GAS PROPORTIONAL SCINTILLATION COUNTER FOR IONIZING RADIATION WITH MEDIUM AND LARGE SIZE RADIATION WINDOWS AND/OR DETECTION VOLUMES

Abstract: The present invention concerns detectors for ionizing radiation (X-rays, gamma rays, electrons, protons, alpha particles, etc. as well as neutrons) of the gas proportional scintillation counter type. The preferred embodiment for X-ray detection consists in a detector, filled with very pure xenon at atmospheric pressure, with a 2.5 cm diameter radiation window (1), a grid with a spherical (2 cm radius) curvature (2) with its edges placed 0.8 cm away from a plane grid which is placed in contact with or close to a photomultiplier tube (4). This photomultiplier has uniform sensitivity in the 170 nm wavelength region. This detector presents an energy resolution of 8.0 % for 5.9 keV X-rays.

Description: Description GAS PROPORTIONAL SCINTILLATION COUNTER FOR IONIZING RADIATION WITH MEDIUM AND LARGE SIZE RADIATION WINDOWS AND/OR DETECTION VOLUMES Scope of the invention The present invention concerns detectors for ionizing radiation, of the gas proportional scintillation counter type for X-rays, gamma rays, electrons, protons, alpha particles as well as other charged particles like deuterons and mesons, or other nuclear radiation like neutrons. With the principles described in the present invention it is possible to built gas proportional scintilla¬ tion counters with a high energy resolution, even if the radiation windows and/or the detection volumes have dimen¬ sions analogous to those of the secondary scintillation sensor, generally a photomultiplier. Prior Art Radiation detectors of the gas proportional scin tillation counter type have been described in the scientific literature since 1967 (C.A.N.Conde and A.J.P.L.Policarpo, "A gas proportional scintillation counter", Nucl. Instrum. Meth. 5_3. (1967) 7) being the most commonly used type the one that uses two parallel grids (C.A.N. Conde, M.C.M. Santos, M.Fatima A. Ferreira and Celia A. Sousa, "An argon gas proportional scintillation counter with uniform electric field", IEEE Trans. Nucl. Sci. NS-22 (1) (1975) 104). The working principle has been described in detail in the scientific literature (C.A.N.Conde, "The gas proportional scintillation counter: evaluation of performances with interest for analysis by ion beam techniques", SUBSTITUTE SHEET Nucl. Instrum. Meth. 149 (1978)685) and is briefly described in the following. When ionizing radiation is absorbed in a very pure noble gas (typically xenon at atmospheric pressure) it produces a primary electron cloud with a number of electrons which is proportional, within a good approximation, to the energy of the absorbed radiation (F.P. Santos, T.H.V.T. Dias, A.D. Stauffer and C.A.N. Conde, "Variation of energy linearity and w value in gaseous xenon radiation detectors for X-rays in the 0.1 to 25 keV energy range: a Monte Carlo simulation study", Nucl. Instrum. Meth. A307 (1991)347). These electrons are subject to a weak reduced electric field (smaller than lV/cm/Torr) produced by the application of a voltage (typically 500V) between a plane first grid and the detector window; then the primary electrons drift towards this grid. The region between the detector and the first grid is usually called "drift region". Next to the first grid and parallel to it there is a plane second grid, about 1 cm away. An electric high voltage (typically 5000 Volts) is applied to the second grid which produces a strong electric field with an intensity below or near the threshold for ionization (i.e. 5 V/cm/Torr) . Once the primary electrons reach the region between the two grids (the so-called "scintillation region") they excite the noble gas atoms, which in the de-excitation process lead to the production of vacuum ultraviolet light, the so-called "secondary scintilla¬ tion". One single electron may excite, successively, hundreds of noble gas atoms. Therefore the intensity of the secondary scintillation will be quite large and proportional to the number of primary electrons, i.e. proportional to the energy dissipated by the ionizing radiation in the gas. The secondary scintillation is detected by a vacuum ultraviolet sensor (generally a photomultiplier) which produces a pulse with an amplitude proportional to the number of primary electrons. Thus, with a multichannel SUBSTITUTE SHEET analyzer it is possible to obtain energy spectra, much in the same way as with a standard NaΙ(Te) scintillation