Neutron-Induced Background

Atmospheric and albedo neutrons can interact within detectors at balloon altitudes and in low-Earth orbit. Target nuclei can emit secondary particles or gamma rays that contribute to the background counting rate of the 

---

As noted earlier, spectroscopic information can be obtained by observing either the inelastically scattered neutrons or the de-excitation y rays. In the y-ray detection mode, i.e. (n,n y), an intrinsic HpGe or Ge(Li) detector is placed at approximately 100 cm from the cylindrical sample (see Fig. 1). Since the beam is pulsed, the neutron-induced events in the detector and time-independent background can be rejected through the use of the TOF... 

It is useful to extrapolate the cosmic ray components of the background in the deep KamLAND detector to shallower depths at which detector construction is more feasible. Most of the cosmic ray induced background comes firom neutrons with energies up to several hundred MeV produced by muon capture and spallation in materials surrounding the detector. Few experiments have been done that give detailed information on the... 

Fig. 5. Neutron counting as detection method for spontanous fission events of superheavy nuclei. The recorded neutron rates (points) were found to follow the relative cross sections of cosmic-ray induced spallation reactions (curve) and were, thus, due to background events. The numbers are rates for natural uranium and thorium. From W. Grimm, G. Herrmann and H.-D. Schiissler [40].

The pH-induced micellization of a DMAEMA-6-DEAEMA diblock copolymer has been studied in detail using dynamic light scattering, small-angle neutron scattering, and fluorescence spectroscopy [166], The DMAEMA constitutes the corona of the micelle, whereas the DEAEMA forms the core. Pyrene was used as a probe to determine the nature of the DEAEMA blocks. It was shown that the hydrophobicity of the micellar cores increased progressively as the solution pH was adjusted from pH 7 to 9. In the presence of an electrolyte, it was possible to observe both individual chains (unimers) and micelles under certain conditions. The critical micellization pH depended on both the copolymer concentration and also the background electrolyte concentration. 

Ar production in the crust is small compared to the ambient background of atmosphere-derived Ar introduced into the crust dissolved in groundwater and is usually neglected. Although a small amount of muon-induced Ar occurs close to the surface, the principle route of production is the P-decay of C1 (Fontes et al. 1991 Hiinemohr 1989). C1 has a half-life of 3.01 x 10 yr and decays to Ar with a branching ratio, R, of 0.95. Cl is only produced in the crust by the thermal neutron reaction... 

O Brien K, Sandmeier HA, Hansen GE, Campbell JE (1978) Cosmic ray induced neutron background sources and fluxes for geometries of air over water, ground, iron, and aluminum. J Geophys Res 83 114-120... 

The main activity induced by neutrons in an aluminum matrix is A1, which is a shortlived radionuclide (Tu2 = 2.25 min) that decays within an hour. However, fast neutrons, also present in a nuclear reactor spectrum, react with Al yielding Na via the Al(n,a) Na reaction. Because of the relatively long half-life (Ti/2 = 15 h) and the high-energy photons that cause significant increase of the background due to Gompton interaction, Na becomes a major interference in the determination of trace elements in Al. 

Since neutron and gamma fields caimot be separated, as is done in NAA, a more complex shielding is needed in this technique. This requires more construction material in the vicinity of the detector, thus increasing the spectral background induced by the scattered neutrons in these materials. (See later.)... 

The SMOPY device can also be operated in active mode using an AmLi source. This has been successfully demonstrated for the underwater verification of canisters containing residues of irradiated HEU. This application is based on total neutron counting and detects the difference between active background and active background plus induced fissions. 

---