M.A.F. Alves

Improved resolution for low energies with gas proportional scintillation counters

Abstract Observation of the light emitted by xenon, when the primary electrons produced by an ionizing particle are accelerated under an electric field, allows the detection of energies, in the ionization chamber region, that, with gas counters, were only previously detectable in the proportional region. For example, for 5.9 keV a full width half maximum of 500 eV was measured. The relative variance of the pulses from the gas proportional scintillation counter is 0.087 E − 1 2 (E in keV ) and can be slightly improved. The possibility of using the counter for fast coincidence work is considered.

Detection of soft X-rays with a xenon proportional scintillation counter

Abstract Soft X-rays were detected with a xenon proportional scintillation counter; for example, full widths at half maximum of 100 eV and 140 eV were obtained for the carbon and oxygen X-rays, respectively. Window sizes up to 100 mm2 were used. The measured value of the Fano factor, 0.170 ± 0.007, implies that the ultimate relative standard deviation of the pulses is 0.06 E − 1 2 (E in keV ) to be compared to the experimental determination 0.087 E − 1 2 .

The argon-nitrogen proportional scintillation counter

Abstract The characteristics of a gas proportional scintillation counter are studied for various argon-nitrogen mixtures at total pressures of around 1000 Torr, using α-particles. Measurements of the delay between the primary and the secondary scintillation pulses show that most of the secondary light is produced close to the central anode. The secondary scintillation amplitude increases sharply with the nitrogen concentration reaching a maximum at about 2.5%. Its variation with the anode voltage is studied and it is shown that for large voltages higher order scintillation components appear. The various factors affecting the energy resolution are considered. The experimental resolution is found to be limited by the appearance of the higher order components, the light collection efficiency and the photomultiplier. The extrapolation of the data shows that if the first factor can be eliminated the resolution will be limited by the total number of photons produced and not by a scintillator intrinsic resolution which is found to give a contribution of less than 1%. The spectral analysis of the secondary light (carried out with optical filters) is made for various voltages and nitrogen concentrations. It shows that most of the light is produced within the 3250 A–3800 A region. The importance of nitrogen on the secondary scintillation mechanism is discussed.

Fano factors of rare gases and their mixtures

Abstract Measurements of FW (Fano factor × mean energy to make an ion-pair) for several non metastable Penning gas mixtures were made using the proportional scintillation process. Taking for W the values that correspond to 5.3 MeV alpha particles, upper limits of F were determined. The following results were obtained: 100% Xe, FW = 3.247, F ⩽ 0.15 ± 0.03 100% Ar, FW = 10.536, F ⩽ 0.40 ± 0.03 Ar + 80% Xe, FW = 4.635, F ⩽ 0.21 ± 0.03 Ar + 5% Kr, FW = 9.397, F ⩽ 0.37 ± 0.06 Ar + 24% Xe, FW = 5.093, F ⩽ 0.23 ± 0.02 Ar + 20% Kr, FW = 3.038, F ⩽ 0.12 ± 0.03 Ar + 20% Xe, FW = 3.582, F ⩽ 0.16 ± 0.02 Ar + 79% Kr, FW = 3.113, F ⩽ 0.13 ± 0.02 Ar + 5% Xe, FW = 3.204, F ⩽ 0.14 ± 0.03 100% Kr, FW = 4.584, F ⩽ 0.19 ± 0.02

Observation of electron multiplication in liquid xenon with a microstrip plate

Abstract We report here on the observation of electron multiplication in liquid xenon in a microstrip chamber with an amplification factor of the order of 10. The measurements were carried out at a temperature between 208 and 215 K (liquid density of about 2.7 g/cm 3 ).

Thresholds for Secondary Light Emission by Noble Gases

This work concerns the hypothesis that direct excitation of gaseous media by migrating electrons under the influence of weak electric fields is the dominating process by which secondary light is emitted from pure noble gases. Determinations both experimental and theoretical, as well as compilations of previous data, are made for the lower limit of the reduced electric field, for which secondary light emission from gaseous media displays a sharp increase. A detailed comparison of the theoretical results with more reliable published experimental data shows, in general, very good agreement.

Liquid xenon multiwire chamber for positron tomography

Abstract The liquid xenon multiwire chamber has been proposed as a detector of 511 keV gamma-photons for positron emission tomography (PET). Both the scintillation and the ionization signal, read from the anode wires, are used. A test chamber of about 300 cm3 and containing six multiwire ionization cells has been built. The cell is formed by two plane cathodes, 10 mm apart, and 20 anode wires spaced by 2.5 mm tied to pairs resulting in 10 channels, read out independently. The first experiments were performed for a single cell. The number of the anode wire on which the signal is induced gives the depth of interaction in the detector with a precision of 5 mm. This is important information allowing one to reduce the parallax error and, therefore, to decrease the tomograph size and improve the solid angle. The position sensitivity across the cell (or along the tomograph ring) is achieved by measurement of the drift time of the electrons from the point of primary ionization to the anode, triggered by the scintillation detected with a photomultiplier tube. The scintillation also supplies a fast trigger for the coincidence analysis.

The gas proportional scintillation counter under X-rays bombardment: Resolution and pulse correlations

Abstract The resolutions of a proportional scintillation counter (“pill box” geometry) with Xe and Xe−0.75% N 2 and the correspondent charge resolutions are measured for 5.9 keV X-rays. Typical values are 20% for the light pulses and 17.5% for the charge pulses. It is shown that there is a strong correlation between charge and light pulses, implying the same intrinsic resolution, in agreement with the mechanism of light production. The practical light output for the 5.9 keV X-rays was about the same as the light output of a NaI(Tl) crystal for the 137 Cs γ-rays.

Large light output gas proportional scintillation counters

Abstract The scintillating properties of the mixtures Ar-5% Xe and He-42.5% Xe are studied under the influence of a radial electric field. The secondary scintillation pulse, which is slow (∼ 10 μs), has an amplitude that exceeds by a factor of 200–300 the pulse amplitude from standard CsI(Tl) and NaI(Tl) scintillators. This is an order of magnitude larger than the one from the previously reported argon proportional scintillation counter. This study is carried out for different conditions of pressure and electric field intensity, with and without wavelength shifter. The maximum pulse amplitude (with no higher order components) is, for the Ar-5% Xe mixture, independent of pressure. These mixtures have inherent interest for nuclear spectroscopy applications.

Gas Proportional Scintillation Counter with Xenon and Xenon Mixtures

The light output of the recently developed gas proportional scintillation counter - a gas scintillation counter with light multiplication produced by a cylindrical geometry electric field - is investigated for heavy gaseous media under alpha particle excitation with a 56 UVP phototube. The gases used are Xe, and Xe-Ar and Xe-N2 mixtures for a wide range of concentrations, at a total pressure of about 965 Torr. The light output of the Xe-Ar mixtures shifts towards the ultraviolet (? ? 3250 A) region as the electric field intensity increases. Xe-Ar mixtures with Xe concentrations ranging from about 1 to 10% give - with a p-quaterphenyl wavelength shifter - a light output more than two orders of magnitude larger than that of a CsI(T1) scintillator. In Xe-N2 mixtures - with wavelength shifter - nitrogen has a quenching effect. For the mixtures of noble gases the rise time of the secondary component of the light pulse is slow, typically 15 ?s. Methane and nitrogen while having a quenching effect, reduce this rise time.