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Resolution and spectral characteristics of ultra high pressure proportional counters using various quench gases
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 527 (2004) 493–511
Resolution and spectral characteristics of ultra high pressure proportional counters using various quench gases D.J. Grey, R.K. Sood*, R.K. Manchanda1 School of Physics, University College, University of New South Wales, Australian Defence Force Academy, Northcott Drive, Canberra, ACT 2600, Australia Received 28 January 2000; received in revised form 28 January 2004; accepted 19 March 2004
Abstract We present the results of a systematic search for the best gas mixture for use in proportional counters operating at high pressures. The data were gathered using a variety of quench gases with argon or xenon as the primary fill gas. The gas gain and the energy resolution broadly follow the complex ionization within the gas volume. The data show that the spectral response for argon does not change significantly with pressure. However, for xenon the spectral response is dominated by secondary shell interaction processes, and charge loss due to recombination. r 2004 Elsevier B.V. All rights reserved. PACS: 07.85.F; 29.40.C; 07.87.+v Keywords: Proportional counters; Energy resolution; Gas gain; High-pressure gas
1. Introduction Proportional counters have traditionally been the preferred detectors for spectral and temporal studies in X-ray astronomy [1–6]. The available large area and the ease of construction make these detectors the most sensitive alternative for lowenergy X-ray observations. In addition, many techniques have evolved to reduce both the *Corresponding author. Tel.: +61-2-6268-8801; fax: +61-26268-8786. E-mail address: [email protected] (R.K. Sood). 1 Permanent address: Tata Institute of Fundamental Research, Mumbai 400 005, India.
internal background of these detectors and also the background generated by Compton scattering of the high-energy ambient photon flux. Xenon and argon are the most commonly used detection gases. A small percentage of quenching gas is, however, essential for the stable operation of a proportional counter. While there is no theoretical or empirical standard for the percentage and the type of quench gas, the choice of the quenching molecule and its concentration can have a significant effect on the operating conditions. The energy range of operation of proportional counters filled with gas at atmospheric pressure is limited to 2–10 keV for argon and to B80 keV for xenon. In addition a strong K-escape peak is
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.03.202
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generated in xenon as 87% of the K-shell interactions for incident photons with energy above 30 keV lead to fluorescent photons which escape the gas cell because of a low attenuation cross-section. This is mainly because the cell size in directions orthogonal to the anode wire is generally restricted to p15 mm to avoid electron– ion recombination during the charge collection process. The use of a fill gas at high pressures in a proportional counter can enhance the detection efficiency for incoming photons. It can also increase the attenuation of the K-escape photons. However, the increased probability of scattering of the incident photons in the gas volume with increase in pressure leads to complex spectra independent of the intrinsic energy resolution of the filling gas mixture. These problems are much more pronounced in proportional counters when compared to ionization chambers, because the secondary multiplication increases the DN=N fluctuations inherent in the primary cloud, where N is the number of electrons produced in the primary photon interaction. N depends not only on the ionization potential of the main fill gas but also on the type of quenching gas. The number of primary electrons can be increased through the secondary ionization of the quenching gas via collision with the long lived metastable states of the excited noble gas and absorption of the UV photons emitted during the de-excitation of the host gas molecules. Therefore, the energy resolution of a proportional counter is a function of both the gas pressure and the type and amount of quench gas. The electron collison processes in gaseous xenon, the resultant ionization yield, and the complexity of the electron kinematics are still not fully understood [7–9]. Monte Carlo simulations of the gamma ray response of low-pressure proportional counters has previously been attempted in order to understand the pulse height spectra from these counters [10,11]. In most of the earlier experiments in X-ray astronomy, methane ðCH4 Þ and carbon-dioxide ðCO2 Þ were the most commonly used quenching additives for both argon and xenon filled proportional counters. Laboratory investigations have, however, been conducted with various polyatomic
quench gas mixtures to search for the optimum resonance Penning mixtures so as to achieve the best possible energy resolution [12,13]. A large majority of these investigations have been confined to test counters operating at low pressures up to about 1 atm: Since many of the gas properties and the molecular band structure appear to change when the pressure is increased [14], a systematic study of different operating conditions is absolutely essential to optimize the future use of these detectors. In this paper, we present detailed investigations of gas gain curves, energy resolution, and the spectral shape of the output of a proportional counter of cylindrical geometry, using different quench gases at various pressures. We compare the results with earlier work and interpret them in terms of the physical mechanism of the energy loss processes in the gas. This work is a continuation of our earlier published work on the characteristics of ultra high pressures proportional counters filled with argon+methane and xenon+methane mixtures [15–17].
2. Experimental arrangement The experimental set up used in the present investigations is similar to the one used in our earlier work. The tests were performed using a conventional cylindrical aluminium counter 30 cm long, 30 mm inside diameter, and 3 mm wall thickness. A 25 mm gold plated tungsten wire was used for the anode. The mechanical structure was hydro-statically tested to 60 atm: The experiments were conducted at filling pressures up to a maximum of 30 atm; and depending upon the nature of the gas mixture, an anode voltage of up to 7:5 kV was employed. A LeCroy TRA100 charge sensitive pre-amplifier was used at the front end, and an Ortec 450 unit was used for further amplification of the incoming signal. Spectra were gathered at various X-ray line energies. The X-ray photons were obtained from collimated radioactive sources namely, 241 Am; 133 Ba and 57 Co: Apart from the principal Ka and Kb line energies of these sources, the escape peak of xenon and other low level gamma transitions in the 20– 500 keV range provided a large dynamic range for
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test and calibration purposes. For each source spectrum recorded, a background spectrum was recorded at the same gain setting and over the same time period. The background was then subtracted from the raw source spectrum. The amplifier electronics noise, which contributes to the final spectral characteristics, was measured as 0:44 fC equivalent RMS. The differentiating and integrating time constants for the pulse shaping electronics were set at 0.5 and 2 ms; respectively [18]. In order to attain optimum counter performance, a rigorous cleaning and purification scheme was used during the entire process of tube assembly. The counter was evacuated and baked at 80–100 C for at least 72 h until a stable vacuum level of B1 107 Torr was achieved. A newly assembled ultra high purity gas purification and filling system was used during these laboratory tests. This system is completely of stainless steel construction and employs a high temperature copper catalyst filter, an Oxisorb filter, and a molecular sieve for the gas purification process. It is designed to reduce the concentration of electronegative impurities ðO2 ; H2 O; CO2 ; COÞ to a level of less than 5 ppb [19].
3. Results and discussion A large variety of argon and xenon based mixtures were tested during the present investigations. We also tested the Penning mixture of argon+xenon, and mixtures of argon and/or xenon with methane ðCH4 Þ; ethylene ðC2 H4 Þ and iso-butane ðiC4 H10 Þ as quenching gases. These tests were motivated by the possibility of the use of such mixtures in detectors for future X-ray astronomy missions. The various results presented in the following sections are based on data obtained for Ar þ CH4 (98:2), Ar þ C2 H4 (98:2), Ar þ iC4 H10 (98:2), Ar þ Xe (90:10), Ar þ Xe (70:30), Xe þ C2 H4 (98:2), Xe þ C2 H4 (90:10), Xe þ iC4 H10 (98:2), and Xe þ Ar þ CH4 (90:8:2). 3.1. Gas gain behaviour Gas gain studies were carried out using the method of current determination in a Faraday
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cage. A 0:116 mCi241 Am source was used with a 5 mm collimator. With such a wide collimator, the gas gain curves presented in this section correspond to the behaviour of the counter averaged over the length of the anode wire. Some of the representative data are shown in Figs. 1(a)–(g). The curves show the gas gain M versus applied voltage V ; for different gas mixtures over a range of pressures. It is seen from the figure that large values of gas gain are possible for some of these mixture but that the incremental gain change with voltage depends strongly on the operating pressure. The values of the optimum operating voltage for a given pressure and the percentage of quench gas are taken as those which yield the best energy resolution. The horizontal dashed lines in Fig. 1 give the upper and lower limits of the gas gain values that correspond to optimum energy resolution at 60 keV for a given mixture. The data in the figure show that xenon based mixtures containing isobutane and ethylene quenching additives, used in the proportions shown, exhibit the lowest % average optimum gain of Mp5507185; while xenon+methane give a mid value of % Mp7507150: Argon based mixtures exhibit the highest optimum gain with the Penning mixture of Ar þ Xe (90:10) giving the highest average value of % Mp9157185: As we will show in the next section the horizontal dashed lines in the figure also imply that the mean value of gain necessary for optimum resolution for a given mixture is independent of the gas pressure. The data presented in Figs. 1(a)– (g) compare well with our earlier data and with those of other workers as reported in the literature [15,16]. For comparison we have shown the data of Agrawal et al. [20] for Ar þ Xe (90:10) and (70:30) in Figs. 1(a) and (b), and those of Ye et al. [17] and Sood et al. [16] in Fig. 1(c). It is also seen from Fig. 1 that the M–V curves show the expected exponential behaviour when considered individually. However, the gradient changes continuously with change in pressure, a fact alluded to in our earlier papers. The large variety of the incremental change in the gradient with pressure, seen in the plots, suggests that no single parameter can be fitted to describe this behaviour. To a first approximation, variations of the gradient with pressure p and of the intercept on
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Fig. 1. Gas gain versus applied voltage at different gas pressures: (a) Ar þ Xe (90:10), (b) Ar þ Xe (70:30), (c) Xe þ CH4 (98:2), (d) Xe þ Ar þ CH4 (90:8:2), (e) Xe þ C2 H4 (98:2), (f) Xe þ C2 H4 (90:10) and (g) Xe þ iC4 H10 (98:2). The data of Agrawal et al. [20] at 1 atm are shown in (a) and (b) by +. Those of Ye et al. [17] and Sood et al. [16] are shown in (c) by symbols that are not underscored.
