A line was scribed on the A1 floorplate below and parallel to the beam axis so that by using a plumb bob it was easy to locate the sphere detector on the axis to within ☒ mm. The collimated neutron beam depicted in figure 1 has a half angle of 4.5°. Pulse height spectra were recorded and output for subsequent analysis. The 3He detector used a similar amplifier-SCA arrangement, but the amplifier output was also supplied to a multichannel analyzer (MCA). This calibration procedure was employed on a daily basis to eliminate possible shifts in the bias level. The discriminator level of the SCA was adjusted to give a selected rate when a calibrated source was positioned in a reproducible location in the sphere. Signals from the sphere detector were preamplified, amplified, and supplied to a single channel analyzer (SCA) operated in the lower level mode. Alignment was facilitated by use of an air bearing that supported the shield and a set of four screws rigidly attached to the floor which located the air bearing. These were aligned, along with the shield, using the same transit that defined the proton beam and the neutron collimator axis. Machined inserts of lithium loaded polyethylene defined the neutron beam. To reduce room scattered neutrons the source was surrounded with a massive shield of paraffin loaded with Li 2CO 3. Target current and currents incident on each collimator were monitored throughout the experiment. The discolored region where the beam struck the target indicated that the effective spot size was less than 4 mm. The target spot was defined by two collimators in the proton beam line which were also used in the neutron collimator alignment. The target thickness was measured to be 17 keV by observing the yield from the target as a function of bombarding energy near threshold and by the width of the neutron peak observed in the black detector TOF spectrum. No evidence for oxidation of surface layers was seen in the threshold measurements. The targets were transported from the evaporator to the beamline in a helium atmosphere and care was taken to minimize their exposure to air. Metallic lithium was vacuum evaporated onto a tantalum substrate that formed the end of the beam transport system. The source of neutrons was the 7Li( p, n) reaction. Since measurements at other laboratories made use of the symmetry mentioned above, this efficiency was defined to include the detector geometry and no correction was made for air transmission between the neutron source and detector. The objective of the procedure was to measure an efficiency for each of the two intercomparison instruments at the two selected energies. Since the 3He detector is a spectrometer, a curve fitting procedure could be used to discriminate against background. The sphere background was determined by studying the radial dependence of the count rate and by interposing a shadow bar between source and detector. Since the transfer instruments were too slow for time-of-flight (TOF) measurements, backgrounds were determined in other ways. Instead, the “black” detector was used to calibrate a secondary monitor at a back angle and the latter was used to normalize intercomparison monitor and “black” detector runs.īackgrounds for the “black” detector were measured using pulsed beam techniques. Since the NBS system employs a single beam, collimated and monitored at 0°, use of this technique was not possible. For these energies the intercomparison monitors used in the program are a 3He gas filled proportional counter and a BF 3 counter imbedded in a polyethylene sphere.Īll other standards laboratories use an open geometry for measurements and the intercomparison is usually performed with the BIPM intercomparison monitor and the laboratory flux monitor symmetrically located equidistant from the neutron source. The NBS flux was intercompared at neutron energies of 250 and 565 keV. Flux monitoring at this facility is done with a “black” neutron detector which has been calibrated through the associated particle technique and calculated using a Monte Carlo program. NBS participation occurred during the period December 1 to 15, 1976 at the neutron flux facility associated with the 3 MV positive ion Van de Graaff. The International Bureau of Weights and Measures (BIPM) is sponsoring an intercomparison of neutron flux monitoring at the major international standards laboratories.
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