Compton Background Research Articles

Coincidence-anticoincidence gamma-ray spectroscopy with a NaI(TI) split annulus and a Ge(Li) detector

A versatile gamma-ray spectrometer system consisting of a Ge(Li) detector surrounded by a large (20.3 cm × 20.3 cm) split NaI(Tl) annulus is described, and experimental results are presented as examples of its use in a number of coincidence and anticoincidence experiments. The annulus as a whole was used as an anti-Compton spectrometer and was found to reduce the Compton backgrounds in the Ge(Li) detector considerably. When the optically-isolated halves of the annulus were used separately, the system was found to be very efficient for triple-coincidence experiments and as a pair spectrometer. As the latter, it yielded double-escape spectra having essentially no underlying backgrounds, even when used with complex gamma-ray emitters such as 56Co. It was also very effective in determining relative β + feedings to various levels in the decay of position emitters. When the system was used as a coincidence spectrometer with narrow gates set on the pulses from the annulus, the Compton backgrounds of coincident gamma-rays detected in the Ge(Li) detector were reduced significantly.

A spectrometer for neutron capture gamma ray studies

Due to the large Compton background from intense gamma lines the usefulness of the Ge(Li)-detector for the investigation of neutron capture gamma ray spectra between 500 and 2000 keV is restricted to the determination of strong gamma lines. By placing the Ge(Li)-detector in the reflected beam from a single crystal spectrometer this background can be completely suppressed within a variable energy interval. This combination has been tested on the complex gamma yield from thermal neutron capture in 167Er. When using the Ge(Li)-detector as a photo effect spectrometer gamma rays up to about 1500 keV have been detected. The energy resolution of the combined spectrometer, using this particular Ge(Li)-detector is better than that of double flat and curved crystal spectrometers above about 700 keV. Recent developments in solid state detector techniques will appreciably lower this limit.

Über eine eichung eines 7.5 × 7.5 cm - NaJ(Tl) - Vollkristalls zur absolutzählung flächenförmiger präparate

The photopeak efficiency and the peak-to-total ratio for a 7.5 × 7.5 cm solid NaI(Tl)-crystal are determined in the range of 0.145–1.33 MeV. The disintegration rates of the disc sources used are known by 4 πβ-and 4 πβ- γ-coincidence measurements. The effect of the disc diameter and the distance source-crystal on the photopeak efficiency is studied thoroughly. The data obtained are compared with those of other authors. The evaluation of the photopeak area is performed in a frequency diagram, a special type of probability scale, in which both parts of the Gaussian curve are represented by straight lines. Hence, corrections for the Compton overlap, bremsstrahlung and background can accurately be made.

A scheme for high luminosity, high resolution internal and external conversion measurements

In charged particle spectrometers relying upon the dispersive power of some kind of imaging system one generally has to compromise between the desire of high resolution and the desire of high intensity at the detector. Starting from a source of given absolute and specific strength, and a given level of resolution, any factor of improvement in resolution can in principle be realized if the accompanying decrease in intensity is accepted. The aim of the present work has been to realize the sufficient experimental conditions for proceeding about an order of magnitude in resolution in the well-known double focusing π√2 β-spectrometer without this normally occurring reduction of intensity. To achieve this with a given absolute source strength requires that the aperture solid angle is at least not reduced when going to the higher resolution. With also the total weight of source material given, the smaller maximum source thickness allowed at the higher resolution requires—in the general case — a possibility of distributing the source material over a larger source area when going to higher resolution. Taken together, the above requirements make it necessary to improve the luminosity, i.e., the product of source area and aperture solid angle, by between two and three orders of magnitude at the higher resolution. In the present case this has been achieved by adding to the magnetic π√2 spectrometer two electrostatic devices. One of these, the corrector, reduces the point source aberrations while keeping the aperture solid angle constant and the other device, a special source arrangement, allows a large source to be used in conjunction with high resolution. This latter possibility is realized by using a non-equipotential source surface. In this way electrons, which are originally monoenergetic, obtain energy changes which depend on the position of their starting point. The potential distribution on the source surface is chosen in such a way that the resulting variation of electron momentum, by means of the dispersion of the magnetic field, compensates for the variation in starting point in such a way that an extended source is imaged as a narrow line in the detector plane. By the combined action of the corrector and the new source arrangement an increase in luminosity by a factor of 120 over that of the normal double focusing spectrometer has been experimentally achieved, with even larger improvement within reach. In the case of external conversion measurements the above methods are also applicable but, however, are not always sufficient to meet the basic aim of fully retaining the intensity when improving the resolution. However, the large source area permitted by the above methods allows a special method to be used for the reduction of the Compton electron background often disturbing external conversion measurements. For weak photolines on an intense Compton background a reduction of this background can compensate for a possible residual reduction in photoline intensity which may accompany an improvement in resolution. The reduction of the Compton background relies upon a geometrical separation of the image of the converter (i.e., the photo electron source) and the image of the γ-ray source with absorber (the main contributor to the Compton background). The possible background reduction factor increases with resolution and the method is hence especially suited to the present high-luminosity, high-resolution methods.

