A new end-window variable geometry x-ray proportional counter

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A N E W E N D - W I N D O W VARIABLE G E O M E T R Y X-RAY P R O P O R T I O N A L C O U N T E R J. A. CAIRNS*, C. L. D E S B O R O U G H t and D. F. HOLLOWAY*

*Solid State Division, t Electronics and Applied Physics Division, Atomic Energy Research Establishment, Harwell, Didcot, Berks., England Received 22 July 1970 The new detector described here §, being of the end-window type, is readily accessible to X-ray sources in a confined environment, and thus offers very favourable X-ray collection geometry. It exhibits extremely good resolution, particularly in the vital soft-to-ultrasoft X-ray region. The anode wire is held in a novel compact unit, which may be interchanged with ease. Other features include a built-in head amplifier and a simple assembly

which allows windows to be replaced and fitted in a gas-tight seal. The variable geometry feature of the instrument arises as a result of the incorporation of a calibrated dial, by means of which the anode-to-window distance may be varied during operation. This allows the effective gas path length to be optimised, so that X-rays of a particular energy may be detected in a more selective manner.

1. Introduction

good energy resolution from such a device, but, because the anode is in the form of a needle, a highly localised intense field is developed at the needle tip. This causes a disturbing variation in efficiency with radial distance at a constant needle-to-window distance, and means that the incoming X-radiation must be very carefully collimated to obtain satisfactory performance; thus a considerable proportion of the X-ray yield is lost. Another consequence of the localised field at the needle tip is that these counters exhibit marked pulse height depression; i.e. there is a reduction in charge output with increase in count rate at a given X-ray energy. This is caused by the build-up round the needle tip of a positive ion sheath, which allows recombination with further electrons produced by the incident X-rays, hence causing a reduction in gas gain.

One of the main disadvantages of the conventional side-window proportional counter is that its shape and bulk often impose restrictions on its ability to be positioned close to targets from which X-rays are being generated, particularly in a vacuum system environment. This places a severe limit on the collection efficiency when non-dispersive X-ray counting techniques are attempted. Such a drawback was particularly apparent in our studies of characteristic X-ray generation by heavy ion bombardment1), which yields X-rays virtually free from "white" background radiation, so that even the lightest elements can be detected. Flow proportional counters are used in this work, since solid state detectors, whilst offering much superior resolution, are currently incapable of detecting X-rays much below 1 keV in energy, and so cannot be employed to identify the characteristic X-rays of elements below sodium in atomic number. In order to overcome the geometrical restrictions of the side-window counter, one can employ an endwindow detector, preferably with its window mounted on the end of a nozzle which can be placed through a suitable orifice in the vacuum chamber wall and positioned close to the specimen, whilst causing a minimum of interference to other components in the vacuum system. The difficulty is that end-window detectors usually exhibit very poor energy resolution, although one possible exception to this is the point anode end-window counter, described by Duncumb 2) and later, in various stages of development, by Mathieson and Sanford a) and Ranzetta and Scott4). These authors have shown that it is possible to obtain § Available from J. & P. Engineering Ltd., Reading, Berks., England. + Patent applied for.

2. Counter construction +

2.1. GENERAL FEATURES Fig. 1 illustrates a general view of the counter, showing its main components. F r o m this it may be seen how the gas flows via a flexible insulating tube which passes through a dial on the end of the counter body and into a stainless steel tube which carries the gas (and the EHT) to the anode unit, and thence back to the exit port. This entire flow path is completely gas-tight, so that the counter may be conveniently operated at reduced pressure, if desired. The head amplifier is mounted on a board inside the screening box, as close to the stainless steel tube as possible, so as to minimize capacitance, and is readily accessible by removal of the cover plate. The screening box is, of course, always at atmospheric pressure, since it is completely separated from the gas flow system by the " O " ring seals. Fig. 1 also illustrates the mechanism by which rotation of the calibrated dial on the

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end of the counter body causes the distance between the anode unit and the window to be accurately varied. When the counter is fitted into a vacuum system, the nose-piece is positioned through a simple adaptor flange, which incorporates an " O " ring to form the vacuum seal. It may be appreciated from fig. 2 that it is possible therefore simply to uncouple the remainder of the counter for inspection or replacement of the anode unit, whilst leaving the nose-piece in position to maintain the vacuum in the target chamber. This

facility means that the whole system can be in operation again within a few minutes of changing the anode unit. Fig. 2 also indicates the prospect for attainment of high X-ray collection efficiency. 2.2. THE ANODE UNIT

