The Duo-Sphere is an instrument developed by BAe Systems Marine to enable the characterisation of relatively low dose neutron fields.
The Duo-Sphere uses the ratio of counts outputs from 127mm and 208mm diameter spheres to derive the correction factors for neutron instrumentation for the impact of the neutron spectrum on detector response. Rather than attempting to produce an instrument with a response that matches the variation of dose equivalent rate with energy over the whole neutron spectrum, the approach adopted was to plot the detector sensitivity against the ratio of the response of the two detectors for a selection of realistic neutron spectra (largely from the SPKTBIB catalogue).
It was found that to the 95% confidence level the actual sensitivities of the detectors used (i.e. Leake detector, detectors with 127mm and 208mm moderator spheres) were within approximately 20% of the value given by the fit for all of the spectra used to derive the fit (over 200). Field comparisons of Duo-sphere predicted response corrections with those predicted from TNS measurements and Monte Carlo calculations have shown good agreement.
It is concluded that the Duo-Sphere approach provides a method of evaluating field correction factors for neutron instrumentation. In principle it could be applied to any neutron instrument or monitoring system provided that the variation of response with energy is known. The Duo-Sphere is sufficiently sensitive to allow accurate field correction factors to be determined in low dose rate neutron fields (within twenty minutes in a field of 1µSvh -1 for a typical low spectrum.
The Duo-Sphere
. 127mm (5") and 208mm (8.25") diameter polyethylene moderators.
. SP9 He-3 proportional counters
. Counting device such the NMS 14
Typical sensitivities in a 241 Am-Be field
. 127mm detector 0.770 cps/µSvh -1
. 208mm detector 1.520 cps/µSvh -1
. Leake design 0.185 cps/µSvh -1
How is it operated?
. Two measurements are made in the same location
. A detector is placed on the tripod and a measurement made
. The first detector is removed then the second detector is placed on the tripod
. Measured data is entered into an MS Excel spreadsheet
. Result is given as mean sensitivity in cps/µSvh -1 for each detector
. Duo-Sphere can be any combination of detector
Principle of operation
. The energy response of the 127mm and 208mm instruments was determined using the Monte Carlo MCBEND code
. Response validated by fixed point energies at NPL
. The response of each detector was calculated for realistic workplace spectra from SPKTBIB catalogue and from spectra measured using TNS
. Some unrealistic spectra were rejected from SPKTBIB but 205 were used
. Detector response in cps/µSvh -1 is plotted against the ratio of the 127mm and 208mm detector response in cps
Principle of operation - 208mm Response
Principle of operation - 127mm Response
Principle of Operation - Leake Design Response
Uncertainty of measurements
. Overall percentage uncertainty includes uncertainties of the fit, counting and calibration
. All detectors used as a Duo-Sphere have a fit of better than ±20% at 95% confidence level for all relevent spectra
. Measurements typically better than ±20% at 95% confidence level
. STF. Leake correction factor 1.86 compared with 2.02 determined by TNS . Neptune reactor at Derby. Leake correction factor 1.70 compared with 1.75 from MCBEND calculations . Realistic field at NPL. 127mm detector response factor 1.06 and 208mm detector 1.08
Conclusions
. Accurate ambient dose equivalent measurements . Used as a means of determining correction factors for other instrumentation . 127mm detector may be used in known fields instead of much heavier monitors . High sensitivity means measurements can be made quickly