CdWO4 scintillator as a compact gamma ray spectrometer for planetary lander missions

The objective of this work is to develop a gamma ray spectrometer (GRS) suitable for use on planetary rover missions. The main characteristics of this detector are low weight, small volume low power and resistance to cosmic ray radiation over a long period of time. We describe a 3 cm diameter by 3 cm thick CdWO₄ cylindrical scintillator coupled to a PMT as a GRS for the energy region 0.662–7.64 MeV. Its spectral performance and efficiency are compared to that of a CsI(Tl) scintillator 2.5 cm diameter by 6 cm thick coupled to a 28 mm×28 mm PIN photodiode. The comparison is made experimentally using ¹³⁷Cs, ⁶⁰Co, 6.13 MeV gamma rays from a ¹³C(,γn)O¹⁶* source, 7.64 MeV thermal neutron capture gamma rays emitted from iron bars using a ²⁵²Cf neutron source, and natural radioactivity 1.46 MeV ⁴⁰K and 2.61 MeV ²³²Th gamma rays. We use a Monte Carlo method to calculate the total peak efficiency of these detectors and the full energy, first and second escape peak efficiencies. The experimental and calculated results agree well. We investigated the usefulness of these detectors for a GRS on a Mars lander mission. Although both detectors meet desired specifications, it was found that CdWO₄ has advantages over CsI(Tl) being a more compact detector of higher efficiency. Using a shaping amplifier of 24 ms, CdWO₄ spectrometer exhibited a 6.8% FWHM at 662 keV. At 6.13 MeV, CdWO₄ detector possesses an intrinsic total and full energy peak efficiencies of 16.7% and 6.3%, respectively. These efficiencies are nearly a factor of 1.6 and 4 greater than the corresponding efficiencies of the CsI(Tl) detector.

A proposed gamma ray spectroscopy system to be placed on a rover, consists of a central detector surrounded by a Compton suppressor shield. The central detector is a cylindrical CdWO₄ detector and the Compton suppressor shield is made of segmented CdWO₄, coupled to PIN photodiodes. The shield also prevents thermal neutron activation of the central detector.     

Simulations of a thin sampling calorimeter with GEANT/FLUKA

The Advanced Cosmic-ray Composition Experiment for the Space Station (ACCESS) will investigate the origin, composition and acceleration mechanism of cosmic rays by measuring the elemental composition of the cosmic rays up to 10¹⁵ eV. These measurements will be made with a thin ionization calorimeter and a transition radiation detector. This paper reports studies of a thin sampling calorimeter concept for the ACCESS thin ionization calorimeter. For the past year, a Monte Carlo simulation study of a thin sampling calorimeter (TSC) design has been conducted to predict the detector performance and to design the system for achieving the ACCESS scientific objectives. Simulation results show that the detector energy resolution function resembles a Gaussian distribution and the energy resolution of TSC is about 40%. In addition, simulations of the detector's response to an assumed broken power law cosmic ray spectrum in the region where the ‘knee’ of the cosmic ray spectrum is believed to occur have been conducted and clearly show that a thin sampling calorimeter can provide sufficiently accurate estimates of the spectral parameters to meet the science requirements of ACCESS.     

Monte Carlo simulation of the high energy cosmic muon background in a resonant gravitational wave antenna

We present the results of a Monte Carlo simulation for the energy loss distribution of high energy cosmic muons crossing a Weber-type gravitational wave (g.w.) antenna. The number of events per day of energy greater than an assigned value, generated in the antenna by the muons, is deduced.

The simulation shows that a rate of 60 events per week due to the cosmic background is expected in a sea-level g.w. detector with an energy sensitivity ten times greater than that of the present antennas. With the sensitivity of the resonant detectors approaching the quantum limit value the rate will increase to 5 × 10⁴ events per days. Therefore it seems unavoidable to carry on the experiment in an underground laboratory.     

A neural network for positron identification by transition radiation detector

A neural network algorithm has been applied in order to distinguish positrons from protons by a transition radiation detector (TRD). New variables are introduced, that simultaneously take into account spatial and energy TRD information. This method is found to be better than the one based on classical analysis: the results improve the detector performance in particle identification for efficiency higher than 90%. The high accuracy achieved with this method is used to identify positrons versus protons with 3 × 10⁻³ contamination, as required by TRAMP-SI cosmic ray space experiment on the NASA Balloon-Borne Magnet Facility.     

