Active Collimator Research Articles

Aiglon, a magnetic spectrometer for low energy electrons

The magnetic spectrometer is designed to detect low energy electrons (5–50 MeV) with good energy (10%) and angular ( < 5 ∘ ) resolutions, and sufficiently large acceptance ( 10 cm 2 sr ), to monitor short term changes of the trapped particle population in the Earth's magnetic field. The influence of multiple Coulomb scattering is reduced by active collimation, filter planes composed of edgeless silicon microstrip detectors. The incident and exit particle trajectories are reconstructed in four planes of scintillating fibers. The spectrometer is a digital device in the sense that the notion of sampling is omnipresent: at the level of the filter plane design to suppress large angle multiply-scattered electrons, and at the level of the frontend electrons, where silicon photomultipliers (SiPM) are used.

Possibility of crystal extraction and collimation in the sub-GeV range

Bent crystal situated in a circulating beam can serve for efficient slow extraction or active collimation of the beams. This technique, well established at 1--1000 GeV, could be efficient also at the energies as low as 0.1--1 GeV according to the computer simulations presented in this paper. Applications might include halo scraping in the spallation neutron sources or slow extraction from medical synchrotrons.

Using the active collimator and shield assembly of an EXIST-type mission as a gamma-ray burst spectrometer

The Energetic X-Ray Imaging Survey Telescope (EXIST) is a mission design concept that uses coded masks seen by Cadmium Zinc Telluride (CZT) detectors to register hard X-rays in the energy region from 10 keV to 600 keV. A partially active or fully active anti-coincidence shield/collimator with a total area of between 15 m2 and 35 m2 will be used to define the field-of-view of the CZT detectors and to suppress the background of cosmic-ray-induced events. In this paper, we describe the use of a sodium activated cesium iodide shield/collimator to detect gamma-ray bursts (GRBs) and to measure their energy spectra in the energy range from 100 keV up to 10 MeV. We use the code GEANT 4 to simulate the interactions of photons and cosmic rays with the spacecraft and the instrument and the code DETECT2000 to simulate the optical properties of the scintillation detectors. The shield/collimator achieves a -sensitivity of erg cm-2 s-1 and erg cm-2 s-1 at 100 keV and 600 keV, respectively. The sensitivity is well matched to that of the coded mask telescope.The broad energy coverage of an EXIST-type mission with active shields will constrain the peak of the spectral energy distribution (SED) for a large number of GRBs. The measurement of the SED peak may be key for determining photometric GRB redshifts and for using GRBs as cosmological probes.

The PIBETA spectrometer for studying rare and forbidden decays of muons and pions

The construction and characteristics of the PIBETA spectrometer are described. This spectrometer is designed to implement a program of precise measurement of pion β decay π+ → π0 + e + + νe at the Paul Scherrer Institute (Switzerland). A spherical calorimeter, consisting of 240 crystals of pure CsI scintillator and embracing a solid angle of ∼3π, is the main detector of the setup. In addition, the spectrometer is composed of an active collimator (which also acts as a beam degrader), a segmented active plastic target, two multiwire cylindrical proportional chambers, a 20-element cylindrical plastic hodoscope, and veto counters of cosmic muons.

The "Radiation Continuity Checker": an instrument for monitoring nuclear disarmament treaty compliance

We describe the design and construction of an instrument designed to monitor compliance with future arms control treaties. By monitoring changes in the gamma-ray spectrum emitted by a stored nuclear weapon, our device is able to sense perturbations in the contents of a weapon storage container that would indicate treaty non-compliance. Our instrument (dubbed the Radiation Continuity Checker or RCC) is designed to detect significant perturbations in the gamma-ray spectra (indicative of tampering) while storing no classified information about the weapon, and having a negligible false alarm rate. In this paper we describe the technical details of two prototype instruments and describe the strategies we have adopted to perform signal processing in these instruments. Our first instrument prototype uses a scintillation spectrometer and a massive tungsten alloy collimator to reject the gamma-ray background. Our second prototype instrument makes use of an active collimation scheme employing a multiple detector Compton scatter approach to reject background radiation. The signal processing method we employ uses linear algorithms applied pulse by pulse. This eliminates the need for storage of pulse height spectra, which are in many cases classified.

