SNS neutron background measurements using a portable 3 He LPSD detector

1. SNS neutron background measurements using a portable 3He LPSD detector

2. Distance from target • Count rates are highest near the target and decline as the distance from the target increases. • The time of flight spectrums decline over a smaller time range close to the target than they do far from the target. • Absorber plate measurements suggest that a similar range of neutron wavelengths exists at each distance. The difference in shape is a result of the neutrons travelling different distances.

3. Direction from target • On some occasions the neutron flux has been measured to be more intense in some directions from the target than it is in other directions. • These differences appear to be related to the amount of shielding in front of the target in that direction. • The addition of shielding blocks in these directions has been observed to decrease the count rate. • While such hot spots in the shielding create some of the background they do not appear to be the primary contribution. Measurements suggest that there is a general glow of neutrons emerging from nearly all of the shielding blocks.

4. Absorber plate measurements • Inserting neutron absorbing plates provides information about how neutron energies relate to the time of flight. • The absorption pattern is similar to what would be expected if the neutron wavelength is proportional to the time of flight, suggesting that this relationship holds approximately.

5. Comparison to theoretical absorption • The expected absorption can be calculated if the neutron wavelength is assumed to be strictly proportional to the time of flight and some distance of travel is assumed. • The actual spectrums display a greater absorption than the theoretical spectrum at low times of flight but a greater than expected absorption at high times of flight.

6. Probable wavelength distribution • A likely cause of the shape differences is that a particular time of flight does not correspond to a single neutron wavelength but rather to a distribution of wavelengths. • This distribution will have a peak intensity at some wavelength, and the wavelength of this peak does appear to be increasing in a roughly linear fashion with the time of flight. • The neutron travel distance that must be assumed to generate theoretical absorption spectrums that best match the observed spectrums provides a crude estimate of the average distance that neutrons are travelling. The difference in shapes forces this matching to be approximate. • Travel distances obtained in this fashion tend to be about half of the distance to the target.

7. What is being detected? • Questions have sometimes been raised as to whether fast neutrons or gammas are being detected. • Distinctions can be made using travel times, response to discriminator settings, and the effectiveness of absorbers. • All measurements occurred within 30 meters of the target. A gamma can travel this distance in a tenth of a microsecond. A .01 angstrom neutron will need 75 microseconds. Fast neutron detection in this time period cannot be ruled out. • The time of flight spectrums extend to much longer times. A gamma or fast neutron detection would require a delayed emission.

8. The time of flight spectrum broadens as the distance from the target increases, which is difficult for a delayed emission assumption to explain. • The absorption plates placed in front of the detector were designed to work for slow neutrons and would be largely ineffective against gammas or fast neutrons. The observed behavior is consistent with slow neutron absorptions. • Gammas generally release less charge within a detector tube than do neutrons and thus create smaller pulse heights. Gamma detections can occur if thresholds are set too low, but the count rates will be more sensitive to changes in threshold settings than is seen for neutron detections. Experiments with threshold changes confirm that neutrons are being detected.

9. CNCS vs HYSPEC external backgrounds • The background count rate is lower at HYSPEC than at CNCS. • The CNCS background decays more rapidly with time than HYSPEC and more nearly approaches a zero count rate at high times of flight. • A time independent background component may be present at HYSPEC • The CNCS measurement is closer to the target than is ideal which may be exaggerating the differences.

10. CNCS vs HYSPEC internal backgrounds • The CNCS background decays more rapidly with time than the HYSPEC background • The external background measurements suggest that a particular time of flight may correspond to a shorter time of flight at CNCS than at HYSPEC. • That mechanism can account for some but not all of the observed difference in decay rates. • The CNCS shielding appears to be more effective than the HYSPEC shielding. • HYSPEC has a time independent background component that is not seen at CNCS.

11. HYSPEC shielding compared to cadmium absorber • The count ratefor the HYSPEC decays with time more rapidly than is seen by a detector measuring the background external to the enclosure, but at about the same rate as the external detector covered by a cadmium plate. • This plate is similar to those used to line the HYSPEC enclosure, suggesting that transmission through the lining accounts for much of the background seen by the HYSPEC detectors. • The external detector covered by 2 borated aluminum plates shows stronger absorption than cadmium for times of flight less than 2000 microseconds.

12. Other points • Current shielding of the detector enclosures appears to be effective in absorbing neutrons of 2 angstrom wavelength or higher. It is the background above 2 angstroms that must be dealt with for any further improvements in shielding. • Experiments varying the detector orientation usually see the strongest intensity in the direction to the target but some neutrons appear to be arriving from other directions. • There is some evidence for neutrons emerging from the surfaces of shielding blocks, perhaps due to the moderation of fast neutrons within them.