the Y -axis may be represented by optimum fits of the type mlnðMÞ ¼ apb and BlnðMÞ ¼ cpd : The derived values of the coefficients a; b; c and d; along with values of the regression coefficient R2
and the chi-test parameter w2 are listed in Table 1 for the different gas mixtures investigated. The variations of the gas gain gradient and the corresponding intercept on the Y -axis are shown
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Table 1 Curve fitting coefficients for the gas-gain gradient m; and intercept B for the curves shown in Fig. 1 Gradient mlnðMÞ
Intercept—BlnðMÞ
Gas mixture
a
b
R2
w2
c
d
R2
w2
Ar þ Xe (90:10) Ar þ Xe (70:30) Ar þ CH4 (98:2) Xe þ CH4 (98:2) Xe þ C2 H4 (98:2) Xe þ C2 H4 (90:10) Xe þ iC4 H10 (98:2) Xe þ Ar þ CH4 (90:8:2)
0.0112 0.0113 0.0080 0.0074 0.0075 0.0069 0.0083 0.0077
0.308 0.384 0.302 0.311 0.416 0.295 0.358 0.319
0.985 0.998 0.981 0.999 0.862 1 0.999 0.998
1 1 1 1 1 0.99 1 1
3.407 4.860 2.348 4.796 3.684 3.547 3.585 3.768
0.302 0.210 0.398 0.250 0.392 0.461 0.408 0.374
0.862 0.992 0.961 0.991 0.993 0.99 0.999 0.999
0.99 1 0.99 0.99 0.99 0.46 0.99 0.99
Optimum parameters were achieved by fitting the gas gain curves with the generalized functional form lnðMÞ ¼ apb V þ cpd : Xe þ C2 H4 (90:10) coefficient values are based only on the first 3 data sets shown in Fig. 1(f).
in Fig. 2. Two points may be noted from this figure. Firstly, the shape of the curves suggests an exponential dependence of the gradient of the gas gain curves on pressure. Secondly, independent of the type of poly-atomic quench gas used, argon and xenon show identical behaviour thereby suggesting a common basic physical process in operation. The Penning mixture of Ar þ Xe (90:10) follows a similar behaviour except for scaling due to increased gain. However, at high pressures, the 70:30 mixture of Ar þ Xe approaches the poly-atomic limit thereby indicating a reduction in the Penning effect at increased pressure. This behaviour is contrary to what is expected. The non-metastable Penning effect in Ar þ Xe occurs due to the collisional de-excitation by Xe atoms of short-lived Ar atoms above the metastable state. Thus an increased concentration of Xe should lead to an enhanced Penning effect. However, it has been reported by Fuzesy et al. [21], that even at lower pressures, an increase in Xe concentration beyond 5–8% leads to a reduction of the Penning effect. We believe that this additional quenching of the signal may be caused by increased fluctuations in the number of clusters of xenon ions that are believed to form at high pressures. The initial charge cloud in the gas counter is formed by the photo-absorption of the incident photon, followed by a combination of electron ionization and photo-ionization processes. The gas amplification in a cylindrical proportional counter
takes place R r close to the anode wire and is given by lnðMÞ ¼ rab a dr; where a is the first Townsend coefficient, and ra and rb are the anode and cathode radii, respectively. For a given pressure and for moderate gas gain M and anode voltage V ; the gas gain can be approximated by M ¼ K expðCo V Þ; where Co is the capacitance of the detector and K is a constant. The pressure dependence of this gain equation is complex and is built in three ways. First, the Townsend coefficient a which controls the ionization in the multiplication region is itself defined as a ¼ ApeBp=EðrÞ ; where A and B are constants, and EðrÞ is the radial electric field. Second, in terms of basic parameters the detector capacitance C per unit length is defined as C ¼ 2peo eg ðp; TÞ=lnðrb =ra Þ where the dielectric constant eg for the gas is a function of both pressure p (atm) and temperature T ð CÞ; and is given as ðeg 1Þp;T ¼ ðpðeg 1Þ1 atm;20 C =1 þ 0:003411ðT 20ÞÞ: Third, as described by Sood et al. [16], the number of primary electrons reaching the multiplication zone changes with pressure, and is given by NðrÞ ¼ % krp=V ; where N% is the mean number of primary Ne ions created. This change arises from the fact that in between the proportional and drift regions, one can envisage an ionization region within the gas volume, where the electric field is such that an electron attains enough energy between two collisions to excite the gas atoms rather than to ionize them [8], thereby reducing the effective distance from the anode at which the multiplication
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Fig. 2. Variation of (a) gas gain gradient mlnðMÞ and (b) the intercept—BlnðMÞ : The curves for Ar þ CH4 are derived from Sood et al. [16].
begins. The increased number of such collisions of the primary electrons will not only give rise to larger variations in the electron statistics but will also lead to a lower overall gain. All these factors therefore lead to a weak exponential dependence of the gas gain on pressure, as seen in Fig. 2(a). Bolotnikov and Ramsey [22] have recently discussed a mechanism for electron–ion recombination in high pressure xenon counters that is dependent on the electric field. The implications
of this model will be addressed in a forthcoming paper, where we discuss the impact of drift time of electrons (up to 1 ms), on the ultimate resolution. Numerical simulations for an argon–methane mixture [23] also suggest a lower value of the first Townsend coefficient a in the region of a lower reduced field. A comparison of the values of the constants a; b; c and d; given in Table 1, does not show any underlying common features. We have also tested
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the applicability of various theoretical gas multiplication models, as described by Ye et al. [17], to our data. The curves representing the relationship between a=p and the reduced electric field lnðMÞ=pra Sa ; where Sa is the reduced electric field at the anode, best fit the functional form of the model of Charles [24]. The a=p versus E=p curves are shown in Fig. 3(a) for various gas mixtures investigated in the present work, while a compar-
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ison with the earlier data from different authors [17,25,26] is shown in Fig. 3(b). It is seen that the curves are highly non-linear and any linear mathematical representation can only be written for small segments of each curve. The nonlinearity arises mainly due to the cooling effect inherent in electrons in inelastic scattering leading to excitation [9]. This process in turn also affects the relative variance of single electron spectra [27].
Fig. 3. Curves representing a=p vesus E=p for the Charles model applied to all gas mixtures used in the present tests are shown in (a). A comparison with the data of Kruithof [29], Sakurai et al. [25] and Ye et al. [17] is shown in (b).