An active collimator system for a high-energy photon beam

Active collimators are designed to both collimate the tagged photon beam and suppress the background of secondary photons. These secondary photons are generated by interaction processes in the collimator medium. They may reach the hadron target with a lower energy compared to the energy determined by the tagging system. Accordingly, those events are tagged with a wrong energy and have to be rejected. By means of a Monte Carlo simulation we investigated the effect and the performance of an active collimator system and developed an optimal design of the collimators to prepare a clean high-energy photon beam. The resulting set-up ensures a precise determination of the tagging efficiency and of the photon flux. The active collimators are a necessary tool to correctly determine photoproduction cross-sections. Results of test measurements at ELSA in Bonn, and MAMI B in Mainz are discussed. Special interest is given to the use of the active collimators in the experiment to test the GDH sum rule at ELSA up to 3.2 GeV .

Energetic neutral atoms in the outer magnetosphere: An upper flux limit obtained with the HEP‐LD Spectrometer on board GEOTAIL

The energetic particle spectrometer HEP‐LD on board the Japanese mission GEOTAIL is the first instrument in the energy range above 77 keV which can discriminate between energetic neutral atoms (ENA) and charged particles. The principle for particle identification is based on a time‐of‐flight (T) and energy (E) measurement which uniquely determines the particle species. A rather compact and highly anisotropic active collimation system in front of the (E,T)‐detector head acts as an effective charged particle rejector (CPR) in the ENA mode. The resulting high‐pass filter function for charged particles allows the detection of ENAs with energies below 200 keV. This report concentrates on ENA observations in the outer magnetosphere on the dayside (GSE x=−3, y=+12, z=+1 RE) and establishes an upper limit for the ENA flux in quiet conditions. The measurements agree reasonable well with predictions, however, the statistical uncertainties are considerable.

Active compensation of flexure on the WHT ISIS spectrograph

ISIS Spectrograph Automatic Active Collimator (ISAAC) was developed at the Optical Science Lab, University College London, as an experimental system to improve the image stability on the detectors of the Intermediate-dispersion Spectroscopic and Imaging System (ISIS) at the William Herschel Telescope (WHT). It is based on the concept of active compensation, where spectrum drifts, owing to the spectrograph flexing under the effect of gravity, are compensated by the movement of an active optical element. ISAAC is a fine-steering tip-tilt collimator mirror; it was tested on the spectrograph in 1995 July. Here we report the details of the design and the laboratory tests together with the results of the experiment on ISIS at the WHT. The experiment shows that active compensation can reduce flexure down to less than 3mum over 4h of telescope motions, dramatically improving the spectrograph A way of implementing ISAAC as a common user facility is briefly discussed, together with a study of the characteristics of flexure in ISIS.

Basic performance of unitized Compton scattering type polarimeter

We have been developing a Compton scattering type polarimeter which consists of nine unit counters to detect the polarization of hard X-rays from stellar objects such as the Crab Nebula. We constructed one unit counter which consists of one plastic scintillator (7/spl times/7/spl times/5 cm/sup 3/) and four phoswich scintillators (6.7/spl times/1.1/spl times/32.2 cm/sup 3/) made of NaI(Tl)+CsI(Na). The characteristics of the unit counter were measured in detail to determine the detection efficiency (20.3%) and the modulation factor (27.5%) at 53.4 keV. The background spectrum of the unit counter in the laboratory is also measured. Based on the results, the background from the outside of the active collimator is reduced by 50.5% when used with the PSD. On the basis of these results, the performance of the planned polarimeter was estimated by computer simulations. The results showed that the polarimeter can detect the polarization of hard X-rays from the Crab Nebula at a MDP (minimum detectable polarization) of 14.0% with a 3-hour balloon observation.

A Compton suppression spectrometer for neutron activation analysis

Abstract A Compton suppression spectrometer consisting of a system of three Ge detectors surrounded by a BGO shield and a CsI(T1) active collimator has been set up for the use in neutron activation analysis. Tests of the performance of the system concerning Compton suppression, peak reduction and effects of the count rate in an energy range 300–2000 keV are described. A practical test for the determination of iodine in iodized salt is presented.