counter, but with much better energy resolution, at least for X-rays. The energy resolution of gas proportional scin¬ tillation counter is limited by the Fano factor (T.H.V.T.Di- as, F.P.Santos, A.D. Stauffer and C.A.N. Conde, "The Fano factor in gaseous xenon: A Monte Carlo calculation for X-rays in the 0.1 to 25 keV energy range", Nucl. Instrum. Meth. A307 (1991) 341) but depends also, amongst other factors on the grid parallelism (C.A.N. Conde, L.F.Requicha Ferreira and M. Fatima A. Ferreira, "The secondary scin¬ tillation output of xenon in a uniform field gas proportio¬ nal scintillation counter", IEEE Trans. Nucl. Sci. NS-24 (1) (1977) 221) and on the non-uniformity of the photomultiplier response (J.M.F. dos Santos, A.C.S.S.M. Bento and C.A.N. Con de, "The effect of the photomultiplier nonuniformity on the performance of GPSCs", Nucl. Instrum. Meth A321 (1992) 238). For detectors with medium size radiation windows (typically 1 or 2 cm diameter) the amount of light reaching the sensor depends on the radial distance of the point where the electrons enter the scintillation region. This is so since, although the intensity of the secondary scintillation is constant (for parallel grids) , the amount that reaches the sensor depends on the solid angle through which it sees the scintillation and on losses due to reflections on the surfaces of the same sensor. These problems have been discussed recently (J.M.F. dos Santos, A.C.S.S.M. Bento and C.A.N. Conde, "The dependence of the energy resolution of GPSC on the scin¬ tillation region to photomultiplier distance", in IEEE Trans. Nucl. Sc. NS-39(4) (1992) and it was shown that they may lead easily to pulse amplitude variations as large as 10 or 15' SUBSTITUTE SHEET or 15%. To these amplitude variations corresponds a deterio¬ ration in the energy resolution for radiation beams entering the detector through the full aperture of the window. Solutions for these problems have been described in the literature (D.F. Anderson, O.H. Bodine, R. Novick and R.S. Wolff, "A focusing gas scintillation proportional counter", Nucl. Instrum. Meth. 144(1977) 485) which use focusing tech¬ niques to bring the drifting electrons closer to the axis of the detector and that through the use to two concentric curved grids vary the scintillation position to photo¬ multiplier distance, in a way that partially compensates the solid angle effects (R.D. Andresen, E.A. Leimann, A.Peacock and B.G. Taylor, "A large area (300 cm2) gas scintillation proportional counter for X-ray astronomy", IEEE Trans. Nucl. Sci. NS-25 (1) (1978) 800) . Another solution calls for the use of large size photomultipliers. Any of these techniques is relatively complex and expensive. Description of the invention In the present invention we present two new solutions to those problems which are simpler and economi¬ cally more advantageous than the ones just described. The solutions here described can even be combined with the above ones, namely the focusing techniques, in the construction of detectors with even larger dimensions. 1- The first technique consists in the replacement of the first grid (plane) by a curved grid having a special shape; the second grid remains plane and may be placed onto the scintillation sensor or a certain distance away (see Fig. 1). The primary electrons produced by the incident radiation that entered the detector through the thin radiation window (1) drift towards the curved grid (2) and once they cross it they enter the scintillation region, between the planar second grid (3) and the curved grid (2) , where they meet a SUBSTITUTE SHEET non-uniform electric field region. This field has an inten¬ sity that increases with the radial distance, r .As the function that represents the intensity of the secondary scintillation versus the electric field intensity is appro¬ ximately a straight line that does not cross the origin of the axis (C.A.N. Conde, L.F. Requicha Ferreira and M. Fatima A. Ferreira, "The secondary scintillation output of xenon in a uniform field gas proportional scintillation counter", IEEE Trans. Nucl. Sci. NS-24 (1) (1977) 221) the amount of light produced by each primary electron while crossing the scintillation region increases with the radial distance. With a proper curvature for the first grid the reduction with the radial distance in the amount of VUV light that reaches the sensor (photomultiplier) (4) due to solid angle and reflection effects, can be exactly compensated