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3.2. Energy resolution The energy resolution of a proportional counter is the key parameter which determines its suitability for astronomical observations. During our tests we collected a large number of spectra for different input energies and at various gas pressures for each of the trial gas mixtures. Gaussian curves were fitted to the data for accurate determination of the mean energy and the corresponding FWHM of each photo-peak. Xray spectra were collected at various pressures up to 30 atm for Ar-based mixtures and up to 10 atm for Xe-based mixtures. A bench mark energy resolution of 9.5% at 59:5 keV was obtained for an Ar þ CH4 (98:2) mixture at 30 atm: This compares well with the value of 12.5% reported by Manchanda et al. [15] that was obtained for a gas gain of B200 and at 27 atm for the same mixture. Results that we obtained at low pressures also compare very well with those obtained by Agrawal and Ramsey [26]. Results obtained at high pressures using other quenching gases failed to achieve the same good energy resolution as achieved with CH4 ; while at low pressures of up to 2 atm; the resolution obtained with iC4 H10 was found to be the best at 4.9% at 59:5 keV: The Penning mixtures of Ar þ Xe at 70:30 and 90:10 ratio failed to improve the energy resolution at high pressures. In the case of Xe-based mixtures with polyatomic quenching gases, the overall energy resolution was found to be slightly worse compared to their Ar-based counterparts. Furthermore, as reported by Ye et al. [17], Xe þ CH4 (98:2) gave the best resolution of 6.5% at 1 atm and 11.9% at 10 atm pressure, respectively. These numbers compare well with the value of 7.3% obtained by Sakurai et al. [25] using a 25 mm anode wire. Using Ar þ CH4 (98:2) and Xe þ CH4 (98:2) mixtures it has been shown by Manchanda et al. [15] and Sood et al. [16] that at low pressures up to 3:5 atm; the energy resolution follows a simple functional form R ¼ 0:46E 1=2 and at higher pressures the functional form changes to DE ¼ aE þ bE 1=2 : In order to look for any unified function dependence and systematic effects between different gas combinations, we studied Xe-
Fig. 4. Energy resolution for 59:5 keV X-rays from 241 Am as a function of gas pressure for (a) Ar þ CH4 (98:2), (b) Ar þ C2 H4 (98:2), (c) Ar þ iC4 H10 ; (98:2), (d) Ar þ Xe (90:10), (e) Ar þ Xe (70:30), (f) Xe þ CH4 (98:2), (g) Xe þ Ar þ CH4 (90:8:2), (h) Xe þ C2 H4 (98:2), (i) Xe þ C2 H4 (90:10) and (j) Xe þ iC4 H10 (98:2).
Table 2 Curve fitting parameters for the curves of energy resolution versus incident photon energy shown in Fig. 4 Gas mixture Xe þ CH4 (98:2)
Pressure a
1 5 7.5 10 Xe þ Ar þ CH4 (90:8:2) 1 5 10 15 Xe þ C2 H4 (98:2) 1 5 Xe þ iC2 H10 (98:2) 1 5
0.0044 0.0186 0.0207 0.0479 0.0014 0.0163 0.0577 0.0242 0.122 0.103 0.0804 0.0197
b
R2
w2
0.5523 1.0013 0.9522 1.3383 0.5584 0.6976 0.7069 0.9475 1.2193 1.6568 1.0734 1.0031
0.93 0.97 0.88 0.88 0.93 0.92 0.47 0.57 0.80 — 0.85 —
0.91 0.88 0.93 0.81 0.95 0.78 0.81 0.68 0.76 — 0.14 —
The data are fitted with the functional form R ¼ aE þ bE 1=2 where E is in keV.
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based and Ar-based mixtures at different pressures, and over a range of X-ray energies. The pressure dependence of the energy resolution was investigated using primarily the 59:5 keV X-ray photo-peak from 241 Am: Energy resolution curves are shown in Fig. 4, where curves (a)–(e) refer to Ar-based mixtures, while curves (f)–(j)
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refer to Xe-based mixtures. We have fitted the curves for all the gas mixtures with a universal functional form as mentioned above. We present in Table 2 the fitting parameters for curves for Xebased mixtures. It is seen from the table that apart from variations in the values of a and b for a given mixture at different pressures, there is no
Fig. 5. Energy resolution for 59:5 keV X-rays versus gas gain for different pressures for (a) Ar þ Xe (90:10), (b) Ar þ Xe (70:30), (c) Xe þ iC4 H10 (98:2), (d) Xe þ C2 H4 (98:2), (e) Xe þ Ar þ CH4 (90:8:2) and (f) Xe þ CH4 (98:2).
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discernible pattern even though Xe was the primary fill gas in all these cases. The best measured resolution figures of 4.5% for Ar þ CH4 (98:2) and 6.5% for Xe þ CH4 (98:2) at 1 atm pressure compare very well with earlier measurements [15,16,20]. It is seen in Fig. 4 that the family of curves form two separate groups corresponding to Ar-based and Xe-based mixtures, with Ar-based mixtures exhibiting superior overall resolution. The effect of the quenching gases, in the proportions used in this experiment, on the overall resolution of the mixtures may be ranked as (best) CH4 oC2 H4 oiC4 H10 (worst). The rate of deterioration of the energy resolution with increased pressures is also a function of the primary gas, while the effect of the quenching gas is similar in the two groups. These changes must, therefore, arise from variations in the energy loss process, the formation and size of the primary electron cloud, the electron transport process, and the effective volume of the secondary ionization process, rather than simple variations in the secondary multiplication process near the anode. In Fig. 5, we have plotted the energy resolution that we obtained at 59:5 keV as a function of gas gain for different gas mixtures at various pressures. The almost parabolic behaviour of all the curves shown in the figure is quite striking. By virtue of the logarithmic scale used for the gas gain, the curves appear to be roughly symmetrical around the resolution minima. However, plots on a linear scale indicate a very sharp deterioration of the resolution at lower gas gain values at lower gas pressures, while a sharp decline in resolution is also seen at high pressures. The dashed vertical line in each of the graphs represents the gas gain value corresponding to the best measured energy resolution for a given mixture. Note that the dashed lines shown in Fig. 1 correspond to gas gain values relevant to this value for optimum resolution. It is also seen from Fig. 5 that although a general range of values for the best operating gain is between 500 and 1000 for pressures right up to 30 atm; much narrower limits apply for each gas mixture at a given pressure. The dependence of the energy resolution on gas gain for each gas mixture is clearly seen in Fig. 5. It is theoretically expected that the resolution should
continue to improve with decreasing gain, ultimately resulting in Fano-fluctuation limited values at the gain of one. However, additional factors that contribute to the observed deterioration of the energy resolution at low gains include the size and orientation of the primary ion cloud, space charge effects, and the inherent electronics systems noise. Such effects have been observed and reported by several groups [16]. Fig. 6 shows the best measured resolution as a function of X-ray energy for a number of Xebased gas mixtures at various gas pressures. Classically, the energy resolution in a given gas, % where P represents the pulse RE ¼ DE=E dP=P; height for photon energy E; was believed to arise from the fluctuations in the average number of % 2 and in the secondary primary electrons ðdN=NÞ 2 % multiplication ðdM=MÞ : However, the observed change in energy resolution with pressure as well as with the fraction of quench gas used cannot be
Fig. 6. A comparison of the energy resolution versus incident photon energy for (a) Xe þ CH4 (98:2) and (b) Xe þ Ar þ CH4 (90:8:2) at different pressures.
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explained by this simple model [15,16]. Sakurai et al. [25] have also noted that even though the variation in resolution with pressure between pure xenon and a xenon–methane mixture may be explained by a reduced charge cloud model based on changes in the diffusion constant, the model fails when the data are compared for mixtures with varying amounts of quenching gas. To explain the deviation of DE versus E from the canonical formula, Sood et al. [16] proposed that the
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variance in the observed pulse height at a given energy E may be written as 2 2 dP dN 1 dM 2 1 dL 2 ¼ þ þ ð1Þ % P% N% N% M N% L% where N% is the mean number of primary ion pairs produced by a photon of energy E and mean track % M % is the mean amplification factor and length L; dP; dN; dM and dL are the standard deviation of the relevant quantities. This equation may be
Fig. 7. Illustration of UV emission spectra for noble gases Ar, Kr and Xe (a) [30], UV emission spectra for the rare gas scintillation process (b) [31], along with the UV absorption spectra of CH4 ; CO2 and C2 H4 (c) [32].
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simplified to give 2 2 2 dP dN 1 dM ¼ þ % P% N% N% Zðp; EÞM which is analogous to function given ðf =Zðp; EÞw=E 1=2 ; where is the average ionization
the familiar resolution as RE ¼ 236½ðF þ F is the Fano factor, w % 2; potential, f ¼ ðdM=MÞ
and Z is energy dependent in a way such that Z ¼ 1 for energies above 100 keV; and ZBE 1=2 for energies below 100 keV: The deviation of the dependence of energy resolution on energy that we observe in our data, from the ideal functional form RE pE 1=2 ; can be seen in Fig. 6. Clearly, the energy resolution even at a fixed energy, shows a significant pressure
Fig. 8. Energy level diagram for various test gases used in the present studies (Ar, Xe, CH4 ; C2 H4 ; and iC4 H10 ), showing ionization potentials IP, average ionization potentials w; with dominant excitation states Ex ; metastable states M; and dissociation energies D [33,34].