MOS structures with “chlorine” grown oxide and tungsten-gold metallization

Oxide layers produced by the thermal oxidation of silicon in an oxidising atmosphere containing trichloroethylene show better properties when used in MOS transistors. The dielectric strength is improved, and the surface state density is reduced from about 2.8 × 10¹¹ cm⁻² to 1.0 × 10¹¹ cm⁻² compared with normally oxidised silicon.     

Heavy ion radiation damage in double-sided silicon strip detectors

A ²⁵²Cf fission fragment source was used to produce heavy-ion radiation damage in a double-sided silicon strip detector. It was found that a good quality fission fragment spectrum (as determined by the peak to valley ration N_L/N_V) could not be achieved for radiation incident on the p⁺ face of the detector. However, for radiation incident on the n⁺ face, the ratio N_L/N_V remained adequate up to an accumulated dose of 4×10⁶ fragments mm⁻². For the measurement of alphas, typical resolution deteriorated from an initial 30 keV FWHM to 50 keV FWHM at a dose of 8×10⁶ fragments mm⁻² for incident on the n⁺ face, and 6×10⁶ for radiation incident on the p⁺ face. The interstrip resistance in one region of the n⁺ face broke down completely after a relatively small radiation doses incident on that face. Further investigation of this is still required.     

Extensive air shower experiment at Kolar Gold Fields

An experiment to determine the nature of primary cosmic rays of energy > 10¹⁴ eV by studying high energy (> 220 GeV) muons and their correlations with other parameters of extensive air showers generated by them, was carried out at Kolar Gold Fields, India (atmospheric depth of 920 g cm⁻²). An accurate estimate of shower parameters in showers as small as 10⁴ particles was achieved by means of a closely packed array of large area detectors and by employing special methods of analysis. In this paper, the details of the array, the data recording system, the procedure of data analysis and error estimates are described.     

Characteristics of the radiation damage seen in the silicon microstrip detector of Fermilab experiment E771

The central region of the silicon microstrip detector used in Fermilab experiment E771 was subjected to a peak fluence of 9.5 × 10¹³ p/cm² induced by 800 GeV protons over a two-month period. Fourteen 300 μm thick planes manufactured by Micron Semiconductor were operated at bias voltages ranging from 84 to 109 V. Analysis of data from low intensity beam triggers taken near the end of the run shows that the mean pulse height from our amplifiers began to decline at a fluence of approximately 2 × 10¹³ p/cm² and fell to near zero by 6 × 10¹³ p/cm². We show that the use of fast amplifiers contributed to this early loss of signal.     

Accelerator test of an angle detecting inclined sensor (ADIS) prototype with beams of Ca and fragments

The measurement of cosmic rays and Solar energetic particles in space is basic to our understanding of the Galaxy, the Sun, phenomena in the Heliosphere and what has come to be known broadly as “space weather”. For these reasons, cosmic ray instruments are common on both scientific spacecraft and operational spacecraft such as weather satellites.

The resource constraints on spacecraft generally mean that instruments that measure cosmic rays and Solar energetic particles must have low mass (a few kg) and low power (a few W), be robust and reliable yet still highly capable. Such instruments must identify ionic species (at least by element, preferably by isotope) from protons through the iron group. The charge and mass resolution of heavy ion instruments in space depends upon determining ions’ angles of incidence. The Angle Detecting Inclined Sensor (ADIS) system is a highly innovative and uniquely simple detector configuration used to determine the angle of incidence of heavy ions in space instruments. ADIS replaces complex position sensing detectors (PSDs) with a system of simple, reliable and robust Si detectors inclined at an angle to the instrument axis.

In August 2004, we tested ADIS prototypes with a ⁴⁸Ca beam at the National Superconducting Cyclotron Laboratory's (NSCL) Coupled Cyclotron Facility (CCF). Among the analyses performed on the data taken at the NSCL, we demonstrate that our prototype design with an ADIS system has a charge resolution of less than 0.25e. We also present a more generalized analytic derivation of instrument response and report on the corresponding analysis of Monte-Carlo modeling data.     

