Eine einfache Energiebetrachtung zeigt, das elektronenangeregte Molekule ein geringeres Ionisierungspotential und eine hohere Elektronenaffinitat besitzen als Molekule im Grundzustand. Dadurch sind Elektronenubertragungsreaktionen zwischen angeregten Molekulen und geeigneten Elektronendonatoren bzw. -akzeptoren moglich, die im Grundzustand nicht stattfinden. Dies wird durch blitzlichtspektroskopische Untersuchungen an Perylenlosungen (+ Amin bzw. Pyridinium) bestatigt.

Die mit Hilfe von Fluoreszenzmessungen an den gleichen Systemen durchgefuhrte kinetische Untersuchung zeigt, das der Elektronenubergang in unpolaren Medien zu einem angeregten Elektronenuberfuhrungskomplex fuhrt, wahrend er in polaren Losungsmitteln offenbar auch ohne direkten Kontakt zwischen den Reaktionspartnern moglich ist.

Elektronenübertragungsreaktionen des angeregten Perylens

Sensitization of charge injection into semiconductors with large band gap

The absorption and fluorescence spectra of crystalline pyrene have been measured at room temperature and 77°K. Only the first absorption region could be studied because of crystal thickness but a Davydov splitting of about 18 cm—1 was detected with the a polarization at higher energy than the b polarization. Calculations of the splitting (dipole‐dipole approximation for the intermolecular potential) give 23 and 13 cm—1, respectively, for the B2u and B3u levels with the observed sense, and so it is not possible to decide between two possible assignments from theory. The polarization ratio shows that mixing with a higher level of opposite polarization is present and this is consistent with the only theoretical prediction of energy levels in the molecule.The fluorescence is displaced to long wavelengths from the absorption spectrum and is a broad band with a maximum which is further displaced on cooling the crystal. The band has been interpreted as a charge‐transfer transition involving the molecules grouped...

Absorption and Fluorescence Spectra of Crystalline Pyrene

The scintillation properties of trans -stilbene crystals grown for the first time by application of the solution growth technique to the scale of 10 cm are reported. Measurements of the scintillation light output, pulse shape discrimination, and neutron detection efficiency were made with sets of crystals cut as 50 cm diameter cylinders of different lengths from 0.3 to 10 cm. Comparison to liquid scintillators and traditional melt-grown stilbene showed that at increasing sizes new solution-grown crystals exhibit better scintillation performance that makes them promising for use in large scale neutron detectors. Results are discussed in relation to structural imperfections attributed to different methods of growth.

Scintillation properties of solution-grown trans-stilbene single crystals

Efficient, readily-available, low-cost, high-energy neutron detectors can play a central role in detecting illicit nuclear weapons since neutrons are a strong indication for the presence of fissile material such as Plutonium and Highly-Enriched Uranium. The main challenge in detecting fast neutrons consists in the discrimination of the signal from the gamma radiation background. At present, the only well-investigated organic crystal scintillator for fast neutron detection, in a n/gamma mixed field, is stilbene, which while offering good pulse shape discrimination, is not widely used because of its limited availability and high cost.

New Organic Crystals for Pulse Shape Discrimination

The scintillation counting behavior of a group of fifty‐five pure crystalline organic compounds has been extensively studied. The data obtained have been analyzed with the goal of developing a better understanding of the scintillation process and of scintillator behavior.The materials were carefully purified, and massive crystals were grown. Relative scintillation average‐pulse‐height efficiencies at 30°C and —70°C for cobalt‐60 gamma‐ray excitation, gamma‐ray excited scintillation decay times, and 2537 A ultraviolet‐excited reflection and transmission photofluorescence spectra have been determined. A few solutions were also studied for comparison purposes. The purification and properties of the different materials are discussed in detail.The experimental data have been analyzed on the basis of Birks' photon cascade theory of the scintillation process. The ratio of the scintillation efficiency to the integrated photofluorescence intensity is shown to be a measure of Birks' primary photon production effici...

Study of Organic Scintillators

Two methods for separating Na22 from magnesium cyclotron targets are described. The first is a direct precipitation of NaMg(UO2)3(C2H3O2)9·6H2O or NaZn(UO2)3(C2H3O2)9·6H2O from an acetic acid solution of the target utilizing the normal sodium contamination of the magnesium for carrier. This is effective for as little as 1 mg Na in 5 g of Mg. If the sodium contamination is too small for precipitation, the bulk of the magnesium can be removed as Mg(NH4)2(CO3)2·4H2O which does not carry sodium. Deuteron excitation functions for Mg26(d,α)Na24 and Mg24(d,α)Na22 and thick target yields for the two sodium isotopes are given. The maximum values of the cross sections are σNa24=0.095×10−24 cm2 at 8.5 Mev and σNa22=0.16×10−24 at 9.5 Mev. With 14‐Mev deuterons the thick target yields are Na24=8.7rd/μah and Na22=65mrd/μah.

Cyclotron Targets: Preparation and Radiochemical Separation. III. Na22

Radioactive krypton (T=34h, e+, γ) has become a very useful gas for tracer experiments. This isotope can be made by Br79,81(d, 2n)Kr79,81. Preparation and separation of this gas have been a routine matter in this laboratory for some time and a brief description of the methods used is of interest. Two different methods will be described. Both are equally good as far as the separation of the krypton is concerned. One method involves very simple manipulation, but requires an apparatus made of monel metal, fused quartz, and Pyrex glass; the other uses only readily obtainable and inexpensive apparatus, but slightly more complicated manipulation.

John W. Irvine

Papers

Study of Organic Scintillators

Cyclotron Targets: Preparation and Radiochemical Separation. III. Na22

Cyclotron Targets: Preparation and Radiochemical Separation. II. Krypton

Preparation of Zn65 of High Specific Activity from Copper Bombarded with 16‐Mev Deuterons