Background Rejection Efficiency Research Articles

The non-linear signal domain selection using a new quality function in neural net training

Applying cuts in the space of event parameters is the traditional technique for background rejection in high energy physics. Obtained by considering the simulation of signal and background events, particular values of these cuts are used to reach required balance between efficiency of signal detection and background rejection. Modern experiments in high energy accelerator physics and astrophysics are operating with multidimensional parametric spaces. Thus, the problem of the best cut selection is of vital interest. Frequently used rectangular cuts are usually too restrictive and can deteriorate the shape of selected multivariate signal distribution. With the aid of the proposed method, it is possible to obtain smooth nonlinear shape of signal cluster which optimizes the ratio of signal to noise. The search of the best γ-cluster on the data files of Crab nebula detection by Atmospheric Cherenkov Telescope of Whipple collaboration proves the superiority of neural techniques upon traditional methods.

The potential of ground based arrays of imaging atmospheric Cherenkov telescopes. I. Determination of shower parameters

Abstract The performance of the arrays of imaging atmospheric Cherenkov telescopes (IACTs) for detection of ≥ 100 GeV γ-rays is discussed. The effective detection area of stereoscopic multi-IACT systems is determined essentially by the total geometrical area of the array which may be covered by independent standard ≥ 3 telescope ‘cells’ with a linear size of about 100 m. We have investigated the characteristics of a quadrangular 100 × 100 m2 ‘cell’ with four ‘100 GeV’ class IACTs in its corners which is recommended as an optimal design for the future IACT arrays. The idea of the ‘cell’ makes straightforward the study of the characteristics of IACT arrays. The conclusions obtained, concerning in particular the angular and energy resolutions, and the cosmic ray background rejection efficiency, are rather general and, do not depend on the specific configuration of the array.

The Gas Imaging Spectrometer on Board ASCA

The Gas Imaging Spectrometer (GIS) system on board ASCA is described. The experiment consists of 2 units of imaging gas scintillation proportional counters with a sealed-off gas cell equipped with an imaging phototube. The performance is characterized by the high X-ray sensitivity (from 0.7 keV to over 10 keV), good energy resolution (7.8% FWHM at 6 keV following E−0.5 as a function of X-ray energy E), moderate position resolution (0.5 mm FWHM at 6 keV with E−0.5 dependence), fast time resolution down to 61 μs, and an effective area of 50 mm diameter. The on-board signal processing system and the data transmitted to the ground are also described. The background rejection efficiency of the GIS is reaching the level achieved by the non-imaging multi-cell proportional counters.

Separating γ- and hadron-induced cosmic ray air showers with feed-forward neural networks using the charged particle information

The success of current and future air shower arrays in detecting point sources of cosmic rays above 10 TeV depends crucially on the possibility of finding efficient methods for separating γ-induced air showers from the overwhelming background of hadron-induced showers. We study the application of computer-simulated feed-forward neural networks in the analysis of cosmic ray data taken with the Geiger towers of the HEGRA air shower array. The combination of these charged particle detectors with the neural net based analysis is characterized by high background rejection and signal efficiency. In contrast to the often-quoted non-transparency of the net technique, the detailed analysis of the net performance gives insight into the physics involved and helps to asses the different information that allow γ/hadron separation

Inflight performance of the position sensitive proportional counter on board the X-ray astronomy satellite ROSAT

Two redundant position sensitive proportional counters (PSPC) operating in the energy range from 0.1 to 2.4 keV are the prime focal plane instruments on board the X-ray astronomy satellite Rosat, Rosat was launched on June 1, 1990 and has operated highly efficiently since then. The authors report on the in-orbit performance of the PSPC. Apart from its food energy and position resolution ( delta E/E=41% and 230 mu m full width at half maximum at 1 keV, respectively) the detector has a very high charged particle background rejection efficiency of 99.8%. The gas gain of the detector is very stable. Short-time variations due to thermal variations over one orbit are less than 1%. The long-term drift over one month is 0.5%. >

SVX test beam results

An SVX sector prototype was operated at Fermilab in a configuration very close to the version to be installed in CDF. The sparse readout features of the SVXD chip [1,2], which provide the local data compaction that is necessary to deal with the detector high number of channels (40K) and typical Tevatron luminosities, were tested for the first time with a 230 GeV/ c pion beam. A good background rejection and high hit efficiency was demonstrated in this mode of operation, with adequate signal to noise ratio and spatial resolution.

