Gridded Ionization Chamber Research Articles

High Resolution Measurements of Double Differential (n, .ALPHA.) Cross Sections of 58Ni and natNi between 4.2 and 6.5 MeV Neutrons.

Double differential (n, α) reaction cross sections of 58Ni and natNi were measured for 4.2–6.5 MeV neutrons with energy resolution good enough to separate α-particles from the low-lying levels of residual nuclei by using a gridded ionization chamber. Angular distribution and excitation functions were derived for α0, α1 and αi≥2 components (α-particles to the ground level, the 1st level and levels higher than the 2nd level, respectively). The experimental results were compared with these obtained from calculation based on Hauser-Feshbach model employing the optical potential and the level density parameters derived to reproduce the experimental values of total, (n,p) and (n,α)cross sections. The calculation showed fair agreement with the experimental data while it underestimated the (n,α)cross section above 6 MeV.

Detailed study of double-differential cross-sections for the 17O(n,α) 14C reaction

The 17O(n,α) 14C reaction was studied in order to clarify discrepancies in the data on the reaction cross-section for the range of 50 keV to 1000 keV and perform α-particles angular distributions corresponding to different resonances measurements. The experiment was carried out using a gridded ionization chamber. The target was niobium oxide enriched in 17O.

Measurement of 14N(n,p) 14C cross section for kT = 25.3 keV Maxwellian neutrons using gridded ionization chamber

We measured the 14N(n,p) 14C reaction cross section for neutrons with a quasi-Maxwellian distribution of kT = 25.3 keV which is one of the key parameters for the s-process nucleosynthesis in astrophysics. Protons were detected with high efficiency using a gridded ionization chamber. We employed a thick Li target and a 1.912 MeV proton beam for neutron production, and a vacuum-evaporated melamine (C 3H 6N 6) film as a 14N sample. The absolute cross section was determined relatively to the 6Li(n,t)α cross section by measuring the triton yield from a 6LiF film in place of melamine. Our result (1.70 ± 0.08 mb) is approximately a factor 2 larger than the value reported in an earlier direct measurement for the same Maxwellian neutron spectrum; it is consistent with results deduced from monoenergetic neutrons or inverse reaction.

Attenuation length of drifting electrons in allene-doped liquid argon

The attenuation length of drifting electrons in liquid argon doped with 50 ppm of allene and its time variation have been investigated by using a dual type gridded ionization chamber. The attenuation length changed from 1.1 m as initial value to longer than 2 m after 20 hours in an electric field of 1.0 kV/cm, while the amount of collected charge decreased with time. The reason why the charge collection yield in liquid argon doped with allene is degraded with time is discussed.

Purification techniques and purity and density measurements of high-pressure Xe

A xenon gas purification system and simple techniques for purity and density measurements of high-pressure (∼60 atm) Xe have been developed. The purification system features two stages. As a first stage of purification an oxisorb and a high-temperature getter are used to purify Xe gas up to a level of that typically required for liquid Xe detectors. As a second stage, a titanium spark purifier is employed to further improve the purity by an order of magnitude. A gridded ionization chamber is used to measure the purity level and density of the xenon. The duration of pulses produced by cosmic muons inside the chamber gives the lower limit of an electron's life time. The capacitance between the mesh and anode, which depends on the dielectric constant of Xe, provides a temperature independent measurement of the density. This work is a part of a development program of high-pressure Xe detectors for hard X-ray and low energy gamma-ray astronomy.

Electronic collimation for precise measurement of resolution, with a gridded ionization chamber

An ionization chamber with a Frich grid is used to determine both the energy (E) of the charged particles emitted from the sample positioned coplanar with the cathodes and the cosine of the emission angle (θ) with respect to the cathode. Using the combined information of cos (θ) and E, problems in particle counting due to sample absorption and scattering effects can be circumvented. An isotropic angular distribution for alpha particles is obtained. Investigations on affected areas using backscattering and self-absorption, and on unaffected areas were carried out and a resolution difference between the two areas was measured. The difference is greater than 0.6%.

A compressed xenon ionization chamber X-ray/gamma-ray detector incorporating both charge and scintillation collection

A high resolution room temperature dual gridded ionization chamber X-ray/gamma-ray detector incorporating both charge and scintillation light collecting capabilities is described. The detector uses highly purified and compressed xenon gas as the detection medium and has an intrinsic energy resolution of 2.4% at 662 keV. The scintillation signal is ideal for triggering, background discrimination, and ionization position determination. The detector design and timing properties are described.