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dependence. In our view the deterioration of the energy resolution must partially arise from a pressure dependence of the parameter Z described above. We note from Figs. 4 and 5, and Figs. 9 and 10 (discussed later) that Ar- and Xe-based mixtures behave differently. While Xe-based mixtures remain more sensitive to ion-recombination problems in the low reduced-field drift regions inherent in high-pressure counters [28], an underlying general dependence on pressure nevertheless remains. We therefore propose that the pressure dependence may be described as a function of a filling factor Q; where Q is a function of the atomic number of the primary gas and the filling pressure. Using the resolution measurements shown in Fig. 4, an empirical relation is derived as Q ¼ ðZgas =ZAr Þ0:5 ðp=1ðatmÞÞ0:2 ; where p is in atm and Zgas and ZAr define the atomic numbers of the host gas and argon, respectively. The generalized functional form for the variation of resolution with pressure and the type of filling gas can then be written as Rp;g ¼ Q Rg :
The data plotted in Fig. 4 also suggest a variation of the energy resolution with the type of quenching gas used. Functionally, a quenching gas absorbs the UV photons emitted during the deexcitation of the gas molecules produced in elastic collisions, relaxes the metastable molecular states via the Penning effect, and improves the electron and ion mobility in the detection gas. The variation in the behaviour of the primary gas with different quenching agents can, therefore, be understood in terms of the differences in the UV emission spectra of the primary gases, the absorption spectra of the quenching additives, and the ionization potentials of the mixture gases. In Figs. 7 and 8, we show the emission spectra and the energy level diagrams for the gases under investigation. It is clearly seen from these figures that while CH4 works very well as a quenching agent for Ar and Kr, it is not the ideal gas for use with Xe. The absorption spectra of C2 H4 appears to be better matched to the emission spectra of Xe. The increase in the gas gain for Xe þ C2 H4 as
Fig. 9. Typical spectra for 241 Am at (a) 30 atm; (b) 20 atm; (c) 10 atm and (d) 1 atm for Ar þ CH4 (98:2).
Fig. 10. Typical spectra for 241 Am at (a) 10 atm; (b) 7:5 atm; (c) 2:5 atm and (d) 1 atm for Xe þ CH4 (98:2).
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Fig. 11. Spectra for 241 Am and 133 Ba obtained for Xe þ CH4 (98:2) at 1 atm: Various identified lines including the Ka fluorescent lines for Xe are marked in the figure.
compared to Xe þ CH4 at a given applied voltage and pressure is seen from Figs. 1c and (e). The probability of an increase in the number of primary electrons due to the Penning effect can be gauged by the energy level diagram of Fig. 8.
Matching of the ionization potentials of various gas mixtures therefore, depends upon the emission spectra which is a function of the population density in various Landau levels achieved during excitation.
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3.3. Spectral behaviour During the present measurements, spectra of our calibration sources were obtained using a variety of X-ray beam widths in order to study both the spatial resolution across the anode wire and the effect of pressure on the spectral shape. In
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Figs. 9 and 10 we present a comparison of the observed average spectra at 1–30 atm for Ar þ CH4 (98:2), and 1–10 atm for Xe þ CH4 (98:2). These spectra were obtained with the collimated 241 Am source located along the vertical axis of the detector, in line with the anode. The pulse height distributions were translated from channel number
Fig. 12. Observed spectra, Gaussian fits and residuals for Ar þ CH4 (98:2) at 10 atm obtained with an
241
Am source.
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to photon energy in order to identify the expected features resulting from the 241 Am source. It is clearly seen from these figures that in the case of argon, an increase in gas pressure does not lead to any major changes in the output spectra except for an enhanced detection efficiency. However, in the case of xenon, the shape of the photo peak is very asymmetric with a pronounced left wing. The good quality of the gas mixture used
and its intrinsic energy resolution is demonstrated in Fig. 11. These data indicate that the peculiar pulse height distribution seen for Xe-based mixtures at higher pressures cannot, therefore, be ascribed to non-standard gas properties. In order to decode the underlying mechanism that is responsible for the asymmetric shapes at higher pressures, we adopted a multiple-fit shape analysis of the spectra. Assuming a Gaussian
Fig. 13. Observed spectra, Guassian fits and residuals for Ar þ CH4 (98:2) at 30 atm obtained with an
241
Am source.
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shape of the intrinsic resolution of the detector we superposed a set of lines to reproduce the observed behaviour. We then proceeded to identify the various line energies and to establish how these could be generated within the gas volume. Figs. 12 and 13 show the results of the fitting analysis for Ar þ CH4 (98:2) at 10 and 30 atm; respectively. The residuals in the figures represent the goodness of fit. It is seen from the figures that the main
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observed lines at 18, 21.5, 27 and 59:5 keV correspond to the Np triplet between 11–21 keV and X-rays at 26 and 59:5 keV from 241 Am: The 14 keV (Np, La ) line is not clearly seen due to the attenuation in the detector body at this energy. The results of a similar analysis for Xe þ CH4 (98:2) at 1 atm are shown in Fig. 14. The analysis is clearly consistent with the Gaussian profiles expected from the features shown in Fig. 11a, with
Fig. 14. Observed spectra, Gaussian fits and residuals for Xe þ CH4 (98:2) at 1 atm obtained with an
241
Am source.
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Fig. 15. Observed spectra, Gaussian fits and residuals for Xe þ CH4 (98:2) at 10 atm obtained with an
a non-Gaussian contribution from Compton backscatter at B50 keV: However, at higher pressures, this type of fitting requires large contributions from Gaussian features at line energies that are difficult to generate within the detector volume. An example is shown in Fig. 15 for Xe þ CH4 (98:2) at 10 atm which exhibits a highly asymmetric photon peak. The origin of the asymmetry, which is also reflected in the escape peak in Fig. 15,
241
Am source.
is best explained by non-perfect charge collection at higher gas pressures for Xe-based mixtures [25]. X-ray photons that interact further away from the collecting anode produce ions that partially recombine during the drift to the anode, thus leading to a lower ‘‘effective’’ energy in the photopeak. The integration time of 2 ms is sufficient to eliminate the effects of partial response to charge collection for the summed
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signal arising from the absorption of the K-escape photon and the main initial interaction. However, it is expected that recombination in the primary electron cloud during the drift process, will alter the gain characteristics of a counter with a given geometry and a 3-d scattering model is necessary to study the charge propagation and lateral diffusion in the very low reduced field region [10]. These effects will be further enhanced at higher pressure, and will be discussed in a future paper.
Acknowledgements It is a pleasure to thank John Panettieri for his valuable technical help in the fabrication and assembly of the counters, and Dr. Vernon Edge for his advice on the chemical aspects of the project.
References [1] E. Costa, G. Auriemma, L. Boccacini, A. Emannuale, R.K. Manchanda, M. Masterpietro, G. Medici, P. Ubertini, Nucl. Instr. and Meth. 156 (1978) 57. [2] A.R. Rao, P.C. Agrawal, R.K. Manchanda, Proceedings of the 18th ICRC, Bangalore, India, Vol. 8, 1983; p. 5. [3] A. Bazzano, L. Boccaccini, C.D. La Padula, M. Mastropietro, R. Patriarca, V.F. Polcaro, P. Ubertini, R.K. Manchanda, Nucl. Instr. and Meth. 214 (1983) 48. [4] R.K. Manchanda, Indian J. Radio Space Phys. 20 (1991) 252. [5] R.K. Sood, R.K. Manchanda, Z. Ye, Adv. Space Res. 11 (8) (1991) 421. [6] R.K. Sood, J. Panettiery, D.J. Grey, G. Wood, J. Hoffmann, R.K. Manchanda, R. Staubert, E. Kendiziorra, G.K. Rochester, Publ. Astron. Soc. Aust. 13 (1996) 156. [7] H. Date, K. Kondo, M. Shimozuma, H. Tagashira, Nucl. Instr. and Meth. A 451 (2000) 588. [8] H. Date, Y. Ishimura, M. Shimozuma, Nucl. Instr. and Meth. B 207 (2003) 373. [9] D. Mazed, M. Baaliouamer, Nucl. Instr. and Meth. A 437 (1999) 381.
