Methods of neutron field spectrometry, other than those depending on the use of pulsed neutron sources, are surveyed. Neutron spectrometers are compared with particular reference to characteristics such as energy resolution, useful energy range, neutron detection efficiency and response functions.

Neutron spectrometry-historical review and present status

The literature relevant to the use of insulators and dielectrics in a space environment is reviewed and discussed in terms of specific aspects of the space environment such as temperature, pressure, radiation, corrosion and meteoroid flux. Whereever possible, data are presented from space experiments or simulators designed to duplicate as closely as possible the space environment. Data from high energy physics, nuclear power and electric utility communities are included where relevant. The need for accelerated aging methodology for a multistimulus/multifactor stressed environment is also discussed.

Electrical Insulation and Dielectrics in the Space Environament

/pdf/scintillation-detectors-for-fast-neutrons-2zd1z7uff0.pdf

Scintillation detectors for fast neutrons

Deuterated liquid scintillation detectors have shown promise as neutron spectrometers in absence of time-of-flight for identifying discrete neutron energy groups. Here, we investigate their advantage in spectrum unfolding of continuous Watt spectra similar to those emitted from special nuclear materials. We compare the deuterated EJ315 with the hydrogen-based EJ309 in identical Scionix detector assemblies. Measurements with a 252 Cf neutron source are used as trial spectra. Results indicate an advantage for the deuterated EJ315. We use measured response matrices and a simple sequential-least-squares algorithm without regularization.

Comparison of spectrum-unfolding performance of (EJ315) and (EJ309) liquid scintillators on measured 252Cf pulse-height spectra

Verification of future warhead-dismantlement treaties will require detection of certain warhead attributes without the disclosure of sensitive design information, and this presents an unusual measurement challenge. Neutron spectroscopy—commonly eschewed as an ill-posed inverse problem—may hold special advantages for warhead verification by virtue of its insensitivity to certain neutron-source parameters like plutonium isotopics. In this article, we investigate the usefulness of unfolded neutron spectra obtained from organic-scintillator data for verifying a particular treaty-relevant warhead attribute: the presence of high-explosive and neutron-reflecting materials. Toward this end, several improvements on current unfolding capabilities are demonstrated: deuterated detectors are shown to have superior response-matrix condition to that of standard hydrogen-base scintintillators; a novel data-discretization scheme is proposed which removes important detector nonlinearities; and a technique is described for re-parameterizing the unfolding problem in order to constrain the parameter space of solutions sought, sidestepping the inverse problem altogether. These improvements are demonstrated with trial measurements and verified using accelerator-based time-of-flight calculation of reference spectra. Then, a demonstration is presented in which the elemental compositions of low-Z neutron-attenuating materials are estimated to within 10%. These techniques could have direct application in verifying the presence of high-explosive materials in a neutron-emitting test item, as well as other for treaty verification challenges.

Warhead verification as inverse problem: Applications of neutron spectrum unfolding from organic-scintillator measurements

A neutron spectrometer consisting of a deuterated anthracene scintillation crystal and a pulse shape discriminator is described. Forward recoiling deuterons are selected by means of their pulse shape signatures, making use of the direction dependence of the pulse height response and scintillation decay characteristics of the crystal. The line shape of the spectrometer for monoenergetic neutrons is a single peak and the neutron energy resolution (FWHM) varies from 7% at 9 MeV to 3.5% at 22 MeV neutron energy.

Deuterated anthracene spectrometer for 5–30 MeV neutrons

The pulse height response of NE102A plastic scintillator films to fragment ions from 252 Cf spontaneous fission has been investigated as a function of fragment time-of-flight and film thickness t in the range 1–15 μm. For velocities close to the two peaks in the fragment velocity spectrum and for films of either t ≳ 9 μ m or t ≲ 3 μ m, an approximately linear relationship between response and fragment velocity is found. For films of thickness 3–6 μm or for fragments from symmetric or highly asymmetric fission. deviations from the linear dependence are observed. The response increases rapidly with film thickness in the range t = 3–6 μ m and saturates at t ≳ 9 μ m for both light and heavy fragments. The possible role of surface effects in these phenomena is discussed.

W.A. Cilliers

Papers

Deuterated anthracene spectrometer for 5–30 MeV neutrons

Response of thin NE102A scintillator films to fission fragments

Differential cross section for n-p radiative capture at En = 63.4 MeV