Nuclear Instruments最新文献

The properties of the aberration coefficients of the mass spectrographs with first order double focusing along a straight image line are examined, of which in a previous paper⁶) was shown that in one point of the image line two or three of the aberration coefficients are zero. It results that for some of these instruments, besides the mentioned advantages, further improvements are possible. There exist not only instruments in which some aberration coefficients vanish in additional points at the photographic plate, but also instruments with small aberration coefficients in a wide range of the plate. The dimensions of especially favourable mass spectrographs are given in a table, the progress of the aberration coefficients belonging to them is shown in diagrams and the positions of the zero points of the coefficients are indicated in a table.

In measurements of ratios of counting rates, the effects of amplitude drift can be eliminated by the use of one amplifier and one pulse height analyser common to all radiation detectors. A mixing and relaying system incorporating this method is described.

The response of a $\text{1}\text{3}\text{4}\text{″ × 2″}\text{NaI(T1)}$ cylinder to neutrons in the pulse height region between 2.5 and 7 MeV gamma energy equivalent was studied as a function of incident neutron energy for a range of neutron energies between room scattered neutrons and 1.162 MeV. The Li⁷(p, n) Be⁷ reaction was used as a source of neutrons since it is relatively free from high energy gamma ray background. Monoergic neutron response curves were obtained for neutron energies of 166 keV, 341 keV, and 469 keV. All other distributions were for proton energies giving rise to two neutron groups. The pulse height distributions were measured for a fixed number of counts of a shielded long counter at each neutron energy. The long counter was calibrated against Ilford C2 emulsions for the case of 1.162 and 0.698 MeV neutrons in order to obtain the neutron flux from the long counter counts. The pulse height distributions are nearly linear between 4 and 6.6 MeV. The extrapolations of these distributions intersect the gamma equivalent axis at approximately the binding energy of the last neutron in I¹²⁸.

A type of current stabilizer for magnet, used in nuclear magnetic resonance experiments (fig. 3), is described. A high dynamic stability (1:20 000 at 1.6 A, input voltage variations ±5%, load variations 10%) is obtained using a diode with a wolfram filament, which represents a stage with an extremely high amplification in the feedback loop. A high thermal stability is provided by means of a diode which is practically insensitive to normal room temperature changes, as it is evident from eq. (3).

In spite of a relatively high time constant of the heater filament—in connection with fig. 3—owing to the great inductance of the magnet, it is possible to obtain a very quick answer to sudden changes of input voltage.

Measurements were made with a precision potentio-meter of an accuracy 1:100 000 and results agree within 20% with theoretical considerations.

A 60° sector type electromagnetic isotope separator with homogeneous magnetic field is described. The radius of the ion path is 75 cm. The ion source of magnetic type with oscillating electrons is used. Characteristics of the separator and results of measurements are given.

In this paper we establish some criteria for the choice of the collimators for a crystal spectrometer for neutron diffraction in order to achieve a good compromise between luminosity and resolution.

General expressions for the full width at half maximum and for the luminosity of the diffraction peaks are developed for powder sample. The advantage of the parallel arrangement is also computed.

Absolute β-counting with radioelements of low decay-energy is difficult because of absorption losses in the source and its backing. If moreover the half-life is very high, extended sources are necessary. It has been furthermore shown, that backings must be electrically conducting to prevent charging of the source and thereby distortion of the electric field of the counter, if counting is done with the source in the interior of the counter.

A method is described for preparing electrically conducting formvar foils of 85 × 30 mm² with an overall thickness of 10 μg/cm². The conductivity is attained by first exposing the foil to a glow discharge in a hydrocarbon atmosphere and then evaporating silver on it.

An improved ring counter is described which can be extended to any number of stages. The use of modern switching tubes allows counting rates up to 1 Mc/s, every stage giving separate output pulses without loading effect on the cycling process. For program switching application more than one state may simultaneously pass around the ring.

An investigation has been made of the relative amounts of scintillation light emitted from mixtures of the noble gases during the 0.5 μsec interval following the stopping of an α-particle. A bellows pump and a hot uranium furnace in a closed circulation system maintained gas purity. A diphenyl stilbene coating on the photomultiplier tube and the walls of the scintillation volume acted as a wavelength shifter for the primary ultraviolet radiation. Data on the efficiency of various proportions of binary combinations of xenon, krypton, argon, neon, and helium are presented. They show a characteristic large drop in the light for mixtures containing a small proportion of the heavier gas in a major fraction of the ligher. These results may be qualitatively interpreted in terms of noble gas molecular ion formation. The large light output of the 10% Xe-90% He mixture might be useful in constructing a neutron spectrometer of high efficiency using the He³(n,p)H³ reaction.

With the experimental set-up, described earlier⁸), further studies of high current linear deuterium pinches have been made. The stabilizing effect of an external magnetic field has been investigated. This effect has appeared in two ways. Firstly, peaks in the voltage curves and dips in the current curves, which are interpreted as due to sausage instabilities, disappear at B_z 〉 400 gauss. Secondly, the number of subsequent pinches is increased by the field and as many as six consecutive pinches have been observed. The dependence of the neutron yield on the magnetic field has been found to be very weak, the yield decreasing only by a factor of $\text{2}\text{3}$ for 760 gauss. The influence of the gas pressure on the number of neutrons produced has been studied in the region 9 to 30 microns. For B_z = 0 a slight decrease in the neutron yield with increasing pressure has been observed. With the field applied the yield seems to be almost independent of the pressure. The resistance of the discharge has been found to decrease to 0.01 ohm within the first tenths of the discharge period. The results are discussed in connection with the results of other investigators.