Large Area Proportional Counters for XRay Astronomy

The observation of the weak photon fluxes from cosmic X-ray sources with nonimaging instruments (detectors with mechanical collimators) requires large area detectors with high background rejection capability. Table 2.1 shows the development of collecting area of several proportional counter experiments for X-ray astronomy within the last decades. Multi anode multilayer proportional counters subdivided in cells by cathode grids as shown in Fig. 2.3 were mainly used for such observations in the energy band up to 50keV. The cell structure of these detectors offers different possibilities for discriminating background events. Using the signals of the detector cells bordering the walls of the detector housing in anticoincidence results in a three-sided anticoincidence. Additional end-veto electrodes of anode or cathode protect the other two sides of the sensitive volume [3,30]. Requiring that a single event must not show signals in neighboring cells reduces charged particle background from the front side. The loss of real X-ray events, because of photoelectron tracks crossing the border of two cells, is less than 10% in the 1.5-35keV band [6]. Another approach was used in the RXTE detector by introducing a separate front anticoincidence layer filled with a low Z gas (propane) separated from the lower detector volume by an aluminized mylar foil [2].

Table 2.1 Several large area proportional counters for X-ray astronomy

Experiment

Year

AE (keV)

Area (cm2)

FOV (FWHM)

Reference

Uhuru

1970

2.0-20

2 x 840

5° x 5°,5°x 0.5°

[9]

HEAO-1 A1

1977

0.15-20

7 x 1350-1900

1°x 4° - 1° x 0.5°

[18]

EXOSAT ME

1983

1.2-50

1800

0.75° x 0.75°

[29]

Ginga LAC

1987

1.5-37

4000

1.1° x 2° elliptical

[30]

RXTE PCA

1995

2.0-60

6250

1° hexagonal

[2]

Collimator field of view (FOV)

Collimator field of view (FOV)

The detection limit of such an instrument for a point source in the presence of a diffuse X-ray background component and a cosmic-ray background can be estimated as follows if the observed quantities are constant. Q = quantum efficiency of the detector Ax = geometric detector area for X-rays (cm2) Ab = geometric detector area for background (cm2)

Bc = cosmic-ray background events not rejected by event selection logic (events cm-2 s-1 keV-1)

Bx = diffuse cosmic X-rays background (events cm-2 s-1 keV-1 sr-1) Q = field of view in (sr)

Fmin = minimum detectable flux of a point source (Photons cm-2 s-1 keV-1) AE = energy band of detector (keV) S = desired number of standard deviations t = observing time (s)

S BcAb + QQBxAx

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