The CPLEAR electromagnetic calorimeter

A large-acceptance lead/gas sampling electromagnetic calorimeter (ECAL) was constructed for the CPLEAR experiment to detect photons from decays of π⁰s with momentum p_π⁰ ≤ 800 MeV/c. The main purpose of the ECAL is to determine the decay vertex of neutral-kaon decays K⁰ → π⁰π⁰ → 4γ and K⁰ → π⁰π⁰π⁰ → 6γ. This requires a position-sensitive photon detector with high spatial granularity in r−, −, and z−coordinates. The ECAL - a barrel without end-caps located inside a magnetic field of 0.44 T - consists of 18 identical concentric layers. Each layer of 1/3 radiation length (X₀) contains a converter plate followed by small cross-section high-gain tubes of 2640 mm active length which are sandwiched by passive pick-up strip plates. The ECAL, with a total of 6X₀ has an energy resolution of and a position resolution of 4.5 mm for the shower foot. The shower topology allows separation of electrons from pions. The design, construction, read-out electronics, and performance of the detector are described.     

The kinematic separator VASSILISSA performance and experimental results

Within the last five years the kinematic separator VASSILISSA [A.V. Yeremin et al., Nucl. Instr. and Meth. A 274 (1989) 528; and A.V. Yeremin et al., Preprint JINR E15-90-347, Dubna 1990] has been used for investigations of neutron deficient evaporation residues (ER) produced in heavy ion fusion reactions. In the course of this work optimization of the ion-optical and other systems of the separator including improvements of the focal plane detector system have been made. As a result, the separation efficiency values ranging from 3 to 30% were achieved for ERs produced in the reactions with heavy ions ranging from ¹⁶O to ⁴⁰Ar. The suppression factors >10¹⁹ for the full energy beam particles and >10⁴ for multinucleon transfer reaction products were achieved. The results of the performed experiments illuminating the separator performance are briefly presented.     

Very high Q microwave spectroscopy on trapped171Yb+ ions: application as a frequency standard

A microwave frequency standard based on buffer-gas cooled ¹⁷¹ Yb⁺ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional frequency instability characterized by an Allan deviation σ_y (τ)=3.7×10^-13τ^-1/2 for τ<3000 s. Microwave Ramsey fringes with a Q factor of 1.5×10¹³ have been observed     

RAP: acoustic detection of particles in ultracryogenic resonant antenna

The resonant-mass gravitational wave detector NAUTILUS has recently recorded signals due to cosmic rays crossing. Very large signals have been observed in the superconductive state of the antenna. In order to investigate this anomalous response at low temperatures, the Rivelazione Acustica di Particelle experiment has been approved. Its purpose is the measurement of the mechanical vibrations in a superconducting (T100 mK) cylindrical aluminium bar when hit by 10⁵ electrons at 510 MeV from the DAΦNE Beam Test Facility, corresponding to the energies released by extensive air showers in the NAUTILUS antenna. The results of this measurement are crucial to understand the interaction of ionizing particles with bulk superconductors and to confirm the results on the thermo-acoustic model of the past experiments.     

Ion-activated interface adsorption of oxygen during silver deposition on silicon

The process of ion-activated oxygen adsorption on silicon has been investigated using an experimental concept with simultaneous deposition of silver films. Auger electron spectroscopy in combination with sputter depth profiling is subsequently performed to determine the amount of oxygen adsorbed at the Ag---Si interface. Noble gas ions (⁴He⁺, ²⁰Ne⁺ and ⁴⁰Ar⁺)with energies between 50 and 175 keV were used, and it was found that for substrate temperatures of 300–700 K the oxygen adsorption depends strongly on ion mass, ion energy and ion flux density. For flux densities of 5 × 10¹¹ cm⁻² s⁻¹ or less, adsorption dominates and, in particular, for light-ion bombardment the majority of adsorbed oxygen atoms form chemical bonds with the silicon surface atoms (Si---O). However, for heavy ions, physical sputtering starts to compete and limits the effective rate of adsorption. At sufficiently high ion fluxes the adsorption starts to decrease, and for all ions and energies used in this work it is found that, if the electronic energy deposition density exceeds a critical value of about 1.2 × 10²¹ eV cm⁻² s⁻¹, dissociation of the Si---O bonds prevails with a corresponding loss in the adsorbed oxygen quantity.     

Detection of γ-rays with a 3.5 l liquid xenon ionization chamber triggered by the primary scintillation light

A gridded ionization chamber with a drift length of 4.5 cm and a total volume of 3.5 l, was operated with high-purity liquid xenon and extensively tested with γ-rays from ¹³⁷Cs, ²²Na and ⁶⁰Co radioactive sources. An electron lifetime in excess of 1 ms was inferred from two independent measurements. The electric field dependence of the collected charge and energy resolution was studied in the range 0.1–4 kV/cm, for different γ-ray energies. With an electric field of 4 kV/cm, the spectral performance of the detector is consistent with an energy resolution of 5.9% at 1 MeV, scaling with energy as E^−0.5. The chamber was also used to detect the primary scintillation light produced by γ-ray interactions in liquid xenon. The light signal was successfully used to trigger the acquisition of the charge signal with a FADC readout. A trigger efficiency of 85% was measured at 662 keV.     