Detector to study low-flux hard X-ray/gamma-ray sources

We have developed a new kind of phoswich counters that will be capable of detecting low flux hard X-rays/gamma-rays from astronomical objects. The new phoswich counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a rectangular well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). Here, the well-shaped shielding part acts as an active collimator as well as an active shield. We have built a detector system consisting of 64 such phoswich counters: newly developed scintillator (GSO) is used for the detection part and CsI( Tl) is used for the shielding part. The total geometrical area of the 64 detection parts is about 740cm 2 and its 3σ sensitivity is expected to reach below 10 −5cm −2s −1keV −1 up to 700keV. With several improvements such detectors will be able to detect hard X-rays/gamma-rays at a flux level around 10 −6cm −2s −1keV −1 upto 2 MeV.

Collimation of a rubidium beam below the Doppler limit

We used counterpropagating beams of diode laser light forming a one-dimensional optical molasses perpendicular to an atomic beam of 85Rb for active collimation. The minimum measured angular spread of the beam we achieved corresponds to a transverse rms speed of 3 cm/s, well below νD = (7ħγ/20M)1/2 = 10 cm/s corresponding to ordinary one-dimensional Doppler molasses.Achieving this low velocity, limited only by experimental geometry, depends on a magnetic field of ≈ 20 μT applied perpendicular to the axis of the circularly polarized light beam.

PNP - A New Class of Coded Aperture Arrays

We report on a new class of coded aperture arrays which has all the desirable imaging characteristics of the Uniformly Redundant Array (URA), yet is constructible in dimensions which are forbidden to that design. In addition, the new arrays (called PNP arrays, for Pseudo-Noise Product) are of self-supporting geometry, simplifying fabrication and making them ideal candidates for situations where active collimators are employed, as for example in high-energy ?-ray imaging. A unique and important feature of all PNP arrays is the ability to produce reconstructed images whose noise is uniform regardless of the original source structure. A comparison of the predicted performance of the PNP, URA, GEOMETRIC, and PINHOLE designs is presented.

Forward-angle proton spectra in the continuum from theNi58(p, xp)reaction at 100 MeV

Forward-angle proton energy spectra in the continuum were measured for the reaction $^{58}\mathrm{Ni}(p, xp)$ at 100 MeV with a plastic scintillator used as an active collimator. With this system it was possible to take measurements at angles as small as 8\ifmmode^\circ\else\textdegree\fi{}. It was found that a previously observed broad peak in the continuum, which is suspected to appear as a result of a quasi-free nucleon-nucleon scattering, becomes even more pronounced as the scattering angle decreases. The active collimators were essential in eliminating contributions due to the penetration of collimator edges by elastically scattered protons. At small angles such effects can obscure the quasi-free peak.

A multicrystal detection system for localization and measurement of photon emitters without external collimation

This paper describes the design and preliminary testing of a photon detecting system which will yield, simultaneously, information proportional to both the magnitude, and the coordinate location in space, of the source of the gamma and/or X-rays impinging on its detection elements. The operating principle of the detector system, “active collimation”, results from the use of pulse-shape discrimination between three different crystal elements [NaI(Tl), CsI(Tl), CsI(Na)] and allows spatial resolution ( X, Y and Z axes) without the use of separate collimating devices.

Hard X-ray observations near Ariel 1118–61

OBSERVATIONS made by Ariel V in the region of Cen X-3 have revealed a new transient X-ray source with a periodicity of 6.75 min (refs 1 and 2). Tentative identifications are: with a Mira-type variable star by Fabian et al.3, who suggest the 6.75 min periodicity is associated with the rotation period of a compact companion; and with a heavily reddened, variable ultraviolet star4. As well as the proportional counter observations during the period 1974 December 17 to 1975 January 31, the Imperial College crystal scintillator, sensitive to the photon energy range 26 keV to 1.2 MeV, viewed the same region of the sky. This detector has a sensitive area of 8 cm2, an opening angle of 8° FWHM defined by an active collimator and is offset from the spacecraft spin axis by an angle of 3°. A source a few degrees from the direction of the spin axis can be detected by determining the modulation in the counting rate seen in four equal angle spin sectors with a time resolution of 512 s. A single channel (50–190 keV) mode with a 48 s time resolution was also used. Since a spurious modulation is induced in the apparatus by cosmic rays, two pointing direction offsets on opposing sides of the source have to be used, and the X-ray contribution is obtained by a subtraction technique.