Semiconductor beta and gamma detectors

The principles of semiconductor nuclear particle detectors and their applications are reviewed, and recent progress is reported, with particular attention to detection of beta and gamma radiations. By a lithium ion drift process, p-i-n structures with 5-mm (millimeter) depletion depth can now be fabricated routinely in silicon. Such counters have about a 5% efficiency for the detection of gamma photons; when cooled they sometimes show energy resolutions of under 10 kev (kiloelectron-volts). Recent results using germanium, which has a much higher photoelectric absorption cross section, show the photoelectric peak is well above the Compton background for the 660-kev gamma radiation from Cs <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> , with a line width less than 7 kev for the 120-kev gamma from Co <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> . The possibilities of other materials for large volume particle detectors are discussed.

Quantitative Determination of Preferred Orientation

Solutions are presented to the problems which usually prevent accurate x-ray determinations of preferred orientation. The relationship between observed and corrected intensities and the angle dependent absorption factors are treated, including the case of displaced and distorted line profiles caused by low absorption in thick specimens. The absorption factors are derived experimentally from measurements of the scattered (Compton) background radiation without recourse to calculations or tables. Two simple methods, either by varying the wavelength λ or the order n of the x-ray reflection, span the region of the pole figure in which data are unreliable or unobtainable if only one wavelength or order is used. Random samples are not required for normalizing the data to ``times random'' units. The intensity for a random sample can be calculated from the corrected data itself and the equation Ihkl=[[double integral operator]Ihkl(α,β) sinαdαdβ]/2π where (α,β) are the coordinates of a point on the usual polar net. It is suggested that textures can generally be represented by Gaussian or superposition of Gaussian orientation distributions.

Gamma ray spectroscopy using a germanium lithium-drifted diode

Ge diodes with Li-drifted depletion layers are shown to be advantageous for gamma ray spectroscopy. The 122 and 136 kev lines of the Co/sup 57/ spectrum are readily resolved from the Compton background. The Cs/sup 137/ spectrum has a photoelectric peak about twice the height of the Compton background. It is pointed out that a Ge Li-drifted diode will require additional drifting after each use. (D.L.C.)

γ-Ray spectroscopy of artificial radioactive samples from atmospheric air

Light air samples were collected in Naples between October 1958 and June 1959 by filtration on paper and analyzed for their gamma radiation. The radiation was analyzed in the range of energies 0.25 to 1.70 Mev; in addition, sample No. 8 was analyzed in the range 0.03 to 0.70 Mev. Samples No. 8 and 7 are considered in detail. The 0.36 Mev peak (due to I/sup 131/, Ba/sup 140/, and La/ sup 140/) decayed more rapidly than the others. Corrections for the Compton background were calculated with the 0.75 Mev peak due to Zr/sup 95/ and Nb/sup 95/ as reference. Use is made of the data to locate the date of the event producing the radioactivity, and it is concluded that the jet stream passing over Naples could have contributed to the radioactivity of sample No. 8. (D.L.C.)

Scintillation Spectrometer with an Anticoincidence Annulus of NaI(Tl)

In the scintillation spectrometer described, the main detecting crystal of NaI(Tl) is enveloped by a hollow cylinder of NaI(Tl) in anticoincidence with the main crystal. This system completely suppresses the peaks associated with the escape of the two quanta from the annihilation following pair production, and it reduces the Compton background by a factor of 7 for 4.43-Mev gamma rays and by a factor of 5 for those of 6.1 Mev.