C.C. Bueno

The performance of low-cost commercial photodiodes for charged particle and X-ray spectrometry

Abstract The energy response of a low-cost commercial silicon photodiodes detectors for alpha particles, fission fragments, internal conversion electrons and X-rays was studied. An alpha energy resolution of 16.7 keV (FWHM) was obtained for 5.486 MeV α-particles from 241 Am which is comparable in performance with the best surface barrier detectors available, that are considerably more expensive.

Rate effects in a proportional counter with resistive cathode

In this work, the rate e⁄ects associated with the gain of detectors with resistive electrodes are considered. To study these e⁄ects it was decided to use the classical proportional counter geometry, with a cylindrical glass tube as the cathode, rather than parallel plate devices. Indeed, the generality of the information obtained is not a⁄ected and the cylindrical proportional counter geometry has several advantages to gather detailed experimental data and to compare it with simple electric field calculations. A self-consistent set of data is presented for the e⁄ects of counting rate from a few tens of Hz up to several hundreds of Hz, using several gas mixtures, di⁄erent anode wires, cathode radii and thickness of the resistive electrodes. Although a few sources of systematic errors can be identified, all the observed experimental data can be quantitatively explained. Indeed, the measured gas amplification, from a few tens up to &5]104 and for the range of counting rates quoted above, depends only on the local electric field as it should be expected. These results are directly relevant to approach the problem of counting eƒciency in resistive plate chambers. ( 1998 Elsevier Science B.V. All rights reserved.

Transient behaviour and rate effects in resistive detectors

Abstract The dependence of the detection efficiency (or charge gain) of a resistive detector on the counting rate has been considered by several authors. The influence of the resistivity of the electrodes, the counter geometry and the gas mixture composition have also been studied. Most of those measurements assume a stationary regime. In the present work we report on studies of the transient behaviour of detectors with a resistive cathode for various counting rates, charge gains and detector geometries. We show that in some cases the time-decay curves can be fitted by a single exponential plus a constant term, while in others (higher charge gains or higher counting rates) a sum of two or even three exponentials plus the constant term is needed to fit the experimental data. A study of the electric properties of the dielectrics used is also presented and a comparison is made between these results and the data obtained under irradiation conditions.

Pressure dependence of secondary NIR scintillation in Ar and Ar/CF/sub 4/

The variation with charge gain of the total number of photons emitted per electron, in the visible and near-infrared (NIR) regions (400</spl lambda/<1000 mm), in Ar/CF/sub 4/, mixtures, is investigated for CF/sub 4/ concentrations below 15%. Measurements were performed in a uniform field configuration and results are presented for charge gains below 100, under X-ray excitation. The maximum number of photons emitted per drifting electron (0.7 and 0.2 ph/e/sup -/ for Ar/sup +/ 2% CF/sub 4/ and Ar+10% CF/sub 4/, respectively) is obtained for a charge gain G/spl sim/4. The spectral distribution of the emitted light in Ar/CF/sub 4/ mixtures is also analyzed. The pressure dependence (1-4 atm) of the secondary light output, in the visible and near infrared regions is studied as a function of the reduced electric field E/P for pure argon and Ar/CF/sub 4/ mixtures. Above the charge multiplication threshold, the maximum number of photons emitted per electron decreases by a factor of about 1.5 when the pressure increases from 1 to 2 atms.

Measurement of the absolute activity of 235U by the generalized coincidence method

The absolute activity of 235U contained in a sample was measured utilizing a sum-coincidence circuit which selects only the α-particles emitted simultaneously with the 143 and 186 keV gamma radiations from the 231Th (product nucleus). The α-particles were detected by means of a new type of a gas scintillating chamber, in which the light emitted by excitation of the gas atoms, due to the passage of a charged incoming particle, has its intensity increased by the action of an applied electric field. The γ-radiations were detected by means of a 1″ × 112″ NaI(T1) scintillation detector. The value obtained for the half-life of 235U, (7.04 ± 0.01) × 108y, was compared with the data available from various observers which used different experimental techniques. It is shown that our results are in excellent agreement with the best data available on this subject.

Manufactured silicon diode used as an internal conversion electrons detector

In this work we have studied the direct detection and spectrometric capabilities for internal conversion electrons of an ion-implanted diode, developed in the framework of R&D programs at CERN, envisaging its use in an electron-gamma coincidence system for nuclear parameters measurements. The best energy resolution obtained until now, 12.6 keV (FWHM) for the 133Ba 320.32 keV electron emission, is sufficiently good to justify the use of this diode for spectrometry of low energy electrons.

On the Origin of the Satellite Peaks in Alpha Particle Spectra

In this work we describe the results obtained with S3590-06 and ion-implanted diodes for alpha particle spectrometry. Satellite peaks were also investigated in order to identify not only the origin of these phenomena, but also the importance of the guard ring region on their relative intensities. The preliminary results indicate that the true cause of the spurious satellite peaks might be associated with the weak electric fields around the guard rings region and changes on the entrance window absorption near the edge of the diodes. Further steps are needed to clear up this problem

Pressure dependence of secondary NIR scintillation

The variation with the charge gain of the total number of photons emitted per electron, in the visible and NIR regions (0.4</spl lambda/<1 /spl mu/m), in Ar/CF/sub 4/ mixtures, is investigated for CF/sub 4/ concentrations below 15%. Measurements are performed in a uniform field configuration and results are presented for charge gains below 100, under X-ray excitation. The spectral distribution of the emitted light in Ar/CF/sub 4/ mixtures is also analyzed. The pressure dependence (1-5 atm) of the secondary light output, in the visible and near infrared regions (NIR) is studied as a function of the reduced electric field E/P for pure argon and argon-tetrafluoromethane mixtures. Above charge multiplication threshold, the number of NIR photons decreases with increasing pressure.

Rate effects in radiation detectors with resistive electrodes

Detectors with highly resistive electrodes, particularly RPCs, have been studied extensively in recent years. Several authors considered the effect of rate on the detector response (efficiency) and a few also associated with the rate, in a rather qualitative manner, the charge per event, i.e., the current. Here, without loss of generality of the results obtained, rate effects are studied with a classical cylindrical counter geometry and different gas mixtures. To a high accuracy, the observed behavior, both for the proportional mode and for the transition to streamers, is quantitatively and accurately interpreted as loss of voltage on the dielectric due to the current generated in the detector, with direct consequences for the local electric field in the avalanche region. Results obtained in the self quenching streamer (SQS) mode fail the previous interpretation and clearly need further research.

Short-term repeatability of a rad-hard EPI diode applied in electron processing dosimetry

The rad-hard EPI diode was characterized envisaging its application in on-line 1.5 MeV electron beam dosimetry in radiation processing. As expected from a current sensitivity loss (13.5%) to the accumulated dose up to 750 kGy, the EPI diode exhibited good short-term repeatability (CV = 1.9%) which is better than that required for CTA dosimeters, routinely applied in radiation processing. The dose-response curve of the EPI diode was fitted by a second order polynomial function for doses up to 775 kGy. In addition, some preliminary studies on the radiation damage effects induced in this diode using a Capacitance Deep Level Transient Fourier Spectroscopy system are presented. The results showed that up to almost 1.5 MGy only 3 defects (VO, V2 and CiOi) can influence the carrier lifetime and current generated. Further radiation damage studies are under way.