The philosophy behind the design of the anode unit was to simulate as far as possible, the theoretically perfectly uniform field distribution which exists in a side-window proportional counter. The extent to which this goal has been achieved by the anode unit incorporated in the instrument will become apparent later when the resolution performance is assessed. For the present, attention may be confined to the construction, which is detailed in fig. 3. This shows that the BERYLLIUM/ COPPER

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VARIABLEGEOMETRYX-RAY PROPORTIONAL COUNTER anode wire is held taut, soldered between two stainless steel support tubes, parallel to (and insulated from) a beryllium/copper earthpiece, which, by virtue of its fingering, makes good electrical contact with the inside walls of the nose-piece. The anode wire is thus surrounded by an earth field distribution, defined by the earthpiece, the nose-piece, and the inside conductive surface of the detector window. It may be appreciated that it is extremely advantageous to have the anode in the form of such a convenient unit. For example, accidental damage to the anode wire need no longer result in the counter being subjected to the time-consuming wire changing operation which is characteristic of so many side-window designs. In addition, the unit may be examined under a lens to ensure that the wire is perfectly taut, and that the wire attachment points are sufficiently smooth to prevent point discharges. Finally the whole unit may be easily cleaned, preferably ultrasonically, to remove any traces of dust. 2.3. WINDOWMOUNTINGASSEMBLIES One of the advantages of having the windows assembled on the end of a nozzle is that it then becomes possible to mount them in a convenient manner in a variety of positions. Two such arrangements are in fact apparent from figs. 6 and 7. In general, it is extremely important to appreciate that any window assembly should incorporate the following features. Firstly, the windows must be held in a gas-tight seal, so as to prevent counter gas from leaking into the vacuum system. In addition, the inside surface of the window must be conductive, and make good electrical contact with the inside wall of the detector, so as to ensure continuity of the electrical field. Furthermore, it is often advantageous to have the window conductive on the outside surface as well, particularly when using the instrument in non-dispersive analysis, so as to

guard against the danger of scattered particles (particularly electrons) striking the window and causing it to charge up. This produces a serious disturbance in the uniformity of the internal field, and so naturally has an adverse effect on the resolution exhibited by the detector. A further point to be taken into consideration arises when using very thin windows to transmit ultrasoft X-radiation such as carbon-K (284 eV) or boron-K (190 eV). When a pressure difference is applied across such windows, they naturally tend to bow out towards the vacuum, and may thus become considerably stretched. Under these conditions, their internal conductive surface may become broken; this causes a dramatic reduction in the internal field strength of the instrument, and hence a reduction in the ability of the anode to collect all of the electrons released during ionisation of the counter gas by the incoming radiation. The result is a fall in count rate; for example, a detector incorporating a 2 /~m polycarbonate window may actually exhibit a lower count rate for, say carbon-K X-rays, than when incorporating a heavily aluminised 6/~m mylar window, although the latter has a much lower carbon-K transmission than the former. We have observed that this anomaly may be rectified by simply attaching a high transmission nickel support grid to the inside of the window, so as to maintain a uniform internal earth field. 3. Electronics

One of the major causes of signal loss from a proportional counter is the presence of connecting cable between the detector and the head amplifier, since this largely determines the added capacitance of the detector, and hence causes signal degradation. Thus, for a given cable, the greater the cable length, the smaller the signal to the amplifier input (for the case of a high input impedance amplifier), with a corresponding reduction in the signal-to-noise ratio of the system. This I

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is of particular concern when designing a detector for ultra-soft X-rays, since these naturally give rise to a very low signal output. Thus, for optimum performance, a low energy X-ray detector should have its head amplifier connected directly to the anode. This has been achieved by using a head amplifier, designed on thick film techniques, which is of sufficiently small size to be connected directly to the steel E H T tube which holds the anode unit. Fig. 4 shows the circuit diagram of the head amplifier and its associated electronics. Another extremely important prerequisite of a good proportional counter head amplifier is that its inherent noise performance should be so low as to degrade to a negligible extent the signal resolution of even the softest X-radiation. The detector's head amplifier also satisfies this requirement, for its maximum noise figure of 500 ion pairs rms is equivalent to a contribution of less than 1°/o to the signal output from ultrasoft boron-K X-rays. Finally, it may be noted that the amplifier used in this instrument has a relatively long time constant ( > 1 ms) and can therefore handle pulse "pile-up" without saturation.

