Gas Scintillation Proportional Counters

Gas scintillation proportional counters (GSPC), developed in 1972 [20], offer the advantage of an enhanced energy resolution when compared with proportional counters. In conventional proportional counters, the charge generated by an ionizing event is multiplied in a high electrical field. The amplification grows exponentially with the number of ionizing collisions of an electron in the high field region. Only 14 ionizing collisions result in a charge multiplication of four orders of magnitude. The variation in the number of ionizing collisions during multiplication degrades the Fano limited energy resolution of proportional counters by almost a factor of two (see Chap. 2.3.2 energy resolution). In gas scintillation proportional counters (see Fig. 2.4), the charge released by an ionizing event is not amplified. Similar to multiwire proportional counters, X-rays are absorbed by the detector gas (usually a

Entrance Field shaping High voltage UV transparent window electrodes grids window

Entrance Field shaping High voltage UV transparent window electrodes grids window

Gas Proportional Scintillation Counter
Fig. 2.4 Gas scintillation proportional counter

noble gas or a mixture of noble gases) in an absorption and drift region. The electrons drift from this low field region into the high field scintillation region where they acquire sufficient energy to excite the scintillation of the detector gas, but not to ionize it. In case of xenon, diatomic molecules, formed by the collision of excited atoms, deexcite by the emission of UV photons in the wavelength band of 150195 nm [14]. The number of scintillation photons increase linearly with the number of exciting collisions of the electrons with the gas atoms. These collisions are independent events. Therefore, the variation of the light output generated during the scintillation process depends on the statistics of the final number of photons registered by the photomultiplier. The integral intensity of the light flash is proportional to the energy of the ionizing event. GSPCs can reach an energy resolution nearly at the Fano limit because of the large amount of scintillation photons.

GSPCs are rather intolerant to gas impurities because of the low electron mobility in xenon. Therefore, the gas cell of GSPCs has to be manufactured with ultrahigh vacuum technology to avoid contamination of the detector gas. In addition, gas purification systems like getter pumps are used. The slow velocity of the electrons in the absorption and drift region with a low electrical field strength intensifies the susceptibility of the GSPC to gas impurities. This effect can be reduced in the so called "driftless" GSPC. The "driftless" GSPC has a common high field absorption-scintillation region located directly below the detector window. The high electrical field mandatory for the scintillation excitation of the gas by the electrons results in a high drift velocity of the electrons from the beginning. The high field reduces in addition the loss of electrons from the ionization cloud of X-ray events absorbed near to the entrance window. But nothing is for free and the light output of an event in this configuration depends on the absorption depth of the X-ray event in the absorption-scintillation region. To recover the original energy of the event, the signal has to be corrected with a burst length factor.

X-ray astronomy with nonimaging detectors requires large apertures and a good background rejection efficiency. Rise time discrimination, burst length discrimination, and limitation of the energy band are the main background suppression

Table 2.2 Characteristics of GSPC instruments for X-ray astronomy

Satellite

BeppoSAX

EXOSAT

Tenma

Year

1996

1983

1983

Experiment

HPGSPC

GS

SPC-A, SPC-B, SPC-C

Effective area

240 cm2 *

-100 cm2

320 cm2, 320 cm2, 80 cm2

FOV

1.1°

0.75°

3.1°, 2.5°, 3.8° mod. collimator

(FWHM)

Energy range

4-120keV

2-40 keV

2-60 keV

AE/E

31 x (E(keV)r0'5

27 x (E(keV)) ~0'5

23 x (E(keV)r0-5

(% FWHM)

Reference

[1]

[16,17]

methods for GSPCs to distinguish background events from real X-ray signals. The background rejection efficiency by the burst length discrimination can be improved substantially by using a gas mixture of xenon with helium. Addition of helium increases the electron drift velocity considerably compared with pure xenon [11]. The full field energy resolution of large area GSPCs with a single photomultiplier readout is degraded because of solid angle variations of the light emission regions for events distributed over the whole sensitive area. A focusing electrical field in a conical absorption and drift region concentrating the electrons on a small scintillation region reduce the effect of solid angle variation. Large aperture detectors with such focusing geometries were operated on the X-ray satellites Tenma and EXOSAT [11,16]. Another approach to overcome the problem of solid angle variations in large area detectors is to view the scintillation region of the GSPC by an Anger camera arrangement of several photomultipliers. The event position derived from the ratio of the photomultiplier signals is used to correct the event energy. This method was used in the HPGSPC experiment aboard BeppoSAX. Table 2.2 gives the characteristics of GSPCs with mechanical collimators operated on several X-ray astronomy satellite missions.

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