Simulations of neutron response and background rejection for a scintillating-fiber detector

We address the problem of detection of fast neutrons at energies of 5–30 MeV in the presence of strong backgrounds of low-energy neutrons and gammas. In an organic scintillator, the neutrons are detected mainly as recoil protons from n-p scattering; the gammas produce electrons primarily by Compton scattering. The recoil protons and electrons can be distinguished because the light output per unit distance near the end of a proton track is much higher than that for an electron. This difference could be used to reject gamma backgrounds such as those in a reactor environment by constructing a segmented detector from a bundle of scintillating fibers connected to a readout device. We present the results of extensive simulations for a module composed of 6400 fibers, each 250 μm square, arranged into a 2×2×2 cm 3 bundle. Considered were fast neutrons with energies of 3, 7, 14 and 28 MeV and backgrounds composed of neutrons and gammas with energy distributions characteristic of delayed emission from a fission source. The simulations indicate that the detector can obtain a good compromise between outstanding background rejection and reasonable detection efficiency for the neutrons of interest.

A background rejection efficiency calculation for a proposed gaseous xenon TPC in a double beta decay experiment

A detailed calculation of the double beta (ββ) decay distribution and probability are presented, together with the results of a Monte Carlo simulation. The expected background rejection efficiency and achievable neutrinoless ββ decay half-life for a proposed high pressure (5–10 atm) gaseous xenon time projection chamber are calculated. The half-life is comparable to or longer than the present limit (∼ 10 23yr). It is found that ∼ 4 π sr active shielding with NaI scintillator is effective in rejecting e -e + events when the energy resolution is 2% or better at 2.5 MeV. Monte Carlo calculations of electron tracks from neutrinoless ββ decay events show that a magnetic field can be used to distinguish ββ decay events from e -e + events when the energy resolution is ≥ 2%.

Performance of different phoswich configurations in a balloon flight experiment

Four different phoswich units were included in a modular hard X-ray balloon experiment launched on August 10, 1982 in order to check the dependence of background level and rejection efficiency of NaI(Tl)/CsI(Na) phoswich detectors on scintillator thicknesses. The results concern the dependence of the background intensity on both the thickness of the active shield, for a fixed primary detector thickness, and on the thickness of the primary detector, given the active shield thickness. A direct comparison of phoswich detectors with passively shielded NaI(Tl) crystals is also given. As a consequence practical hints for designing new phoswich detectors are derived and the limiting sensitivity of these detectors for hard X-ray observations of celestial sources is inferred.

The gas scintillation proportional counter

The gas scintillation proportional counter has seen rapid development in recent years as a broad-band spectrometer for X-ray astronomy. It may be used in the conjunction with grazing incidence optics or coded aperture masks to produce spectral images of the X-ray sky. Its performance characteristics such as spatial resolution, energy resolution and background rejection efficiency will be described. Reference will be made in large part to work conducted over the last few years at SSD/ESTEC and to work undertaken by the Columbia and Los Alamos groups and at MSSL. The gas scintillator may be coupled to a variety of readout systems including an array of photomultipliers, a microchannel plate or a photoionisation detector. Their relative merits will be reviewed. At high pressures (∼ 3–5 atm) the imaging gas scintillator can be used in nuclear medicine, where its spatial resolution and energy resolution can provide higher resolution images with better contrast than the standard gamma camera.

A Further Development of the Fluorescent Gated Gas Scintillaticn Prdportional Counter

Recent results fran a K gated gas scintillation proportional counter filled with a Xenon/Helium mixture to a pressure of ~ 3 bar have already shown that a background rejection efficiency in excess of 99.3% can be achieved in the energy range 35-100 keV. In addition the application of the technique of fluorescent gating also provides energy resolutions below 3% at ~ 60 keV. Experimental results are presented for a gas scintillation counter filled with a Krypton/Xenon mixture which, through the use of the Krypton K fluorescent gate, enables a high background rejection and improved energy resolution, for the energy range above 14 keV, to be achieved.

