NEW METHODS OF NEUTRON DOSE RATE EVALUATION
A multisphere unit and a single 10-in. sphere instrument were developed for evaluating neutron dose rates. Using either of these methods, the dose rate from reactor leakage neutrons can be determined, including those neutrons in the intermediate energy region. The multisphere method utilizes three cadmium- covered polyethylene spheres of various sizes that are placed over a LiI thermal detector. Because of the different amounts of moderation and absorption of the impinging neutrons, each of the spheres gives a different counting rate for a given neutron spectrum. The efficiency of each sphere over a wide energy region is known; and by assuming thnt the neutrons in the intermediate energy region have a 1/E dependence, it is possible to determine the simplified spectrum from the observed count. From this spectrum one obtains the fraction of intermediate neutrons, the fraction of fast neutrons, and the approximate average energy of the fast neutrons. The dose rate delivered by the neutrons can then be calculated. The single sphere method utilizes a 10-in. polyethylene sphere placed over a LiI thermal detector. This sizy sphere has a response approximately proportional to the dose rate for neutron energies from thermal to 7.0 Mev. The 10-in. sphere responds to thermal neutrons within 4%, and to fast neutron energies below 7.0 Mev within 15% of the actual dose rate. The intermediate energy neutrons produce a weighted overestimate of 80% for a 1/E spectrum extended to 1 Mev. The error in the total dose rate ranges up to an observed maximum of 65% when all energy groups are measured. No assumptions concerning the spectrum are required for the single sphere instrument; it can be used as a survey instrument for neutron energies from thermal to 7.0 Mev. The total neutron dose rate is reported with reasonable accuracy, irrespective of the neutron spectrum, and the dose rate is given by direct readout. A comparison of the total dose rate determined by both techniques and the Hurst type absolute fast neutron detector is given for the various types of sources and for reactor leakage neutrons. (auth)
- Research Organization:
- Los Alamos Scientific Lab., N. Mex.
- DOE Contract Number:
- W-7405-ENG-36
- NSA Number:
- NSA-17-003306
- OSTI ID:
- 4764617
- Report Number(s):
- LADC-5427
- Country of Publication:
- United States
- Language:
- English
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