Detector/source angular response

C. Detector/Source Angular Response 1. Directional Hemispherical Reflectometers... 

Figure 10 shows the angular response of the detector. The 1461 and 2614 keV peak areas are relatively uniform as expected, but the source peak areas show a reduction in the 90 and 180 directions. This reduction is about 15% and can not be explained from the detector construction. The average data collection time was 7 hours. 

The effective reflectance of the detector can be further reduced by using Paschen s (11) method. In particular, an inverted CPC, when positioned in front of a detector (9/2it) or source (2ti/0) to reduce variations from ideal angular response as described in Section V,C, will also enhance the effective absorptance. Radiation reflected from the detector or source at angles larger than the CPC s critical angle is reflected back to the detector or source. If the detector or source is a Lambertian reflector and if the illumination of the CPC is Lambertian, then the effective reflectance of a CPC-detector combination is given by... 

It is also possible to use a calibration program (e.g. ISOCS) to model the relative response of a detector to changing angles of incident radiation from point sources. Such an approach is essential for a collimated detector. This is illustrated in Figure 4 for an angular response model produced for a collimator that might typically be used in the field. 

Figure 3.2. Normalized Angular Response of Detector to a Cf Source Enclosed by...

Figure 3.2 shows the normalized angular response of the detector to a Cf source located inside a polyethylene box at a distance of 1 m above the ground. The detector was positioned at two different distances (3 m and 30 m) from the source. At each location, the detector was rotated about a vertical axis to various angles with respect to the direction to the source (0° is toward the source). At each angle, coimts were collected for 1000 s. As expected, the count rate was at its maximum when the detector was aimed at the source, and the count rate decreased with increasing rotation angle. This decrease in counts resulted from the collimator on the front of the detector and the smaller surface area exposed to the source as the result of... 

Figure 3.3 shows the normalized angular response of the detector to a bare source... 

We see that the response of interferometers is inherently broadband. The beam pattern of the antenna is almost nondirectional. An interferometer is very different from a telescope, which can be pointed to a specific location in the sky. The position of the source in the sky is determined measuring the difference in the arrival time of signals in detectors at distant locations. The broad angular response of the interferometer, however, is an advantage for a survey of the sky. 

We evaluated this detector s performance with a series of tests to determine its response to distant neutron sources and the angular sensitivity achieved by the collimator and shielding. Additional tests have explored applications such as detecting a neutron source passing by the detector in a vehicle and using the detector to monitor human intrusion into the neutron radiation of a storage vault. The results of these tests are provided in later sections of this report. 

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