Development and performance of a position sensitive proportion counter for the rosat project

A multiwire proportional counter capable of imaging soft X-rays has been constructed. The position of an incident photon is obtained by measuring the induced charge signals on two orthogonal sets of cathode wire grids. The anode signal is used as trigger and energy signal. At a photon energy of 0.94 keV the achieved position resolution is 300 μm (FWHM), and the energy resolution is 45% (FWHM). With a five sided anticoincidence the background rejection efficiency is 90–95%.A small version of the counter has successfully been flown twice on sounding rocket payloads. X-ray images of the supernova remnants Puppis A and Cassiopeia A have been taken. As the prime focal plane detector for the ROSAT project we are building a version with an aperture of 80 mm.

A Fluorescent Gated Imaging Gas Scintillation Specftroeter for High Energy X-Ray Astronomy

Results are presented on the development of a position sensitive gas scintillation proportional counter for use in high energy X-ray astronomy. This detector has a higher background rejection efficiency and a better energy resolution above 35 keV than conventional gas scintillators, gas proportional counters and crystal scintillators. These improvements arise directly fran the application of a discrimination technique which makes use of the detection of the fluorescent Xenon K shell photon emitted in most interactions involving the photoabsorption of genuine X-rays with energies greater than the K shell binding energy. Detectors incorporating this technique, and also having a position sensing capability to reject the induced Compton electron background fran the detector walls, will provide unique information on the spectral characteristics of cosmic X-ray sources. Such characteristics include high energy cyclotron lines fran neutron stars. The inherent imaging properties of these detectors when operated at high X-ray energies are also investigated. Such a detector when used in conjunction with a coded aperture mask will form a camera with an imaging capability of a few arc minutes at X-ray energies beyond the practical range of grazing incidence telescopes.

High pressure gas scintillation spectrometers for x-ray astronomy

A comparison of the performance of a gas scintillation counter operating at pressures above one atmosphere is presented. The factors which influence the background rejection efficiency are investigated in detail. In particular the response at higher X-ray energies has been found to improve significantly making this a useful spectrometer for higher energy X-ray astronomy.

Performance characteristics of a gas scintillation spectrometer for X-ray astronomy

The determination of the optimum electrical parameters of a spherical electric field gas scintillation spectrometer for X-ray astronomy is described. Detailed measurements using a collimated 55Fe 5.9 keV X-ray source are presented leading to trade-offs of drift field, shaping and scintillation field in an endeavour to provide a uniform response in terms of energy and time resolution over the full aperture of 314 cm 2. The optimized uncollimated energy resolution at 6 and 60 keV was 11.0 and 4.5% fwhm respectively. The total background rejection efficiency for 60Co gamma ray induced events was 97 and 70% in the energy range 1–10 and 1–60 keV respectively. Finally design improvements which yield and energy resolution as low as 7.5%K at 6 keV are outlined.

Preliminary Studies CF Gas Fillings in Gas Scintillation Proportional Counters

Results are presented on improvements in the performance of medium energy large area gas scintillation proportional counters for use in X-ray astronomy. By a suitable choice of gas mixture the electron transit time through the drift region can be significantly lowered with a consequent reduction in the detector dead time. In addition the electron diffusion is also reduced and this should result in an increase in the background rejection efficiency.

Investigation of pulse shape analyzers for phoswich detectors in space-borne hard x-ray experiments

A low-background telescope for hard X-ray astronomy (15–250 keV), comprising arrays of NaI(Tl)/CsI(Na) phoswiches as photon collectors, was recently developed by our group in collaboration with the Center for Space Research at MIT. The background rejection efficiency of such a telescope, and hence the minimum source detectability in a given time, critically depends on the performance of the phoswich pulse shape analyzer (PSA) in a space radiation environment. Results from theoretical and experimental work on analyzer configurations based on zero-crossing detection are presented. This led to the selection of an optimum configuration for space application. The in-situ performance of this analyzer was evaluated in a balloon-borne hard X-ray experiment, showing excellent discrimination efficiency throughout the entire energy regime.

A simple method for obtaining high background rejection in large area proportional counters

This paper describes a simple means of improving the background rejection efficiency of large area, multi-wire proportional counters used in X-ray astronomy. An efficient end anticoincidence is realised by means of cathode strips supported above the ends of the anode wires. The signals induced in these are used to reject background events emanating from the end walls. A prototype Exosat Medium Energy detector of 250 cm 2 sensitive area has been fitted with this system in a 5-sided anticoincidence configuration. With the addition of rise time discrimination, the efficiency of rejection of 60Co induced background is 99% in the 2–6 keV energy range.