by the increasing amount of light produced and due to the radially increasing electric field intensity. For exact compensation, the amount of light, I (r) , detected in the sensor (4) which is produced by a single electron entering the scintillation region at the radial distance, r, must remain constant as r varies from 0 to a certain maximum value, Rmax. This implies that the following integral remains constant within the same radial range: I(r) = w(s) h(s) R(s) ds (eq. 1) This integral is calculated along the path, s, of an electric field line which starts in the point A in grid 1 that is at the radial distance, r, and ends in the point B in grid 2, as shown in fig. 2. dn/ds is the number of secondary scintillation photons, n, produced by unit of path length, s; dn/ds is not constant since the electric field intensity varies along the electric field line but it can be calculated using expressions of the type of those already published (C.A.N. Conde, L.F. Requicha Ferreira and M. Fatima A. Ferreira, "The secondary scintillation output of SUBSTITUTE SHEET Xenon in a uniform field gas proportional scintillation counter", IEEE Trans. Nucl. Sci. NS-24 (1) (1977) 221). w(s) is the solid angle through which the electron sees the optical sensor (photomultiplier) from position s and R(s) is the average reflectivity of the surfaces of the window of the sensor measured from the same position (J.M.F. dos Santos, A.C.S.S.M. Bento and C.A.N. Conde, "The depen¬ dence of the energy resolution of GPSC on the scintillation region to photomultiplier distance", in IEEE Trans. Nucl. Sc. NS-39(4) (1992) ). h(s) is the average quantum efficien¬ cy of the sensor as measured from position s. In fig. 1 we plot the equipotential curves in the regions between the window (1) and the curved grid (2) and between this grid and the plane grid (3) . All events due to the ionizing radia¬ tions referred to above (X' rays, gamma rays, electrons, photons, etc.) that produce primary electrons within an electric field tube starting in the front wall of the detector and ending in the curved grid at r= Rmax, all of them will produce a secondary scintillation pulse in the sensor, with an amplitude that depends only on the number of primary electrons and not on their position. If the distan¬ ces travelled by the primary electrons are too large, diffusion has to be taken into account. If R n is the radius of a diffusion cloud at r=Rmax near the curved grid, the electric field tube being discussed is smaller by the amount R Q . The detection volume of the detector is equal to the volume of that electric field tube. Whenever the sensor (4) has a uniform quantum efficiency ho, equation 1 reduces to: I(r) =h w(s) R(s) ds (eq. 2) The calculation of the shape of grid 1 that keeps I(r) constant between 0 and Rmax can be made by a successive approximation techniques using commercially available electrostatic CAD programmes for the calculation of electric fields between the grids and using the straight line appro¬ ximation for dn/ds (C.A.N.Conde, L.F. Requicha Ferreira and M.Fatima A. Ferreira, "The secondary scintillation counter", SUBSTITUTE SHEET IEEE Trans. Nucl. Sci. NS- 24 (1) (1977) 221)).The calcula¬ tion of W(s) and R(s) has already been described (J.M.F. dos Santos, A.C.S.S.M. Bento and C.A.N. Conde, "The dependence of the energy resolution of GPSC on the scintillation region to photomultiplier distance", IEEE Trans. Nucl. Sc. NS39(4) (1992) ). 2 - An alternative solution to keep I(r) constant consists in the use of a gas proportional scintillation counter with parallel grids, which implies dn/ds constant, but using a sensor with an efficiency ho that increases radially. In this case we have dn rvi I(r) = h(s) R(s) ds (eq. 3) The required radial variation for h (r) may be obtained starting with a uniform efficiency sensor, or with an uniformized one using the techniques of Mead and Martins (J.B.Mead and J.P. Martins, "Improvement of resolution in large area photomultipliers", Nucl. Instr. Meth. 36(1965)13) and covering it with a mask of small opaque dots (for example using photolithography and/or vacuum evaporation techniques to deposit small metallic (e.g. chromium) cir¬ cles. The density of dots onto the sensor will be adjusted so that h Q (r) will have the required variation. Once W(s) and R(s) are calculated (usually by the Monte Carlos method) the quantum efficiency of the sensor along its surface h 0 (r) may be calculated using successive