29
ionization yield formation
in argon-isobutane mixtures as measured by
a proportional-counter method
Ines KRAJCAR BRONIC1, Bernd GROSSWENDT2
1 Rudjer Boskovic Institute, P.O.Box 1016, 10001 Zagreb, CROATIA
2Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, GERMANY
C-768
revised version
address for correspondence (since Feb. 1, 1996):
Dr. Ines Krajcar Bronic
Abt. 6.11
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 BRAUNSCHWEIG
GERMANY
phone: +49 531 592 6252
fax: +49 531 592 6015
e-mail:
or:
29
ionization yield formation
in argon-isobutane mixtures as measured by
a proportional-counter method
Ines KRAJCAR BRONIC1, Bernd GROSSWENDT2
1 Rudjer Boskovic Institute, P.O.Box 1016, 10001 Zagreb, CROATIA
2Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, GERMANY
Abstract
By using the proportional-counter method, mean ionization yields produced by 5.9keV photons in argon-isobutane mixtures were measured as a function of the mixture composition, total gas pressure, and gas gains between about 103 and 2´104. It was found that the yield for a given mixture is gain dependent: an initial increase of the yield at low gas gains is followed by almost constant yield values for gains between about 2´103 and 8´103; at still higher gains the yield decreases. Moreover it was observed that the ionization yield at constant gas gain depends on the total pressure of the gas mixture. This dependence leads to a rather clear correlation between yields and isobutane partial pressure.
To explain the measurements a simple model of the mean energy W per ion-pair formed in Penning gas mixtures is also given. A direct comparison of calculated W values and measured data showed that the results of our proportional-counter experiments performed at high gas gains were strongly influenced by secondary effects induced during the formation of avalanches in the counter when using low isobutane partial pressures. Therefore, Penning mixtures with incomplete quenching should not be used as a counting gas in proportional counters if both high gas gain and good energy resolution are required, as for instance, in high-resolution low-energy (sub-keV) X-ray spectroscopy.
1. Introduction
The complete understanding of ionization yields produced by ionizing radiation in gas-filled detectors, such as ionization chambers and proportional counters, is of main importance for their application in radiation fields because, as far as the measurement of energy deposited is concerned, a conversion of ionization yields into absorbed energy is necessary. The conversion coefficient commonly used for this purpose is the so-called W value, which is defined as the mean energy required per ion pair formed upon the complete slowing-down of ionizing particles in matter and depends on the particle type, on its energy and is a characteristic quantity of the stopping gas.
If N(T0) is the mean number of ion pairs produced in a gas during the complete dissipation of the energy T0 of an ionizing particle, the corresponding W value is given by Eq.(1).
(1)
Because of its definition, the applicability of W must be carefully checked in practice with respect to the required complete slow-down of the primary particles and of all their secondaries. The fulfillment of this requirement depends on particle ranges, on the dimensions of the sensitive volume of a detector, and on the properties of the used stopping gas.
The applicability of W for converting ionization yields into absorbed energy is to be questioned, for instance, if UV photons could be produced during particle degradation because such photons, if not completely absorbed within the sensitive gas volume, could cause secondary electrons in its surroundings and therefore additional ion pairs not contained in the W value for the stopping gas. UV photons are usually produced in rare gases, and a certain amount of a polyatomic gas (quenching gas) should be added to the rare gas to quench (absorb) the UV photons. In this case the applicability of W may depend on the composition of a gas mixture and also on its total pressure. This fact is of particular importance from the practical point of view since a variety of different regular and irregular gas mixtures are commonly used in proportional counter experiments.
Regular gas mixtures are those mixtures in which the total number of ion pairs (Nreg) formed by an ionizing particle is a weighted sum of the number Ni of ion pairs produced in each gas component of type i. For binary gas mixtures Nreg is given by Eq.(2).