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[10] H. Pruchova, B. Franek, Nucl. Instr. and Meth. A 366 (1995) 385. [11] N. Takeda, K. Kudo, T. Sugita, G. Dietze, X. Yang, Appl. Radiat. Isot. 53 (2000) 893. [12] B.D. Ramsey, P.C. Agrawal, Nucl. Instr. and Meth. A 273 (1988) 326. [13] B.D. Ramsey, P.C. Agrawal, Nucl. Instr. and Meth. A 278 (1989) 576. [14] P. Laporte, V. Saile, R. Reininger, et al., Phys. Rev. A 28 (1983) 3613. [15] R.K. Manchanda, Z. Ye, R.K. Sood, Nucl. Instr. and Meth. A 292 (1990) 373. [16] R.K. Sood, Z. Ye, R.K. Manchanda, Nucl. Instr. and Meth. A 344 (1994) 384. [17] Z. Ye, R.K. Sood, D.P. Sharma, R.K. Manchanda, K.B. Fenton, Nucl. Instr. and Meth. A 329 (1993) 140. [18] V. Radeka, Annu. Rev. Nucl. Part. Sci. 38 (1988) 217. [19] D. Grey, Ph. D. Thesis, University College, University of NSW, ADFA, 1999. [20] P.C. Agrawal, B.D. Ramsey, M.C. Weisskopf, Nucl. Instr. and Meth. A 277 (1989) 557. [21] R.Z. Fuzesy, J. Jaros, L. Kaufman, J. Marriner, S. Parker, V. Perez-Mendez, S. Redner, Nucl. Instr. and Meth. 100 (1972) 267. [22] A. Bolotnikov, B. Ramsey, Nucl. Instr. and Meth. A 428 (1999) 391. [23] A. Kundu, E.J. Morton, Nucl. Instr. and Meth. A 422 (1999) 286. [24] M.W. Charles, J. Phys. E. 5 (1972) 95. [25] H. Sakurai, B.D. Ramsey, M. Weisskopf, Nucl. Instr. and Meth. A 307 (1991) 504. [26] P.C. Agrawal, B.D. Ramsey, Nucl. Instr. and Meth. A 277 (1988) 331. [27] I.K. Bronic, B. Grosswendt, Nucl. Instr. and Meth. B 117 (1996) 5. [28] P.J.B.M. Rachinhas, T.H.V.T. Dias, F.P. Sntos, A.D. Stauffer, F.P. Santos, C.A.N. Conde, IEEE Trans. Nucl. Sci. NS-43 (1996) 2399. [29] A.A. Kruithof, Physica 7 (1940) 519. [30] Y. Tanaka, A.S. Jusrsa, F.J. LeBlanc, Opt. Soc. Am. 48 (5) (1958) 304. [31] M. Suzuki, S. Kabata, Nucl. Instr. and Meth. 164 (1979) 197. [32] D.H. Wilkinson, Ionization Chambers and Counters, Cambridge University Press, Cambridge, 1950. [33] S.J. Buckman, P. Hammond, G.C. King, F.H. Read, J. Phys. B 16 (1983) 4219. [34] A.V. Zarubin, Nucl. Instr. and Meth. A 283 (1989) 409.
Nuclear Instruments and Methods in Physics Research A 527 (2004) 493–511
Resolution and spectral characteristics of ultra high pressure proportional counters using various quench gases D.J. Grey, R.K. Sood*, R.K. Manchanda1 School of Physics, University College, University of New South Wales, Australian Defence Force Academy, Northcott Drive, Canberra, ACT 2600, Australia Received 28 January 2000; received in revised form 28 January 2004; accepted 19 March 2004
Abstract We present the results of a systematic search for the best gas mixture for use in proportional counters operating at high pressures. The data were gathered using a variety of quench gases with argon or xenon as the primary fill gas. The gas gain and the energy resolution broadly follow the complex ionization within the gas volume. The data show that the spectral response for argon does not change significantly with pressure. However, for xenon the spectral response is dominated by secondary shell interaction processes, and charge loss due to recombination. r 2004 Elsevier B.V. All rights reserved. PACS: 07.85.F; 29.40.C; 07.87.+v Keywords: Proportional counters; Energy resolution; Gas gain; High-pressure gas
1. Introduction Proportional counters have traditionally been the preferred detectors for spectral and temporal studies in X-ray astronomy [1–6]. The available large area and the ease of construction make these detectors the most sensitive alternative for lowenergy X-ray observations. In addition, many techniques have evolved to reduce both the *Corresponding author. Tel.: +61-2-6268-8801; fax: +61-26268-8786. E-mail address: [email protected] (R.K. Sood). 1 Permanent address: Tata Institute of Fundamental Research, Mumbai 400 005, India.
internal background of these detectors and also the background generated by Compton scattering of the high-energy ambient photon flux. Xenon and argon are the most commonly used detection gases. A small percentage of quenching gas is, however, essential for the stable operation of a proportional counter. While there is no theoretical or empirical standard for the percentage and the type of quench gas, the choice of the quenching molecule and its concentration can have a significant effect on the operating conditions. The energy range of operation of proportional counters filled with gas at atmospheric pressure is limited to 2–10 keV for argon and to B80 keV for xenon. In addition a strong K-escape peak is
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.03.202
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generated in xenon as 87% of the K-shell interactions for incident photons with energy above 30 keV lead to fluorescent photons which escape the gas cell because of a low attenuation cross-section. This is mainly because the cell size in directions orthogonal to the anode wire is generally restricted to p15 mm to avoid electron– ion recombination during the charge collection process. The use of a fill gas at high pressures in a proportional counter can enhance the detection efficiency for incoming photons. It can also increase the attenuation of the K-escape photons. However, the increased probability of scattering of the incident photons in the gas volume with increase in pressure leads to complex spectra independent of the intrinsic energy resolution of the filling gas mixture. These problems are much more pronounced in proportional counters when compared to ionization chambers, because the secondary multiplication increases the DN=N fluctuations inherent in the primary cloud, where N is the number of electrons produced in the primary photon interaction. N depends not only on the ionization potential of the main fill gas but also on the type of quenching gas. The number of primary electrons can be increased through the secondary ionization of the quenching gas via collision with the long lived metastable states of the excited noble gas and absorption of the UV photons emitted during the de-excitation of the host gas molecules. Therefore, the energy resolution of a proportional counter is a function of both the gas pressure and the type and amount of quench gas. The electron collison processes in gaseous xenon, the resultant ionization yield, and the complexity of the electron kinematics are still not fully understood [7–9]. Monte Carlo simulations of the gamma ray response of low-pressure proportional counters has previously been attempted in order to understand the pulse height spectra from these counters [10,11]. In most of the earlier experiments in X-ray astronomy, methane ðCH4 Þ and carbon-dioxide ðCO2 Þ were the most commonly used quenching additives for both argon and xenon filled proportional counters. Laboratory investigations have, however, been conducted with various polyatomic
quench gas mixtures to search for the optimum resonance Penning mixtures so as to achieve the best possible energy resolution [12,13]. A large majority of these investigations have been confined to test counters operating at low pressures up to about 1 atm: Since many of the gas properties and the molecular band structure appear to change when the pressure is increased [14], a systematic study of different operating conditions is absolutely essential to optimize the future use of these detectors. In this paper, we present detailed investigations of gas gain curves, energy resolution, and the spectral shape of the output of a proportional counter of cylindrical geometry, using different quench gases at various pressures. We compare the results with earlier work and interpret them in terms of the physical mechanism of the energy loss processes in the gas. This work is a continuation of our earlier published work on the characteristics of ultra high pressures proportional counters filled with argon+methane and xenon+methane mixtures [15–17].
2. Experimental arrangement The experimental set up used in the present investigations is similar to the one used in our earlier work. The tests were performed using a conventional cylindrical aluminium counter 30 cm long, 30 mm inside diameter, and 3 mm wall thickness. A 25 mm gold plated tungsten wire was used for the anode. The mechanical structure was hydro-statically tested to 60 atm: The experiments were conducted at filling pressures up to a maximum of 30 atm; and depending upon the nature of the gas mixture, an anode voltage of up to 7:5 kV was employed. A LeCroy TRA100 charge sensitive pre-amplifier was used at the front end, and an Ortec 450 unit was used for further amplification of the incoming signal. Spectra were gathered at various X-ray line energies. The X-ray photons were obtained from collimated radioactive sources namely, 241 Am; 133 Ba and 57 Co: Apart from the principal Ka and Kb line energies of these sources, the escape peak of xenon and other low level gamma transitions in the 20– 500 keV range provided a large dynamic range for
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test and calibration purposes. For each source spectrum recorded, a background spectrum was recorded at the same gain setting and over the same time period. The background was then subtracted from the raw source spectrum. The amplifier electronics noise, which contributes to the final spectral characteristics, was measured as 0:44 fC equivalent RMS. The differentiating and integrating time constants for the pulse shaping electronics were set at 0.5 and 2 ms; respectively [18]. In order to attain optimum counter performance, a rigorous cleaning and purification scheme was used during the entire process of tube assembly. The counter was evacuated and baked at 80–100 C for at least 72 h until a stable vacuum level of B1 107 Torr was achieved. A newly assembled ultra high purity gas purification and filling system was used during these laboratory tests. This system is completely of stainless steel construction and employs a high temperature copper catalyst filter, an Oxisorb filter, and a molecular sieve for the gas purification process. It is designed to reduce the concentration of electronegative impurities ðO2 ; H2 O; CO2 ; COÞ to a level of less than 5 ppb [19].