Observations on diffuse cosmic X-rays in the energy range 20–120 keV

In this paper we present observations of the diffuse background X-rays in the energy range 20–120 keV, based on two balloon experiments carried out from Hyderabad (latitude 17.6°N, longitude 78.5°E), India. The flights were made on April 28, 1968 and December 22, 1968. The detector used was a NaI(Tl) crystal of effective area 97.3 cm2and thickness 4 mm. The crystal was surrounded both by active and passive collimators. The passive collimator was a cylindrical graded shield of lead, tin, and copper, and the active collimator was a plastic scintillator surrounding the shield. The FWHM of the telescope was 18.6° and the geometrical factor for isotropic radiation 13.2 cm2sr. The pulses from the NaI crystal were sorted into ten contiguous channels extending from 17 to 124 keV. An Am241source came into the field of view of the telescope periodically and provided in-flight calibration of the detector. All the information was recorded on photographic film.

Observations of High-Energy X-Rays from Sagittarius

During a recent balloon flight from Mildura, Australia, the region of the Galaxy between −10° &lt; lII &lt; +20°; −5° &lt; bII&lt; +5° was surveyed for high-energy X-rays of photon energy &gt; 17 keV (λ &lt; 0.7 Å) using an active collimation scintillation detector described elsewhere. The narrow opening angle of the telescope enables two sources to be resolved in this region of the sky. The position of one of these sources agrees well with the source GX3+1 observed at rocket wavelength by Bradt et al. while the other source is probably the same as that observed by Gursky et al. and designated GX−5.6. We present herein improved positional information for this latter source which hereafter will be designated GX354−5.

The X-Ray Spectrum of Sco XR-1 to 100 keV

This paper presents the results of a measurement of the high-energy (E ≥ 17 keV) X-ray flux from Sco XR-1 extending to higher energies than has previously been reported. The measurement was made during a balloon flight launched from Mildura, Australia, on 29 February 1968. The X-ray observatory contained two independent X-ray detectors, one being similar in principle to the active collimator detector pioneered by Peterson et al., the other being basically similar to the graded shield detector developed by Boldt et al. Our two detecting systems are described in more detail by Buselli et al. and also in previous papers of this conference.

Observations of cosmic X-ray sources in the 10–250 keV range

Observations of cosmic X-ray sources have been made from high-altitude balloons over Palestine, Texas, using actively collimated detectors. In this technique, a thin NaI central counter 10 to 50 cm2 in area is surrounded by a CsI well crystal shield several centimeters thick. The aperture, about 8° to 20° FWHM, is determined by either the well opening or an active honeycomb collimator. The background is determined mainly by diffuse cosmic and atmospheric X-rays entering the forward aperture. The detector is usually either servo-controlled to track the source or operated as a meridian device. Data are telemetered over the 20–250 keV range in a digital format from a 128-channel pulse-height analyzer. Several strong sources in the northern hemispheric sky have been observed. The Crab nebula has a power-law differential number spectrum with an index of –1.9 ± 0.1 and an intensity of about 10−2 photons/cm2-s-keV at 20 keV. Two observations in September 1965 and September 1966 on this object give the same flux and spectral index within about 5% over the 20–100 keV range. The source Cygnus XR-1 also has a power-law shape, very similar in slope and intensity to the Crab, which extends above background to at least 180 keV. These measurements are in general agreement with those of other workers. The power-law spectrum of the Crab and Cygnus XR-1 may be contrasted with that of SCO XR-1, which has an exponential spectrum, typical of a hot gas at 50 × 106 °K.