(2)
Here, z is the energy partition parameter. It is a function of the mixture composition and depends on the approach used to describe the energy partition between the gas components [1-5]. Several models for W value calculation in regular mixtures were recently summarized and analyzed [6]. According to the conclusions in Ref. [6] we will apply a model of Inokuti and Eggarter [5] which gives the following expression for z:
(3)
where si, Ci, and Wi, i=1,2, are the total ionization cross section, the concentration fraction, and the W value of the component i, respectively.
Irregular gas mixtures are those in which the energy spent on excitation of one gas component can be efficiently transferred to the other gas producing additional new ion pairs [1]. The total number of ion pairs is higher than it would be without the energy transfer, and consequently the W value is smaller. The models for regular mixtures cannot be applied any more because they do not take into account the additional ion pairs. The energy transfer is possible in a mixture of a rare gas with an admixing gas which may be another (heavier) rare gas (for He and Ne as the main gases), or, as in most cases, a molecular gas. The ionization potential of the admixed gas must be lower than the metastable state of the main rare gas. The effect of additional ionization of the admixture was first discovered in neon-argon mixtures by Penning [7,8] and is called the Penning effect. The effect of lower W values for certain concentration ratios in the mixtures due to the Penning effect is usually called the Jesse effect [9]. It should be noted that smaller W values were observed also in mixtures in which the admixture had an ionization potential above the metastable state, but still lower than a resonant state of the main rare gas [2,10]. In such a case we talk about the non-metastable Penning effect, as opposed to the formerly mentioned metastable Penning effect.
The lowest W value in a gas mixture with Penning effect is given in ref. [11] by
(4)
where Wreg=T0/Nreg is the W value for the regular mixture neglecting the Penning effect, and Nreg is defined in Eq.(2). N*/N is the ratio of the number of excited states and the number of ion pairs in the main rare gas. In Eq.(4) h is the ionization efficiency of the admixed molecules. Its value is 1 for a rare-gas admixture, and h1 for molecular gases which after absorption of energy higher than the ionization energy may either dissociate or autoionize [12].
It was found experimentally (by measuring ionization produced by alpha particles in ionization chambers) [9,10] that the W value has a minimum, given approximately by Eq.(4), for a certain concentration fraction of the admixture. In mixtures with the metastable Penning effect these concentration fractions are of the order of 0.1 - 1%, and the W value is lowered by up to 20%. In mixtures with the non-metastable Penning effect [2,10] the changes in W value (the Jesse effect) are much smaller (not more than 5%) and the required concentration fraction of the admixture is higher (3-5%).
The experiment by Parks et al. [13] showed for the first time that the Jesse effect depends on the total pressure of the mixture. Jarvinen and Sipila [14] observed a small increase of W value for several irregular mixtures with increasing total pressure and pointed out that this effect could be explained by a pressure dependence of the destruction of metastable rare gas states. Using the proportional-counter method, Krajcar-Bronic et al. [15] found that in argon - butane mixtures W value depends on the partial pressure of butane. Excepting these (fragmentary) experimental results, no systematic study of W value in Penning gas mixtures at various pressures and over a wide range of mixture compositions exists.
Motivated by this lack of data and by the practical importance of irregular gas mixtures for proportional-counter applications, it was the aim of the present work to study the pressure dependence of ionization yield formation in Penning gas mixtures in more detail. This was done in two parts.
In the first parts we developed a simple model with respect to the pressure dependence of W in argon-based Penning gas mixtures taking into account the most important energy-transfer interaction mechanisms which influence the ionization yield formation. This model can simply be applied also to other irregular mixtures consisting of rare gases and admixtures of molecular gases.
In the second part we measured the ionization yield produced by 5.89 keV photons within a proportional counter filled with argon-isobutane mixtures of different composition and total pressure. The reason of using such mixtures was the fact that the Penning effect in argon-isobutane mixtures is possible since the ionization potential of isobutane (10.67 eV) is lower than the excitation energies of metastable argon states (11.54 eV and 11.72 eV). The measurement should check the applicability of using the proportional-counter method for determining W values for soft X-rays in irregular gas mixtures and, vice-versa, to check the applicability of W values for converting measured ionization yields into energy deposited when using proportional counters. This part of the present paper was one aspect of a comprehensive investigation of various properties of argon - isobutane mixtures in a proportional counter, which included the gas gain determination, the study of the variance of the gas amplification (single electron spectra), and the energy resolution [16].