Rapid Monitoring of Soil, Water, and Air Dusts by Direct Large-Area Alpha Spectrometry

During retrieval and disposition of wastes containing transuranium elements, continuous monitoring of the air, water, and soil for alpha emitters was required to ensure that safety limits were not exceeded and that the waste itself was not disturbed unknowingly. Direct measurements by alpha spectrometry were particularly promising because of their potential speed, sensitivity, and their ability to identify transuranium radionuclides under field conditions. Soil samples or settled dusts were finely ground, suspended in 80% ethanol, sprayed onto circular stainless steel pans, and dried on a hotplate. Water samples were mounted directly by spraying. Air dusts were collected with a high-volume air sampler on 20- by 25-cm membrane filters. The samples were then analyzed directly in a large pressurized gridded ionization chamber without further sample preparation. The lower limits of detection for 10-min counting times were 1.5 Bq g-1 (40 pCi g-1) for 100-mg soil samples, and 4 x 10(-2) Bq m-3 (10(-12) microCi mL-1) for a 10-min air sample taken at 0.4 m3 min-1 (14 cubic feet per minute) and counted without waiting for decay of radon progeny.

Prescission neutron emission in 235U(nth,f) through fragment-neutron angular correlation studies.

Measurements of prompt neutron energy spectra and angular distributions from mass and kinetic energy selected fission fragments were carried out in the thermal neutron fission of $^{235}\mathrm{U}$. Neutron energy was determined by the time-of-flight technique and fission fragment energy and angle were measured using a back-to-back gridded ionization chamber. The measured angular distributions of neutrons emitted from fragment pairs of various mass and kinetic energy were compared with results of Monte Carlo calculations assuming neutron emission from fully accelerated fragments to determine the component of neutrons which may be emitted in the prescission stage. The calculations were carried out using as inputs the measured center-of-mass neutron energy spectra and multiplicites and assuming isotropic emission of neutrons in the center-of-mass frame of both the fission fragments, and a three source fitting of the angular distributions was done to deduce the component of prescission neutrons. The value of the prescission neutron multiplicity averaged over all fragment masses is found to be 0.25\ifmmode\pm\else\textpm\fi{}0.05 (about 10% of the total neutron multiplicity). The present results have been discussed on the basis of the energy damping and time scale of the saddle to scission transition in the thermal neutron induced fission process.

High resolution room temperature ionization chamber xenon gamma radiation detector

A unique thin walled dual-type gridded ionization chamber gamma radiation detector using ultra pure Xe gas as the detection medium is described. The detector was operated at room temperature and the energy spectra of 60Co, 137Cs, 22Na and 133Ba were obtained. An energy resolution of (16 keV) 2.4% FWHM was determined for the 662 keV 137Cs gamma peak which is substantially better than the resolution of conventional NaI(Tl) detectors. The effect of electric field strength, grid field ratio, gamma energy, and electronegative impurities on energy resolution are discussed.

Large solid angle spectrometer for the measurements of differential (n, charged-particle) cross sections

A charged-particle spectrometer with a large solid angle close to 4 π has been developed for studies of fast neutron induced charged-particle emission reactions. On the basis of a high-pressure gridded ionization chamber employing high- Z structural elements, the spectrometer permits the measurements of energy-angular distribution of secondary charged-particles with very high geometrical efficiency and a good signal-to-noise ratio. It has been applied successfully for the measurements of double-differential cross sections of (n, xα) reactions for incident neutrons up to 14 MeV, and of (n, p) reactions up to 6 MeV by use of the data reduction and particle selection methods developed in this study. The present spectrometer proved to be an efficient means for studies of differential (n, charged-particle) cross sections.

Measurement of attenuation length of drifting electrons in liquid xenon

To realize a long attenuation length of drifting electrons in liquid xenon, a purification system which consists of Oxisorb, molecular sieves and a Zr-V-Fe alloy getter has been constructed. A dual type gridded ionization chamber is used for the measurement of the attenuation length. An attenuation length longer than 2 m is achieved in the purified liquid xenon.

