3.4.3 Liguid Nitrogen
The HPGe detectors used in the IMPs operated with liquid nitrogen at a temperature of -196°C. In
the early months of the program the liquid nitrogen was air lifted from Honolulu on scheduled MAC
cargo flights. Two military surplus 500-gallon containers were used. Shipping regulations required
that the pressurized containers be vented outside the aircraft cabin. The condition of the
containers, combined with these regulations, resulted in excessive nitrogen loss before reaching
Enewetak. The on-atoll transfer containers were military surplus, wheeled, horizontal 50-gallon
liquid oxygen carts, all of which had a high liquid nitrogen loss rate. This system was rather
expensive and inconvenient.
An
improved
system
was
devised,
and
better
containers
purchased.
A
military
surplus,
trailer-mounted liquid oxygen/liquid nitrogen plant was obtained, and the base operating contractor
had people trained to operate it.
About every two weeks, the plant was activated and two of the
three on-atoll liquid nitrogen containers were filled. The containers were Linde LS-160B models,
each holding 160 liters. This scheme successfully supplied the IMP and Radiation Lab with liquid
nitrogen.
3.4.4 Detector Performance
Three detectors were purchased for use in the project and a fourth was ordered a few monthslater,
when the effects of Enewetak conditions on the detectors were confirmed. Two other detectors had
been procured for a similar measurement program at the Nevada Test Site (NTS). Detectors were
assigned by DOE to Enewetak or NTS, based on priority and scheduling of the two projects.
Detectors were transferred informally and expeditiously, in response to DOE direction. All six
detectors were used at Enewetak at various times.
All detectors used at Enewetak were initially calibrated in Las Vegas, as discussed in Section 3.2.3.
Starting in July 1978, a calibrated 241 4m source was available on-atoll and periodic remeasurements
of effective detector area were made. These were used to provide an effective area correction
factor for data handling. Field calibration sources, consisting of 4lam, 187g and 60Co, were used
for three-times-daily detector performance monitoring. Field calibration was performed to set the
gain of the detector electronics, and to generally track detector behavior. Tech Notes 5.2 and 11
discuss effective area factor and field calibration. For the field calibration measurements, the
percentage standard deviation for the 241 4m value was 2 to 5 percent. The mean error in a series of
effective area measurements was 1.1 + 0.8 percent.
In the first months of the project, gradual loss of detector resolution with usage was noted. This was
traced to water vapor entering the liquid nitrogen dewar during refilling in the field, causing an ice
layer to form at the bottom of the dewar. This in turn partially insulated the detector, causing
higher than design operating temperature. The problem was solved by the following maintenance
procedure. About once each month, the detector was brought to room temperature, and ethanol used
to remove water from the detector dewar. The dewarinterior was then dried using a stream ofair.
The dewar wasthen refilled with liquid nitrogen.
Operational history of the detectors is summarized in Appendix D. The average detector life span
when installed in an IMP was about four months, with a range of less than a month to over seven
months. Causes or symptoms of failure were: preamp corrosion, vibration sensitivity, no signal
transmission, wide peaks and noise at low energy, and the dewarfailure. The last three items listed
can probably all be classed as dewar failure, and were ultimately traced near the end of the project
to corrosion of the 22 mil beryllium entrance window, or the beryllium-aluminum epoxy seal. An
all-aluminum window was ordered on repaired detectors, but was not available in time to be used on
the Enewetak project.
106