost.This separation is provided by two gas-tight transparent covers which are removed during the course of the experiment.A schematic drawing of the apparatus QPR No. 105 (V.GRAVITATION RESEARCH) composed of the product of the spectral response of cold window, collimating cone, Plexiglas chopper, and detector.SR2 through SR5 are obtained by multiplying SRI by the appropriate interference filter transmission spectra.1.1 1.0 0.9 UL Z 0 0.8 au 0.7 LU 0.6 "' 0.5 LU 0.4 N 2 0.3 z 0.2 0.