Detector for Rn measurements in air at the 1 mBq/m level

The emission of and β radiation in the decay of long lived (22.3 yr) ²¹⁰Pb to ²⁰⁶Pb gives rise to background events in the fiducial volume of the Borexino solar neutrino detector that fall in the energy window of the ⁷Be neutrino signal. Consequently, ²²²Rn as the predecessor of ²¹⁰Pb must be controlled in the central part (inner vessel) of the Borexino detector and in the air of all rooms where parts of the inner vessel are prepared or handled. The Rn monitor designed for Borexino uses electrostatic collection of the ²²²Rn daughter ²¹⁸Po on an detector. The detector (418 l active volume) operates at 30 kV. The detection limit for ²²²Rn in air is below 1 mBq/m³. The method of electrostatic collection has the principal disadvantage that the collection efficiency of the daughter ions depends on the degree of contamination of the air sample with water, hydrocarbons or other components. This causes an additional systematic error if the degree of pollution is unknown. A method to correct for this is outlined.     

The sat calorimeter of the DELPHI experiment at LEP; results of a module test

The construction of an electromagnetic calorimeter using scintillating plastic fibres and lead plates is described. The calorimeter is part of the Small Angle Tagger (SAT) of the DELPHI experiment at the LEP collider, recording high-energy electrons, positrons and photons. Results from a test of a module of similar construction are presented. The module was found to have a linear energy response when exposed to electrons of 10–70 GeV, with an energy resolution given by σ/E[%] = (1.16² + (11.4/√E[GeV])²)^1/2.     

Influence of Si doping level on the Raman and IR reflectivity spectra and optical absorption spectrum of GaN

The optical absorption (hν) and Raman and Infra Red (IR) spectra of Si doped GaN layers deposited on sapphire through buffer layers have been recorded for electron concentrations from 5×10¹⁷ to 5×10¹⁹ cm⁻³. The (hν) values deduced from photothermal deflection spectroscopy (0.5–3.5 eV) and IR absorption (0.15–0.5 eV) vary between 50 and 10⁴ cm⁻¹ showing doping dependant free electron absorption at low energy, doping independant band gap at high energy, and slowly doping dependant defect absorption in the medium energy range. In our micro Raman geometry, maxima appear or can be deduced near the frequency expected for either the A₁(LO⁻) or the A₁(LO⁺) modes split from the A₁(LO) mode by plasmon phonon interaction. There is a large systematic evolution in the expected way for the IR reflectivity.     

Enhancement of Er I13/2 population in Y2O3 by energy transfer to Ce

We report measurements of the energy transfer between Er³⁺ and Ce³⁺ in Y₂O₃. The transition between the Er^{3+ 4}I_11/2 and ⁴I_13/2 excited states can be stimulated by energy transfer to Ce³⁺, augmenting the population in the ⁴I_13/2 state at the expense of that in the ⁴I_11/2 state. Experiments were performed on Y₂O₃ planar waveguides doped with 0.2 at.% erbium and 0–0.42 at.% cerium by ion implantation. From measurements of Er³⁺ decay rates as a function of cerium concentration we derive an energy transfer rate constant of 1.3×10⁻¹⁸ cm³/s. The efficiency of the energy transfer amounts to 0.47 at 0.42 at.% cerium. The energy transfer rate constant measured in Y₂O₃ is two times smaller for Er³⁺→Ce³⁺ than that for Er³⁺→Eu³⁺ in the same material.     

Measurement of cascade showers in lead with thermoluminescent sheets

A new type of detector, thermoluminescent (TL) sheets (BaSO₄:Eu), and a readout system for the TL sheets have been developed to study electromagnetic cascade showers in ultrahigh energy interactions. To perform a measurement of showers recorded in TL sheets, the longitudinal and radial development of an electromagnetic cascade shower in lead produced by accelerator electrons has been studied for comparison. Simulations of the longitudinal and radial development of cascade showers were also performed using the EGS code system.

This study is useful for the design of a thermoluminescent calorimeter (TLC) for the measurement of electromagnetic cascade showers originating from ultrahigh energy (above 10¹⁵ eV) interactions.