3. Results and discussion A large variety of argon and xenon based mixtures were tested during the present investigations. We also tested the Penning mixture of argon+xenon, and mixtures of argon and/or xenon with methane ðCH4 Þ; ethylene ðC2 H4 Þ and iso-butane ðiC4 H10 Þ as quenching gases. These tests were motivated by the possibility of the use of such mixtures in detectors for future X-ray astronomy missions. The various results presented in the following sections are based on data obtained for Ar þ CH4 (98:2), Ar þ C2 H4 (98:2), Ar þ iC4 H10 (98:2), Ar þ Xe (90:10), Ar þ Xe (70:30), Xe þ C2 H4 (98:2), Xe þ C2 H4 (90:10), Xe þ iC4 H10 (98:2), and Xe þ Ar þ CH4 (90:8:2). 3.1. Gas gain behaviour Gas gain studies were carried out using the method of current determination in a Faraday
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cage. A 0:116 mCi241 Am source was used with a 5 mm collimator. With such a wide collimator, the gas gain curves presented in this section correspond to the behaviour of the counter averaged over the length of the anode wire. Some of the representative data are shown in Figs. 1(a)–(g). The curves show the gas gain M versus applied voltage V ; for different gas mixtures over a range of pressures. It is seen from the figure that large values of gas gain are possible for some of these mixture but that the incremental gain change with voltage depends strongly on the operating pressure. The values of the optimum operating voltage for a given pressure and the percentage of quench gas are taken as those which yield the best energy resolution. The horizontal dashed lines in Fig. 1 give the upper and lower limits of the gas gain values that correspond to optimum energy resolution at 60 keV for a given mixture. The data in the figure show that xenon based mixtures containing isobutane and ethylene quenching additives, used in the proportions shown, exhibit the lowest % average optimum gain of Mp5507185; while xenon+methane give a mid value of % Mp7507150: Argon based mixtures exhibit the highest optimum gain with the Penning mixture of Ar þ Xe (90:10) giving the highest average value of % Mp9157185: As we will show in the next section the horizontal dashed lines in the figure also imply that the mean value of gain necessary for optimum resolution for a given mixture is independent of the gas pressure. The data presented in Figs. 1(a)– (g) compare well with our earlier data and with those of other workers as reported in the literature [15,16]. For comparison we have shown the data of Agrawal et al. [20] for Ar þ Xe (90:10) and (70:30) in Figs. 1(a) and (b), and those of Ye et al. [17] and Sood et al. [16] in Fig. 1(c). It is also seen from Fig. 1 that the M–V curves show the expected exponential behaviour when considered individually. However, the gradient changes continuously with change in pressure, a fact alluded to in our earlier papers. The large variety of the incremental change in the gradient with pressure, seen in the plots, suggests that no single parameter can be fitted to describe this behaviour. To a first approximation, variations of the gradient with pressure p and of the intercept on
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Fig. 1. Gas gain versus applied voltage at different gas pressures: (a) Ar þ Xe (90:10), (b) Ar þ Xe (70:30), (c) Xe þ CH4 (98:2), (d) Xe þ Ar þ CH4 (90:8:2), (e) Xe þ C2 H4 (98:2), (f) Xe þ C2 H4 (90:10) and (g) Xe þ iC4 H10 (98:2). The data of Agrawal et al. [20] at 1 atm are shown in (a) and (b) by +. Those of Ye et al. [17] and Sood et al. [16] are shown in (c) by symbols that are not underscored.
the Y -axis may be represented by optimum fits of the type mlnðMÞ ¼ apb and BlnðMÞ ¼ cpd : The derived values of the coefficients a; b; c and d; along with values of the regression coefficient R2
and the chi-test parameter w2 are listed in Table 1 for the different gas mixtures investigated. The variations of the gas gain gradient and the corresponding intercept on the Y -axis are shown
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Table 1 Curve fitting coefficients for the gas-gain gradient m; and intercept B for the curves shown in Fig. 1 Gradient mlnðMÞ
Intercept—BlnðMÞ
Gas mixture
a
b
R2
w2
c
d
R2
w2
Ar þ Xe (90:10) Ar þ Xe (70:30) Ar þ CH4 (98:2) Xe þ CH4 (98:2) Xe þ C2 H4 (98:2) Xe þ C2 H4 (90:10) Xe þ iC4 H10 (98:2) Xe þ Ar þ CH4 (90:8:2)
0.0112 0.0113 0.0080 0.0074 0.0075 0.0069 0.0083 0.0077
0.308 0.384 0.302 0.311 0.416 0.295 0.358 0.319
0.985 0.998 0.981 0.999 0.862 1 0.999 0.998
1 1 1 1 1 0.99 1 1
3.407 4.860 2.348 4.796 3.684 3.547 3.585 3.768
0.302 0.210 0.398 0.250 0.392 0.461 0.408 0.374
0.862 0.992 0.961 0.991 0.993 0.99 0.999 0.999
0.99 1 0.99 0.99 0.99 0.46 0.99 0.99
Optimum parameters were achieved by fitting the gas gain curves with the generalized functional form lnðMÞ ¼ apb V þ cpd : Xe þ C2 H4 (90:10) coefficient values are based only on the first 3 data sets shown in Fig. 1(f).
in Fig. 2. Two points may be noted from this figure. Firstly, the shape of the curves suggests an exponential dependence of the gradient of the gas gain curves on pressure. Secondly, independent of the type of poly-atomic quench gas used, argon and xenon show identical behaviour thereby suggesting a common basic physical process in operation. The Penning mixture of Ar þ Xe (90:10) follows a similar behaviour except for scaling due to increased gain. However, at high pressures, the 70:30 mixture of Ar þ Xe approaches the poly-atomic limit thereby indicating a reduction in the Penning effect at increased pressure. This behaviour is contrary to what is expected. The non-metastable Penning effect in Ar þ Xe occurs due to the collisional de-excitation by Xe atoms of short-lived Ar atoms above the metastable state. Thus an increased concentration of Xe should lead to an enhanced Penning effect. However, it has been reported by Fuzesy et al. [21], that even at lower pressures, an increase in Xe concentration beyond 5–8% leads to a reduction of the Penning effect. We believe that this additional quenching of the signal may be caused by increased fluctuations in the number of clusters of xenon ions that are believed to form at high pressures. The initial charge cloud in the gas counter is formed by the photo-absorption of the incident photon, followed by a combination of electron ionization and photo-ionization processes. The gas amplification in a cylindrical proportional counter
takes place R r close to the anode wire and is given by lnðMÞ ¼ rab a dr; where a is the first Townsend coefficient, and ra and rb are the anode and cathode radii, respectively. For a given pressure and for moderate gas gain M and anode voltage V ; the gas gain can be approximated by M ¼ K expðCo V Þ; where Co is the capacitance of the detector and K is a constant. The pressure dependence of this gain equation is complex and is built in three ways. First, the Townsend coefficient a which controls the ionization in the multiplication region is itself defined as a ¼ ApeBp=EðrÞ ; where A and B are constants, and EðrÞ is the radial electric field. Second, in terms of basic parameters the detector capacitance C per unit length is defined as C ¼ 2peo eg ðp; TÞ=lnðrb =ra Þ where the dielectric constant eg for the gas is a function of both pressure p (atm) and temperature T ð CÞ; and is given as ðeg 1Þp;T ¼ ðpðeg 1Þ1 atm;20 C =1 þ 0:003411ðT 20ÞÞ: Third, as described by Sood et al. [16], the number of primary electrons reaching the multiplication zone changes with pressure, and is given by NðrÞ ¼ % krp=V ; where N% is the mean number of primary Ne ions created. This change arises from the fact that in between the proportional and drift regions, one can envisage an ionization region within the gas volume, where the electric field is such that an electron attains enough energy between two collisions to excite the gas atoms rather than to ionize them [8], thereby reducing the effective distance from the anode at which the multiplication
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Fig. 2. Variation of (a) gas gain gradient mlnðMÞ and (b) the intercept—BlnðMÞ : The curves for Ar þ CH4 are derived from Sood et al. [16].
begins. The increased number of such collisions of the primary electrons will not only give rise to larger variations in the electron statistics but will also lead to a lower overall gain. All these factors therefore lead to a weak exponential dependence of the gas gain on pressure, as seen in Fig. 2(a). Bolotnikov and Ramsey [22] have recently discussed a mechanism for electron–ion recombination in high pressure xenon counters that is dependent on the electric field. The implications
of this model will be addressed in a forthcoming paper, where we discuss the impact of drift time of electrons (up to 1 ms), on the ultimate resolution. Numerical simulations for an argon–methane mixture [23] also suggest a lower value of the first Townsend coefficient a in the region of a lower reduced field. A comparison of the values of the constants a; b; c and d; given in Table 1, does not show any underlying common features. We have also tested
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the applicability of various theoretical gas multiplication models, as described by Ye et al. [17], to our data. The curves representing the relationship between a=p and the reduced electric field lnðMÞ=pra Sa ; where Sa is the reduced electric field at the anode, best fit the functional form of the model of Charles [24]. The a=p versus E=p curves are shown in Fig. 3(a) for various gas mixtures investigated in the present work, while a compar-
499
ison with the earlier data from different authors [17,25,26] is shown in Fig. 3(b). It is seen that the curves are highly non-linear and any linear mathematical representation can only be written for small segments of each curve. The nonlinearity arises mainly due to the cooling effect inherent in electrons in inelastic scattering leading to excitation [9]. This process in turn also affects the relative variance of single electron spectra [27].