Attention has already been drawn (fig. 1) to the calibrated dial, which, on rotation, allows the distance between the anode wire and the window to be accurately varied whilst the counter is in operation. This feature is useful, because it permits the anode to be moved to a position which corresponds to the mean penetration depth of the incoming radiation, thus optimising the detection efficiency for X-rays of a particular energy, and so resulting in a degree of X-ray selectivity which could not be achieved in proportional counters of convenional design. As an illustration of the energy range within which this X-ray selectivity may be applied, consider copper-L X-rays ( ~ 930 eV) and silicon-K X-rays ( ~ 1.7 keV). These X-rays were in fact generated from a copper/silicon target by 100 keV proton irradiation, and detected non-dispersively by the counter, which was filled with an argon/methane gas mixture, and operated at atmospheric pressure• Fig. 5A shows the lzerformance when the anode-to-window distance has been adjusted so that both sets of X-rays were detected with approximately equal efficiency• Fig. 5B illustrates the consequence of reducing the gas path length to detect the copper-L X-rays preferentially; whereas increasing the gas path length, so as to detect the silicon-K X-rays at the expense of the copper-L X-rays, resulted in the spectrum shown in fig. 5C. In order to understand the reason for this selectivity, it is necessary to consider the electrical field which exists within the active volume of the detector at these three anode-to-window distances. Accordingly, fig. 6 illustrates the three anode-to-window distances in question, and represents, by means of lines of force, the field distribution in each case. It then becomes apparent that since the less energetic copper-L X-radiation produces ionisation over only a very short gas path length, it may be detected with most efficiency by reducing the anode-to-window distance, as shown in fig. 6B. On the other hand, increasing this distance, as in fig. 6C, results in a very weak field between anode and window, with a consequent reduction in efficiency for detection of the soft X-rays. However, it is also apparent from fig. 6C that the more penetrating silicon-K X-radiation can still be detected with reasonable efficiency, because of the constant field between anode and earthpiece. It therefore becomes apparent why the field distribution shown in fig. 6A represents the optimum condition (resulting in the approximately equal detection efficiency shown in fig. 5A) because virtually all ionisation taking place anywhere between window and earthpiece

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is detected. It is of significance that in this situation the distance between the window and the anode wire is approximately equal to the distance between the anode wire and the earthpiece - i.e. the field approximates to that which would be expected in a conventional sidewindow design. However, the field distribution departs

from this ideal in fact, due to intrusion of the window support within the active gas volume, leading to the serious distortion of the lines of force between window and anode wire shown in fig. 6A. This causes difficulty in detecting ultra-soft X-rays such as carbon-K, which produce ionisation very close to the window. Fortunately, it has been possible to rectify this situation in a very simple manner, as described in the following section. 4.2. UNIFORM ELECTRICAL FIELD ENVIRONMENT

i Fig. 7. Uniform electrical field distribution obtained by using alternative window assembly.

Fig. 7 illustrates how an alternative arrangement of the window mounting assembly produces an electrical field distribution which is almost perfectly symmetrical with respect to the anode wire - i.e. the field approximates closely to the idealised situation which is presumed to exist in a good side-window detector. This may be confirmed by reference to fig. 8, which illustrates the extremely good resolution (full width at half maximum) obtainable, particularly in the soft and ultra-soft regions, as shown by copper-L: fig. 8b, and carbon-K: fig. 8a, respectively. In addition, fig. 8c illustrates that this resolution is maintained over a wider energy range than would have been possible with the window assembly shown in fig. 6. It should be noted that the detector used in these measurements was operated at atmospheric pressure and incorporated a 6/~m mylar window, aluminised on both sides. The X-rays, shown in fig. 8a, b,c, were generated by 100 keV proton bombardment of the appropriate targets and detected non-dispersively. Fig. 8d, illustrating the main peak and argon escape peak of a standard SSFe source is included for completeness, because of the

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widespread practice of routinely using this source-tomonitor propoitional counter resolution performance. Grateful thanks are due to B. Benneworth for design draughtsmanship, to Dr. G. V. T. Ranzetta for most useful discussion on his point anode counter, and to J. G. E. Paget, J. C. Woodmore, C. G. Barber and S. J. Ashby for assistance with the engineering construction.

References 1) j. A. Cairns, D. F. Holloway and R. S. Nelson, Intern. Conf. Atomic collision phenomena in solids, University of Sussex (North-Holland Publ. Co., Amsterdam, 1970). 2) p. Duncumb, X-ray microscopy and X-ray microanalysis ('Elsevier, Amsterdam, 1960) p. 365. 3) E. Mathieson and P. W. Sanford, J. Sci. Instr. 40 (1963) 446. 4) G. V. T. Ranzetta and V. D. Scott, J. Sci. Instr. 44 (1967) 983. 5) j. A. Cairns, C. L. Desborough and D. F. Holloway, Nature 223 (1969) 1261.