Compressed xenon gas near its critical point as an ionization medium

The authors report results obtained from a dual gridded ionization chamber filled with highly purified gaseous xenon operating near its critical point where the density, rho /sub c/=1.09 g/cm/sup 3/, approaches that of the liquid phase. The data obtained suggest that xenon near its critical point is an important addition to the presently available high-resolution radiation detector media. Both the ability to drift ionization electrons over large distances without attenuation and stability with time have been demonstrated. The energy resolution of the 976-keV K-conversion electron peak of /sup 207/Bi was measured to be 20 keV FWHM in 1.4 g/cm/sup 3/ (62 atm) of xenon, independent of the distance over which the charge drifts. These findings indicate that attachment to electronegative impurities is not a problem. An additional effect was observed as the density of the xenon gas was lowered from 1.4 to 0.5 g/cm/sup 3/: the energy resolution improved without an increase in the average charge collected. >

Back-to-back gridded ionization detector technique for the determination of fragment angle and its use for the measurement of neutron angular distributions in 235U (n th, f)

A back-to-back gridded ionization chamber was used to measure the prompt neutron angular distributions after determining the fission fragment angle with incorporation of energy loss correction in the target and backing material. The correspondence of the ratio ( V g/ V c) of the grid pulse height to the collector pulse height with cos θ is made use of to derive energy loss corrected V g and V c values. The fragment angle is obtained from the corrected grid pulse heights after determining the calibration constants for 0° and 90° emission angles as a function of energy and mass of the fission fragments. An angular resolution of 2° to 3° [FWHM] for each fragment could be obtained over a large angular range on each side of the ionization chamber. The chamber was used along with a NE213 neutron detector to measure the angular distributions of the prompt neutrons emitted in the thermal neutron induced fission of 235U, and results of these are reported as an illustration of the present method.

Charge collection and energy resolution studies in compressed xenon gas near its critical point

We have built a 4.5 l high purity Xe gas ionization chamber and operated it at pressures of up to 62 atm (1.4 g/cm 3) at room temperature. This thermodynamic regime in pure xenon has not previously been studied. Ionization from a 207Bi internal conversion electron source was drifted over distances of 2.7 and 5.0 cm in two separate gridded ionization chambers within the gas volume. We observed that there was less than 1% difference in the charge collected from these two detectors at the same electric field strength. Energy resolution measurements were performed over the range of xenon densities from 0.49 to 1.40 g/cm 3 at several electric field strengths. An electronic noise subtracted resolution of 16 keV FWHM was measured for the 976 keV 207Bi internal conversion line at an electric field strength of 1.3 kV/cm in compressed xenon gas near its critical density.

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in235U(n th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.

Measurements of rise times in liquid argon and xenon gridded ionization chambers

The distributions of rise time for gridded ionization chambers filled with liquid argon and xenon are measured by means of two different techniques.

Fano factor in xenon.

The Fano factor and W value in xenon have been measured with a gridded ionization chamber for 5.3-MeV \ensuremath{\alpha} particles. We obtained a value for the Fano factor of 0.29\ifmmode\pm\else\textpm\fi{}0.02 and the W value of 21.9\ifmmode\pm\else\textpm\fi{}0.3 eV. This W value is nearly equal to those for \ensuremath{\alpha} and \ensuremath{\beta} particles measured by Jesse and Sadauskis [Phys. Rev. 90, 1120 (1953); 97, 1668 (1955)]. The Fano factor is considerably larger than the value reported for x rays using a proportional scintillation counter. The reason for this difference is discussed.

The W Values and Fano Factors in Helium and Argon-Doped Helium

The W values and Fano factors for 5.3 MeV alpha-particles have been measured with a gridded ionization chamber and are found to be 43.3±0.3 eV and 28.9±0.2 eV, and 0.24±0.02 and 0.11±0.02 for pure helium and helium+argon (1%), respectively. The W values for both gases agree fairly well with theoretical estimates as well as with previous experimental results and especially with the results of Jesse. The corresponding Fano factors, however, are significantly large compared with those theoretically calculated for the case of electron incidence. The reasons for this difference are discussed.

Determination of fission fragment angle by the methods of grid pulse heigh and the time difference between grid and collector pulses in a back-to-back gridded ionisation chamber

Measurement of the angle of fission fragments in 235U(nth, f) using a back-to-back gridded ionisation chamber is described by both the grid pulse height method and the method of the time difference between grid and collector pulses. By adopting a mass and energy dependent angle calibration procedure, angle resolution of 3 to 4 degrees (FWHM) could be achieved for light fragments in both methods. Fragment mass and kinetic energy correlations are studied after electronically collimating the fragments to a cone perpendicular to the target foil to reduce target thickness effects. The results show that for the fragment masses around ML = 123 amu and MH = 123 amu two kinetic energy groups separated by about 13 MeV are present.