approximation techniques and a Monte Carlo method to calculate equation 3 and keep it constant. A preferred embodiment of the present invention has cylindrical geometry and is shown in fig. 2. The enclo¬ sure of the detector, pieces (5) , (6) and (9) is made of a metal, in the present case stainless steel, with a low outgassing so to reduce the impurification of the filling gas. Pieces (5) and (6) are tightened against each other with screws (10) and the system is made vacuum tight with an SUBSTITUTE SHEET indium gasket. Piece (9) is a stainless steel cylinder that encloses the photomultiplier (4) and that fits in the lower part of piece (6) with a thread. Piece (11) is made in a insulating material with good characteristics for vacuum in the present case Macor; this piece is epoxied to the lower part of piece (6) with a low vapour pressure epoxy (TRACON Model 2116) . The radiation window (1) is made of a 50 microns thick Kapton film, epoxied (with the low vapour pressure epoxy) to the central part of piece (5) , and with a vacuum deposited thin aluminum film in the inner side for electri¬ cal conductivity. The first grid (2) is made of a square mesh with stainless steel wire (80 microns in diameter) and with the centers of the wire 930 microns apart (made by GANTOIS in France) . This meshed grid is assembled by com¬ pressing a circular mesh piece between the two stainless steel pieces (12) and (13) and the proper spherical calotte shape is achieved by compression the grid between stainless steel moulds with the proper shape. In the present case the calotte radius is 20 mm and the highest point of the first grid (2) is placed 15 mm away from the second grid (3) . The sensor is a photomultiplier with 5.1 cm diameter having a high purity quartz window (so to let most of the vacuum ultraviolet secondary scintillation of the gas, xenon, to pass) . The photomultiplier is in the present case an EMI D676QB one. The second grid (3) is a chromium grid vacuum deposited onto the photomultiplier window and has 100 microns wide lines, spaced 1500 microns, in two perpendicu¬ lar directions. This grid allows a good optical transmission (about 87%) . The grid pattern is made by vacuum depositing first the "horizontal" lines with a mask with parallel bars and afterwards the "vertical" lines with the same mask turned 90 degrees. The lateral surface of the photomulti¬ plier is fully covered with a chromium film which is used to feed the high voltage from the outside to the second grid (3) . Pieces (8) are stainless steel tubes for gas felling, SUBSTITUTE SHEET in the present case xenon at room temperature and at a pressure of 1.2 bar. The xenon gas is continuously purified with getters (SAES ST 707) and circulates by convection. However if instead of using epoxy techniques for joining the different pieces. We use metal to glass and metal to ceramic joining techniques together with ultra high vacuum techniques with outgassing, no continuous purification of the noble gas will be required. For the embodiment just described concerning the shape and geometry of the grids and xenon. For the shape and geometry of the grids and xenon pressure of the embodiment just described, the condition of equation 2 is achieved when to the first grid it is applied a positive voltage of 700 V (fed with a metallic contact (14) through the Macor piece (11) ) . and when to the second grid it is applied a positive voltage of 4400 V. Under these conditions I(r) remains constant up to a radial distance Rmax=14mm. The photomultiplier is biased with a positive voltage of 5100 V to the anode and 4400 V to the photo¬ cathode, so that there is no voltage difference across the photomultiplier window. The dynodes of the photomultiplier are polarized with a classical resistances circuit. The curved grid gas proportional scintillation counter embodiment just described achieves an energy resolution of 8.0% for 5.9 keV uncollimated X-rays with radiation windows 25mm in diameter. Concerning technique 2 (a parallel plane grids detector with a sensor with radially variable efficiency) calculated results for parallel grids 10 mm apart with grid 2 deposited onto the 2mm thick quartz photomultiplier window having a photocathode 46 mm in diameter, show that the condition for equation 3 to remain constant up to SUBSTITUTE SHEET Rmax=46 mm is that the efficiency ho(r) increases with the radius according the numerical expression ho (r) = 1.6671414 x 10 -11 r 8 + 0.48 (eq. 4) SUBSTITUTE SHEET

