A. Pansky

The scintillation of CF4 and its relevance to detection science

Abstract The scintillation properties of CF 4 are presented in comparison with those of Xe and CH 4 . Alpha-particle induced photon emission was measured with vacuum phototubes and with a CsI-based gaseous photomultiplier. The latter method provides an absolute sensitivity of such devices to particle-induced UV-photon background in CF4 and CH4 gaseous Cherenkov radiators. Integrated CF 4 scintillation yields over the range of 150–220 nm are, on the average, 315±95 to 242±60 photons/MeV, in the respective pressure range of 0.063 to 0.75 atm, compared to CH 4 which emits 0.06±0.01 photons/MeV at 1 atm. The total photon yield, integrated over the full emission spectrum of CF 4 (150–500 nm), is of the order of 1200 photons/MeV × 4 π. The primary scintillation photon yield of CF 4 is about 16(±5)% of that of Xe. No proportional secondary scintillation was observed in CF 4 . The avalanche-induced photon yield was measured to be of the order of 0.3 photons per electron. The implications of this considerable photon emission, are discussed.

New approaches to spectroscopy and imaging of ultrasoft-to-hard X-rays

Abstract We propose new techniques of X-ray spectroscopy and imaging, based on the use of low-pressure multistep gaseous electron multipliers. Ultrasoft X-rays are detected by counting single-electron clusters induced in the gas. X-ray induced UV-photons in gas scintillation chambers are read out with wire chambers coupled to CsI photocathodes. X-rays converted in foil-electrodes are imaged by fast multistep avalanche electron multipliers. We discuss the advantages of the various techniques and present experimental results and Monte Carlo simulations.

On the two-stage operation mechanism of low-pressure microstrip gas chambers

A two-stage multiplication mechanism has been observed in microstrip gas chambers (MSGC) operated in a pressure range of 10–50 Torr of isobutane. The resulting high gain (> 104) of single electrons photo-produced on a CsI photocathode is attributed to a preamplification in the gas gap followed by anode strip multiplication. The large and fast rise of the induced pulse in this mode leads to an efficient single electron detection at relatively low charge gain. The reduced positive ion feedback preserves radiation convertors coupled to such electron multipliers from sputtering damage. MSGCs operated in this mode are expected to have a subnanosecond time resolution and very high rate capability. Some potential applications are briefly discussed.

Ionization measurements in small gas samples by single ion counting

Abstract A new method for highly efficient measurements of the ionization statistics in small, wall-less, well-defined low density gas samples is proposed. It is based on counting ions, induced by radiation in a sensitive gas volume. The high resolution permits the measurement of spatial correlations between the number of ions induced in two distanced small sensitive volumes. Using tissue- or solid-equivalent gases, the method allows the accurate determination of the ionization statistics in the corresponding sub-nanometer volume of condensed matter. These data are of relevance to the modeling of microscopic phenomena related to the interaction of radiation with matter, such as in nanodosimetry and studies of radiation damage to solid state devices.

A new technique for studying the Fano factor and the mean energy per ion pair in counting gases

A new method is presented for deriving the Fano factor, F, and the mean energy per ion pair, Wi, in counting gases. It is based on the technique of individual counting of single ionization electrons induced in low‐pressure gas samples by soft x‐ray photons. A correlation of the experimental data with a detailed simulation of the electron deposition and counting process permits the extraction of the Fano factor and the mean energy per ion pair values. We present data of F and Wi for C2H6 and Ar/C2H6 over the energy range of 100–1500 eV. The energy dependence of these parameters reflects the atomic level structure of the gases. We discuss in detail the accuracy of this technique and its advantages and limitations. Ways are proposed for improving the technique and for broadening the energy range.