Fig. 3. Curves representing a=p vesus E=p for the Charles model applied to all gas mixtures used in the present tests are shown in (a). A comparison with the data of Kruithof [29], Sakurai et al. [25] and Ye et al. [17] is shown in (b).
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3.2. Energy resolution The energy resolution of a proportional counter is the key parameter which determines its suitability for astronomical observations. During our tests we collected a large number of spectra for different input energies and at various gas pressures for each of the trial gas mixtures. Gaussian curves were fitted to the data for accurate determination of the mean energy and the corresponding FWHM of each photo-peak. Xray spectra were collected at various pressures up to 30 atm for Ar-based mixtures and up to 10 atm for Xe-based mixtures. A bench mark energy resolution of 9.5% at 59:5 keV was obtained for an Ar þ CH4 (98:2) mixture at 30 atm: This compares well with the value of 12.5% reported by Manchanda et al. [15] that was obtained for a gas gain of B200 and at 27 atm for the same mixture. Results that we obtained at low pressures also compare very well with those obtained by Agrawal and Ramsey [26]. Results obtained at high pressures using other quenching gases failed to achieve the same good energy resolution as achieved with CH4 ; while at low pressures of up to 2 atm; the resolution obtained with iC4 H10 was found to be the best at 4.9% at 59:5 keV: The Penning mixtures of Ar þ Xe at 70:30 and 90:10 ratio failed to improve the energy resolution at high pressures. In the case of Xe-based mixtures with polyatomic quenching gases, the overall energy resolution was found to be slightly worse compared to their Ar-based counterparts. Furthermore, as reported by Ye et al. [17], Xe þ CH4 (98:2) gave the best resolution of 6.5% at 1 atm and 11.9% at 10 atm pressure, respectively. These numbers compare well with the value of 7.3% obtained by Sakurai et al. [25] using a 25 mm anode wire. Using Ar þ CH4 (98:2) and Xe þ CH4 (98:2) mixtures it has been shown by Manchanda et al. [15] and Sood et al. [16] that at low pressures up to 3:5 atm; the energy resolution follows a simple functional form R ¼ 0:46E 1=2 and at higher pressures the functional form changes to DE ¼ aE þ bE 1=2 : In order to look for any unified function dependence and systematic effects between different gas combinations, we studied Xe-
Fig. 4. Energy resolution for 59:5 keV X-rays from 241 Am as a function of gas pressure for (a) Ar þ CH4 (98:2), (b) Ar þ C2 H4 (98:2), (c) Ar þ iC4 H10 ; (98:2), (d) Ar þ Xe (90:10), (e) Ar þ Xe (70:30), (f) Xe þ CH4 (98:2), (g) Xe þ Ar þ CH4 (90:8:2), (h) Xe þ C2 H4 (98:2), (i) Xe þ C2 H4 (90:10) and (j) Xe þ iC4 H10 (98:2).
Table 2 Curve fitting parameters for the curves of energy resolution versus incident photon energy shown in Fig. 4 Gas mixture Xe þ CH4 (98:2)
Pressure a
1 5 7.5 10 Xe þ Ar þ CH4 (90:8:2) 1 5 10 15 Xe þ C2 H4 (98:2) 1 5 Xe þ iC2 H10 (98:2) 1 5
0.0044 0.0186 0.0207 0.0479 0.0014 0.0163 0.0577 0.0242 0.122 0.103 0.0804 0.0197
b
R2
w2
0.5523 1.0013 0.9522 1.3383 0.5584 0.6976 0.7069 0.9475 1.2193 1.6568 1.0734 1.0031
0.93 0.97 0.88 0.88 0.93 0.92 0.47 0.57 0.80 — 0.85 —
0.91 0.88 0.93 0.81 0.95 0.78 0.81 0.68 0.76 — 0.14 —
The data are fitted with the functional form R ¼ aE þ bE 1=2 where E is in keV.
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based and Ar-based mixtures at different pressures, and over a range of X-ray energies. The pressure dependence of the energy resolution was investigated using primarily the 59:5 keV X-ray photo-peak from 241 Am: Energy resolution curves are shown in Fig. 4, where curves (a)–(e) refer to Ar-based mixtures, while curves (f)–(j)
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refer to Xe-based mixtures. We have fitted the curves for all the gas mixtures with a universal functional form as mentioned above. We present in Table 2 the fitting parameters for curves for Xebased mixtures. It is seen from the table that apart from variations in the values of a and b for a given mixture at different pressures, there is no
Fig. 5. Energy resolution for 59:5 keV X-rays versus gas gain for different pressures for (a) Ar þ Xe (90:10), (b) Ar þ Xe (70:30), (c) Xe þ iC4 H10 (98:2), (d) Xe þ C2 H4 (98:2), (e) Xe þ Ar þ CH4 (90:8:2) and (f) Xe þ CH4 (98:2).
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discernible pattern even though Xe was the primary fill gas in all these cases. The best measured resolution figures of 4.5% for Ar þ CH4 (98:2) and 6.5% for Xe þ CH4 (98:2) at 1 atm pressure compare very well with earlier measurements [15,16,20]. It is seen in Fig. 4 that the family of curves form two separate groups corresponding to Ar-based and Xe-based mixtures, with Ar-based mixtures exhibiting superior overall resolution. The effect of the quenching gases, in the proportions used in this experiment, on the overall resolution of the mixtures may be ranked as (best) CH4 oC2 H4 oiC4 H10 (worst). The rate of deterioration of the energy resolution with increased pressures is also a function of the primary gas, while the effect of the quenching gas is similar in the two groups. These changes must, therefore, arise from variations in the energy loss process, the formation and size of the primary electron cloud, the electron transport process, and the effective volume of the secondary ionization process, rather than simple variations in the secondary multiplication process near the anode. In Fig. 5, we have plotted the energy resolution that we obtained at 59:5 keV as a function of gas gain for different gas mixtures at various pressures. The almost parabolic behaviour of all the curves shown in the figure is quite striking. By virtue of the logarithmic scale used for the gas gain, the curves appear to be roughly symmetrical around the resolution minima. However, plots on a linear scale indicate a very sharp deterioration of the resolution at lower gas gain values at lower gas pressures, while a sharp decline in resolution is also seen at high pressures. The dashed vertical line in each of the graphs represents the gas gain value corresponding to the best measured energy resolution for a given mixture. Note that the dashed lines shown in Fig. 1 correspond to gas gain values relevant to this value for optimum resolution. It is also seen from Fig. 5 that although a general range of values for the best operating gain is between 500 and 1000 for pressures right up to 30 atm; much narrower limits apply for each gas mixture at a given pressure. The dependence of the energy resolution on gas gain for each gas mixture is clearly seen in Fig. 5. It is theoretically expected that the resolution should
continue to improve with decreasing gain, ultimately resulting in Fano-fluctuation limited values at the gain of one. However, additional factors that contribute to the observed deterioration of the energy resolution at low gains include the size and orientation of the primary ion cloud, space charge effects, and the inherent electronics systems noise. Such effects have been observed and reported by several groups [16]. Fig. 6 shows the best measured resolution as a function of X-ray energy for a number of Xebased gas mixtures at various gas pressures. Classically, the energy resolution in a given gas, % where P represents the pulse RE ¼ DE=E dP=P; height for photon energy E; was believed to arise from the fluctuations in the average number of % 2 and in the secondary primary electrons ðdN=NÞ 2 % multiplication ðdM=MÞ : However, the observed change in energy resolution with pressure as well as with the fraction of quench gas used cannot be
Fig. 6. A comparison of the energy resolution versus incident photon energy for (a) Xe þ CH4 (98:2) and (b) Xe þ Ar þ CH4 (90:8:2) at different pressures.
