The advantage of , -coincidence spectroscopy for the measurement of radiochemically separated nuclides of low radioactivity is demonstrated. The background is reduced by about three orders of magnitude, and its main component, the natural radioactivity of the laboratory walls, is reduced by about four orders of magnitude. The contribution of cosmic rays becomes important at higher energies and within large shields.
With this article we remember the discovery of solid and liquid scintillators 50 years ago.H. Kallmann andL. Herforth found out that aromatic hydrocarbons are suitable to convert the absorbed energy of nuclear radiation especially from β-particles
into light. They found the quenching of impurities, the reduced photon yield of α-emitters and the different fluorescent decay
time of α- and β-particles.
After publication of an article about the discovery of organic scintillators by H. KALLMANN and L. HERFORTH more details came to the authors knowledge which point to the need to add I. BROSER to the discoverers of organic scintillators.
Neutrons contribute to the background of low radioactivity measurements, especially with detectors in large lead shields above
ground, and to a lower extent in underground laboratories of medium depth. We measured the natural neutron flux in the energy
region of fission by commercial 3He filled proportional counters at different places above ground and in an underground laboratory, which is covered by 47
m of rock, and demonstrated the drastic reduction of the neutron flux in the underground and the influence of the materials
of the environment.
The determination of impurities in semiconductor silicon by nondestructive and destructive NAA is described. To improve the
detection limit, a multiple beta—single gamma detector assembly is used. It is shown that24Na is also produced from silicon by a (n, αp) reaction with reactor neutrons. The cross-section with fission neutrons is 1.8·10−9 barn.
Using a layer etching technique impurity profiles of Cu and Fe in silicon samples are measured by n.a.a. Radiation enhanced
diffusion of Cu and Fe impurities from the surface into the bulk material has been observed. The samples were etched before
irradiation to remove the impurity profiles caused by the sampling procedures.
Impurity profiles of selected samples are determined by neutron activation analysis. After irradiation and cleaning with a
mixture of hydrochloric and nitric acid a layer of 20 μm was etched with hydrofluoric and nitric acid. In the layer 0.5 ng/cm2 Cu and 3 ng/cm2 Fe were found. In cutted slides of zonefloated silicon we found a deep profile expressed by the equation c=co·exp-(x/a)2, co=2 ppm, a=190 μm.
A survey of problems connected with the activation analysis of extremely pure materials is given. The treatment of the surface
of the samples influences the results. The detection limit of counting radiochemically pure nuclides are determined by efficiency,
background and counting interval. The background, which usually is reduced by selected construction materials, large shields
and anticoincidence techniques also may be reduced by the βγ-coincidence technique. It is possible to arrange several small
β-ray detectors closely to one large γ-ray detector. Several samples are measured at the same time with a long counting interval
without interferences. The method was applied to the analysis of semi-conductor silicon.
Beta-gamma-coincidence spectroscopy is a sensitive measurement technique in reactor neutron activation analysis. A simple
counting device with a 4″×3″ NaI(Tl) crystal and a 32 mm×2 mm plastic scintillator is described and the advantage of the β-γ-coincidence
technique, which has been successfully applied to the determination of iron and other impurities in semiconductor silicon,