Claims: CLAIMS 1.- Gas proportional scintillation counter for ionizing radiations with medium and large size radiation windows and/or detection volumes, with the characteristic that, for compensation of the loss of scintillation light due to solid angle and reflection effects, utilizes, instead of the two parallel plane grids usually employed in these apparatus, one curved grid with the curvature calculated in a way so to increase radially the amount of scintillation light pro¬ duced, or else utilizes the two referred to above parallel grids but with a sensor with an efficiency that increases radially in a way that compensates the losses due to solid angle and reflection effects. 2.- Gas proportional scintillation counter for ionizing radiations which, in agreement with the previous claim, is characterized by the property that the loss with the radial distance of the amount of light that reaches the sensor due to solid angle and reflection effects, is properly compen¬ sated by an increasing amount of light due to an electric field whose intensity increases radially in a suitable way, provided the curvature of the first curved grid is properly calculated, which can be determined from the condition that the integral remains constant as the radial distance, r, increases from zero up to a maximum value r = Rmax, being I(r) the amount of light detected in the sensor and produced by a single electron which enters the scintillation region at the radial distance, r, being the integral calculated along the path, s, of an electric field line that starts in the point where the field line meets the surface of the curved grid (at the potential VI) that is at the radial distance r from the axis and ends in the second plane grid in the point where the field line meets the surface of this grid (at potential SUBSTITUTE SHEET V2) , being dn/ds the number n, of secondary scintillation photons, produced by a single electron drifting along the unit of path length, s, being w(s) the solid angle through which the electron sees the detector, R(s) the average reflectivity of the surfaces of the sensor and h(s) the average efficiency of the sensor as calculated from the position, s, where the electron is located. 3.- Gas proportional scintillation counter for ionizing radiation in agreement with claim I, characterized by the compensation of losses due solid angle and reflection effects being achieved with parallel grids, which implies that the number of secondary scintillation photons are constant, but that through the use of a sensor with an efficiency that increases radially, starting with a uniform, or uniformized, detector and adjusting its efficiency with a mask of small opaque dots covering its surface, so that it varies radially in the convenient way which is determined from the condition that the integral I'(r) = w(s) h(s) R(s) ds remains constant as the radial distance, r, increases from zero up to a maximum value r=Rmax, being the variables the same ones as those in claim 2 and the integral calculated along the electric field lines between the two parallel grids at potential V, and V2. 4.- Gas proportional scintillation counter for ionizing radiation which, in agreement with the previous claim, is characterized by the property that any ionizing event produced by the incident radiation, for example X-rays, gamma rays, electrons, protons, alpha particles as well as neutrons, which leads to the formation of a primary electron cloud within an electric field tube of force starting in the front wall of the detector and ending in the first grid (either curved as in claim 1 or plane, as in claim 2) at the SUBSTITUTE SHEET radial distance r=Rmax, will produce a scintillation pulse at the output of the sensor with an amplitude which depends only on the number of primary electrons and to which corres¬ ponds a good energy resolution determined mainly by the fluctuations in the number of primary electrons produced. SUBSTITUTE SHEET

Inventors: MARQUES FERREIRA DOS SANTOS JO, NABAIS CONDE CARLOS ALBERTO, SENA SAO MIGUEL BENITO ANTONIO, MARQUES FERREIRA DOS SANTOS, JOAQUIM, NABAIS CONDE, CARLOS, ALBERTO, SENA SAO MIGUEL BENITO, ANTONIO, CARLOS
Applicants: MARQUES FERREIRA DOS SANTOS J, NABAIS CONDE CARLOS ALBERTO, SENA SAO MIGUEL BENITO ANTONIO, MARQUES FERREIRA DOS SANTOS, JOAQUIM, NABAIS CONDE, CARLOS, ALBERTO, SENA SAO MIGUEL BENITO, ANTONIO, CARLOS
Application year: 1993
Main classification: H01J47
IPC codes: G01T1, H01J47