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explained by this simple model [15,16]. Sakurai et al. [25] have also noted that even though the variation in resolution with pressure between pure xenon and a xenon–methane mixture may be explained by a reduced charge cloud model based on changes in the diffusion constant, the model fails when the data are compared for mixtures with varying amounts of quenching gas. To explain the deviation of DE versus E from the canonical formula, Sood et al. [16] proposed that the
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variance in the observed pulse height at a given energy E may be written as 2 2 dP dN 1 dM 2 1 dL 2 ¼ þ þ ð1Þ % P% N% N% M N% L% where N% is the mean number of primary ion pairs produced by a photon of energy E and mean track % M % is the mean amplification factor and length L; dP; dN; dM and dL are the standard deviation of the relevant quantities. This equation may be
Fig. 7. Illustration of UV emission spectra for noble gases Ar, Kr and Xe (a) [30], UV emission spectra for the rare gas scintillation process (b) [31], along with the UV absorption spectra of CH4 ; CO2 and C2 H4 (c) [32].
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simplified to give 2 2 2 dP dN 1 dM ¼ þ % P% N% N% Zðp; EÞM which is analogous to function given ðf =Zðp; EÞw=E 1=2 ; where is the average ionization
the familiar resolution as RE ¼ 236½ðF þ F is the Fano factor, w % 2; potential, f ¼ ðdM=MÞ
and Z is energy dependent in a way such that Z ¼ 1 for energies above 100 keV; and ZBE 1=2 for energies below 100 keV: The deviation of the dependence of energy resolution on energy that we observe in our data, from the ideal functional form RE pE 1=2 ; can be seen in Fig. 6. Clearly, the energy resolution even at a fixed energy, shows a significant pressure
Fig. 8. Energy level diagram for various test gases used in the present studies (Ar, Xe, CH4 ; C2 H4 ; and iC4 H10 ), showing ionization potentials IP, average ionization potentials w; with dominant excitation states Ex ; metastable states M; and dissociation energies D [33,34].
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dependence. In our view the deterioration of the energy resolution must partially arise from a pressure dependence of the parameter Z described above. We note from Figs. 4 and 5, and Figs. 9 and 10 (discussed later) that Ar- and Xe-based mixtures behave differently. While Xe-based mixtures remain more sensitive to ion-recombination problems in the low reduced-field drift regions inherent in high-pressure counters [28], an underlying general dependence on pressure nevertheless remains. We therefore propose that the pressure dependence may be described as a function of a filling factor Q; where Q is a function of the atomic number of the primary gas and the filling pressure. Using the resolution measurements shown in Fig. 4, an empirical relation is derived as Q ¼ ðZgas =ZAr Þ0:5 ðp=1ðatmÞÞ0:2 ; where p is in atm and Zgas and ZAr define the atomic numbers of the host gas and argon, respectively. The generalized functional form for the variation of resolution with pressure and the type of filling gas can then be written as Rp;g ¼ Q Rg :
The data plotted in Fig. 4 also suggest a variation of the energy resolution with the type of quenching gas used. Functionally, a quenching gas absorbs the UV photons emitted during the deexcitation of the gas molecules produced in elastic collisions, relaxes the metastable molecular states via the Penning effect, and improves the electron and ion mobility in the detection gas. The variation in the behaviour of the primary gas with different quenching agents can, therefore, be understood in terms of the differences in the UV emission spectra of the primary gases, the absorption spectra of the quenching additives, and the ionization potentials of the mixture gases. In Figs. 7 and 8, we show the emission spectra and the energy level diagrams for the gases under investigation. It is clearly seen from these figures that while CH4 works very well as a quenching agent for Ar and Kr, it is not the ideal gas for use with Xe. The absorption spectra of C2 H4 appears to be better matched to the emission spectra of Xe. The increase in the gas gain for Xe þ C2 H4 as
Fig. 9. Typical spectra for 241 Am at (a) 30 atm; (b) 20 atm; (c) 10 atm and (d) 1 atm for Ar þ CH4 (98:2).
Fig. 10. Typical spectra for 241 Am at (a) 10 atm; (b) 7:5 atm; (c) 2:5 atm and (d) 1 atm for Xe þ CH4 (98:2).
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Fig. 11. Spectra for 241 Am and 133 Ba obtained for Xe þ CH4 (98:2) at 1 atm: Various identified lines including the Ka fluorescent lines for Xe are marked in the figure.
compared to Xe þ CH4 at a given applied voltage and pressure is seen from Figs. 1c and (e). The probability of an increase in the number of primary electrons due to the Penning effect can be gauged by the energy level diagram of Fig. 8.
Matching of the ionization potentials of various gas mixtures therefore, depends upon the emission spectra which is a function of the population density in various Landau levels achieved during excitation.
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3.3. Spectral behaviour During the present measurements, spectra of our calibration sources were obtained using a variety of X-ray beam widths in order to study both the spatial resolution across the anode wire and the effect of pressure on the spectral shape. In
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Figs. 9 and 10 we present a comparison of the observed average spectra at 1–30 atm for Ar þ CH4 (98:2), and 1–10 atm for Xe þ CH4 (98:2). These spectra were obtained with the collimated 241 Am source located along the vertical axis of the detector, in line with the anode. The pulse height distributions were translated from channel number
Fig. 12. Observed spectra, Gaussian fits and residuals for Ar þ CH4 (98:2) at 10 atm obtained with an
241
Am source.
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to photon energy in order to identify the expected features resulting from the 241 Am source. It is clearly seen from these figures that in the case of argon, an increase in gas pressure does not lead to any major changes in the output spectra except for an enhanced detection efficiency. However, in the case of xenon, the shape of the photo peak is very asymmetric with a pronounced left wing. The good quality of the gas mixture used
and its intrinsic energy resolution is demonstrated in Fig. 11. These data indicate that the peculiar pulse height distribution seen for Xe-based mixtures at higher pressures cannot, therefore, be ascribed to non-standard gas properties. In order to decode the underlying mechanism that is responsible for the asymmetric shapes at higher pressures, we adopted a multiple-fit shape analysis of the spectra. Assuming a Gaussian
Fig. 13. Observed spectra, Guassian fits and residuals for Ar þ CH4 (98:2) at 30 atm obtained with an
241
Am source.
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shape of the intrinsic resolution of the detector we superposed a set of lines to reproduce the observed behaviour. We then proceeded to identify the various line energies and to establish how these could be generated within the gas volume. Figs. 12 and 13 show the results of the fitting analysis for Ar þ CH4 (98:2) at 10 and 30 atm; respectively. The residuals in the figures represent the goodness of fit. It is seen from the figures that the main
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observed lines at 18, 21.5, 27 and 59:5 keV correspond to the Np triplet between 11–21 keV and X-rays at 26 and 59:5 keV from 241 Am: The 14 keV (Np, La ) line is not clearly seen due to the attenuation in the detector body at this energy. The results of a similar analysis for Xe þ CH4 (98:2) at 1 atm are shown in Fig. 14. The analysis is clearly consistent with the Gaussian profiles expected from the features shown in Fig. 11a, with
Fig. 14. Observed spectra, Gaussian fits and residuals for Xe þ CH4 (98:2) at 1 atm obtained with an
241
Am source.
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Fig. 15. Observed spectra, Gaussian fits and residuals for Xe þ CH4 (98:2) at 10 atm obtained with an
a non-Gaussian contribution from Compton backscatter at B50 keV: However, at higher pressures, this type of fitting requires large contributions from Gaussian features at line energies that are difficult to generate within the detector volume. An example is shown in Fig. 15 for Xe þ CH4 (98:2) at 10 atm which exhibits a highly asymmetric photon peak. The origin of the asymmetry, which is also reflected in the escape peak in Fig. 15,
241
Am source.
is best explained by non-perfect charge collection at higher gas pressures for Xe-based mixtures [25]. X-ray photons that interact further away from the collecting anode produce ions that partially recombine during the drift to the anode, thus leading to a lower ‘‘effective’’ energy in the photopeak. The integration time of 2 ms is sufficient to eliminate the effects of partial response to charge collection for the summed
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signal arising from the absorption of the K-escape photon and the main initial interaction. However, it is expected that recombination in the primary electron cloud during the drift process, will alter the gain characteristics of a counter with a given geometry and a 3-d scattering model is necessary to study the charge propagation and lateral diffusion in the very low reduced field region [10]. These effects will be further enhanced at higher pressure, and will be discussed in a future paper.
Acknowledgements It is a pleasure to thank John Panettieri for his valuable technical help in the fabrication and assembly of the counters, and Dr. Vernon Edge for his advice on the chemical aspects of the project.
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