2. Model of W in irregular mixtures
The processes occurring in Ar - molecular gas mixtures after irradiation by electrons (or, in general, by any ionizing radiation) are the following:
Ar + e ® Ar+ + e + e / NAr / (5a)M + e ® M+ + e + e / NM / (5b)
Ar + e ® Ar* / N* / (5c)
Ar* + Ar + Ar ® Ar2* + Ar / km / (5d)
Ar* + M ® (Ar M)¢ / kQ / (5e)
(ArM)¢ ® Ar + M(fragments) / 1 - h / (5f)
(ArM)¢ ® Ar + M+ + e / h / (5g)
where M represents the molecule of the admixed gas with an ionization potential below the metastable state of Ar. Other symbols are defined below.
The first two processes (5a) and (5b) represent the direct ionization of an Ar atom or a molecule M. The number of Ar ions formed by process (5a) is NAr, and that of molecular ions formed in process (5b) is NM. The total number of ions formed in this first step is given by Eq.(2) and would lead to the W for regular mixtures, if there were no additional processes of electron formation.
The process (5c) represents an excitation of argon to a metastable or any other excited state, and the total number of excited states is N*. The ratio N*/NAr of the number of excited states and the number of argon ions is about 0.5 according to Platzman [17]. It is clear that not all the argon atoms are excited to the same excitation level and the distribution of excitations is determined by the distribution of the oscillator strengths. However, higher excited states are quickly deexcited (by emission of radiation) to either the lowest metastable states or the lowest resonant states. Due to the trapping of the resonant radiation, the resonant states, usually very short-living, are effectively long-living states [18]. In further discussion it will therefore be assumed that (i) all the excited levels come from the first excited configuration of argon (resonant states 1P1, 3P1, metastable states 3P0, 3P2), (ii) they are all considered to be long-living ("metastable"), and (iii) no difference among them (in ionization efficiency h, rate constants for various processes, etc.) will be taken into account. Practically, all four excited states from the first excited configuration are replaced by a single representative level. This assumption is justified by other investigations, see for example [19,20].
Argon excited/metastable states Ar* may be destructed by several processes [21]. Under conditions of relatively high pressure the diffusion and the radiative decay in two-body collisions may be neglected, and the two most important destruction processes are the formation of diatomic molecules in three-body collisions (5d) and collisions of the second kind with admixture molecules (5e). In previous studies of proportional counters filled with Penning mixtures no attention has been paid to reaction (5d). However, studies of VUV emission of rare gases and gas mixtures showed the importance of formation of excited rare gas molecules [22] and the effects of some quenching reactions other than the Penning ionization on the proportional counter operation [20].
The process (5d) is a three-body destruction of argon metastables in which argon excited molecules (eximers) are formed with a rate constant km. The reaction rate constant was measured by [18,20,22,23]. The Ar2* molecule decays within 3.5ms emitting a radiation energy of 10eV, which is not sufficient to ionize most of the admixed molecules. The formation of eximers, thus, leads to the loss of the energy in non-ionizing processes. Since it is a three-body process, it is proportional to the square of the number of argon atoms, and therefore also to the square of pressure. Experiments showed that the intensity of the light emitted after a radiative decay of rare gas eximers increases with increasing gas pressure [20].
In a collision of Ar* with an admixture (quenching) molecule M, which occurs with a rate constant kQ, the intermediate neutral superexcited molecular state (ArM)¢ is formed (process 5e). Superexcited states were first introduced by Platzman [24], and they represent highly excited electronic states (above the ionization threshold). Superexcited states have an important role as reaction intermediates in a variety of collision processes [25], among which is also the Penning process.