Fano factor and the mean energy per ion pair in counting gases, at low x-ray energies

The mean energy per ion-pair (Wi) and the Fano factor (F) are provided with high accuracy (2% and 3%-4%, respectively), in C2H6, C3H8, i-C4H10, CH4, DME, Ar/C2H6(20:80), Ar/i-C4H10(20:80) Ar/DME (20:80) and Ar/Xe/i-C4H10(66.6/16.7/16.7), in the x-ray energy range of 0.11–1.5 keV. These parameters were extracted from precise measurements of the number and temporal distribution of x-ray induced electrons, accompanied by extended simulations of the detection process. A decrease in these parameters with increasing x-ray energy was observed, accompanied by sharp increases at x-ray energies just above some atomic shells. The effect is discussed in relation to Auger electron emission. A Penning process in Ar/C2H6(20:80) and Ar/i-C4H10 (20:80) is observed on the basis of comparative measurements of Wi and F in these mixtures and in the pure hydrocarbons. Ways are proposed for further improving the accuracy provided by the electron counting technique to better than 1%.

Detection of X-ray flourescence of light elements by electron counting in a low-pressure gaseous electron multiplier

Abstract Ionization electrons deposited by soft X-rays in a low pressure (10 Torr) gas medium are efficiently counted by a multistage electron multiplier, providing an accurate measurement of the X-ray photon energy. Energy resolutions of 56-28% FWHM were measured for X-rays of 110–676 eV, recording electrical induced charges or visible photons emitted during the avalanche process. It is demonstrated that a combined analysis of the number of electrons and the electron trail length of an event, provides a powerful and competitive way of resolving ultra-soft X-rays. We present the experimental technique, discuss the advantages and limitations of the Primary Electron Counter, and suggest ways to improve its performances.

ON THE HIGH GAIN OPERATION OF LOW-PRESSURE MICRODOT GAS AVALANCHE CHAMBERS

Abstract Microdot avalanche chambers (MDOT) equipped with thin semitransparent Cr photocathodes, were characterized with UV photons at low gas pressure. Gains superior to 104 were reached with gas multiplication at the dots. In a mode where preamplification in the gas volume precedes the additional dot multiplication, gains superior to 106 were measured at 30–60 torr of propane. The fast amplification mechanism results in narrow high amplitude pulses with 2–3 ns rise time, visible with no further electronic amplification means. We present here our preliminary results and briefly discuss potential applications.

Specific primary ionization induced by minimum ionizing electrons in CH4, C2H6, C3H8, i-C4H10, argon, dimethylether, triethylamine, and tetrakis(dimethylamino)ethylene

Specific primary ionization induced by minimum ionizing electrons has been measured in several gases and vapors. Charges deposited by β‐electrons in a low‐pressure gas were collected, amplified by a multistep gaseous electron multiplier, and counted. The high counting efficiency of the multiplier provided results of systematically higher values as compared to existing data. The respective values of the specific primary ionization in CH4, C2H6, C3H8, i‐C4H10, argon, dimethylether, triethylamine, and tetrakis(dimethylamino)ethylene are: 0.034, 0.065, 0.095, 0.12, 0.03, 0.082, 0.195, and 0.370 clusters/cm Torr. The experimental method is presented and the results and their accuracy are discussed.

Application of the electron-counting method to low-Z elemental analysis

Abstract We applied the electron counting method, in low-density gas, for ultra-soft X-ray spectroscopy. The method is based on the spatial expansion of photon-induced ionization electron clusters, and the recording of pulse trails formed by individual electron avalanches in a fast multiplication element. A detector, operating at room temperature with low gas pressure, was characterized with Particle-Induced X-ray Emission (PIXE) and with electron-induced X-ray emission in a Scanning Electron Microscope (SEM). High detection efficiency for soft X-rays is obtained due to the very thin polymer window between the detector and the radiation source. The low-density detector is blind to Bremsstrahlung and charged-particle background. We present the results of the spectral analysis of characteristic X-rays emitted from low-Z elements, in the energy range 100–1500 eV. The recorded X-ray spectra, using both the information of the number of counted electrons and the length of the electron pulse trail, are unfolded with the help of a computer simulation of the detector response to X-ray photons. This detailed simulation of the electron detection and counting process provides an efficient means for a quantitative spectral analysis and permits the precise reconstruction of complex energy spectra.