DE3050716C2 - Method for operating a position-sensitive proportional counter tube - Google Patents

Description

ten exists and in which a continuous delay line the counting wire or wires detected, characterized in that to achieve a high spatial resolution of the energy loss the ionizing radiation in the counter tube is used, which is directly proportional to the track length is in the counter tube and with which the pulse height increases by only such pulses from the detection electronics be selected, the pulse height of which the

due to reflection on the collimator walls. The desire for increased spatial resolution the known proportional counter tube thus ultimately leads to a loss in the sensitivity of the arrangement must be taken into account that this device appears to be unsuitable for very weak radiation permit.

In contrast, it is the object of the invention to provide a method according to the preamble of claim 1

Radiation πύί to the counting wire (7) perpendicular incidence 20, in which on the one hand an effective KoUimation corresponds to the radiation to be measured is achieved, but without 2. Method according to claim 1, characterized on the other hand, the sensitivity of the arrangement to denote that the gas amplification with the help of the amicable.

The operating voltage and the counting gas are selected so that this object is achieved by the invention through the features

Space charge effects occur which cause 25 le of the characterizing part of claim 1.

Incident radiation perpendicular to the counting wire (7) It is generally known that in the case of the

a significantly smaller pulse level results than the resulting ionization processes

Primary ionization caused by the impulse height / shape of various physical

from

The invention relates to a method for operating a position-sensitive proportional counter tube according to the preamble of claim 1.

Such a method is from »Journal of Chroma-

speaks, whereas inclined to the counting beam (7) depend on the incident parameters. This is how a publication describes Radiation has a pulse height proportional to its output ("Nuclear Radiation Detection", Mc Graw Primary ionization generated. Hill Book Company INC, 1958, pp. 103-107) that im

In the case of an ionization chamber with parallel electrode plates, especially a so-called electron pulse ionization chamber, the pulse height and the pulse shape depend on the place where the primary ionization occurs, so that in the case of ion paths with the same number of Λ / νοη ions, the voltage pulses generated depend on the center of charge of these N ions during the ionization and thus from the distance of the Sanvndelectrode. Dar-

tography «, 150 (1978) page 411 and in its 40 it follows that in ion trajectories with the same La details in a brochure “Chromelec 101” the focus of the conversation but the different inclinations Numelec in 78 005 Versailles.

A general problem with location-sensitive
There is proportional counter tubes with this measuring system
in that the spatial resolution is limited. that in 45
the counter tube also such particles enter and there
cause a count and thus a corresponding pulse induction on the delay line, the
do not get vertically on the shortest path from the surface to be measured to the counting wire, but do not run at an angle depending on the location of the primary ionization. Even with the most precise electronic role, more plays a role.

Measurement of the transit time difference is based on the spatial resolution or the distinction between the orbital inclinations

Due to these physical conditions, therefore, to the extent that the generated avalanche of charges from the secondary set, since the measured location of the induced im-electron is proportional to the primary ionization, this pulses on the delay line are only proportional to the energy loss of the tuned Limits with the actual location of the emitting particle, / ί particle. The loss of energy

the same pulse shape occurs that consequently the one dealt with here Effect cannot be used to solve the stated problem.

In contrast, the effect utilized by the operation method according to the invention is based on a proportional counter tube first of all, on the occurrence of secondary electrons in the immediate vicinity of the counting wire, making the above described

sion of the ionizing particle matches, conditional through the geometric relationships.

In the case of the known proportional counter tube, attempts have been made to improve the spatial resolution to remove the disruptive, Eliminate particles falling on an inclined path by opening the entry window of the counter tube with a mechanical collimator device. This collimator is made of fine laminate

but is greater, the longer the particle path in the counter tube. The number of the pulse height in the counting wire (and thus in the delay line) determining secondary electrons arriving at the counting wire is therefore a direct measure of the orbit inclination of the avalanche causing this cargo avalanche to be detected, particles passing through, with the result that particles passing through perpendicular to the counting beam are smaller

mell. which are perpendicular to the counting wire cause b "> pulses on the delay line than

and which have to have a certain height in order to be able to move a particle that is actually at an angle.

to achieve the same coordination. This principle is particularly favorable to apply

Such collimators, however, have some night cides, if by additional measures a high one

Sensitivity of the counter tube is guaranteed. so that dispensing with the evaluation of particles with inclined particle trajectories does not have a negative effect affects the sensitivity of the counter tube.

Dispensing with a mechanical collimator opens up the possibility of using the proportional counter tube to operate with an open entry window and to increase the height of the interior of the counter tube reduce, which on the one hand already contributes to increasing the sensitivity, on the other hand but again a contribution to the higher spatial resolution provides, since with decreasing distance of the surface to be measured it is independent of the counting wire of the counting tube From the evaluation pnnzip, the spatial resolution is better because the deviations of the Radiation location from the location of the corresponding induced pulse proportional to the distance Surface — Remove the counter tube.

The operating method according to the invention, so to speak represents an "electronic collimator", so it allows additional ones to be grasped at the same time Measures to increase the sensitivity of the counter tube to such an extent that this can be achieved by dispensing with the Evaluation of pulses lying above the specifiable pulse threshold is not impaired.

Discriminator stages of the evaluation circuit required to carry out the operating method according to the invention can advantageously be a single-channel discriminator use that only those pulses of the pulse height spectrum of the delay line selects for measuring the activity distribution that lie in a predeterminable »impulse window«, whereby the The upper threshold of this impulse window can be set so low that one can be sure that the Impulses can only come from particles whose trajectory is practically perpendicular to the surface to be measured runs

An embodiment of the invention provides. that the operating voltage of the proportional counter tube (and thus whose gas amplification) is selected as high as possible, so that the pulse heights are reduced from perpendicular rays impinging on the counting wire relative to the pulse heights of rays impinging obliquely occurs.

This effect can be explained by the fact that in the case of perpendicular rays, because of the locally concentrated rays Charge avalanche due to the (positive) ions generated by the space charge shielding resp. Recombinations some of the secondary electrons do not reach the counting wire and are therefore much smaller Pulses occur than would be expected after the primary ionization. This space charge effect occurs at an angle incident rays apparently not or only in weak form. The one to achieve high spatial resolution unwanted inclined incoming rays can therefore be made even easier by this design Eliminate the measuring process in the discriminator stages.

A circuit example for performing the operating method according to the invention will now be based on a Briefly explained in the block diagram:

The two ends of the delay line 9 are each connected to a preamplifier 14, the output signals of which are fed to the zero passage discriminators 16 via “shaping amplifiers” 15. The zero crossing measurement carried out enables transit time measurements of one ns (nanosecond).

With three further discriminators 17, the pulse heights measured. By setting the thresholds of these discriminators accordingly, rays are under more of the less perpendicular angle of incidence to the Counting wire 7 selected for measuring the activity distribution. The signals from the discriminators 16 and 17 Coincidence stages 18 are supplied, which then when the coincidence condition is met (zero crossing and pulse window) provide a start and a stop signal for the runtime measurement.

ίο The further data evaluation then takes place via a Time-to-digital converter or via a time-to-amplitude converter 19 with a conventional pulse height analyzer 20. There the measurement data of the activity distribution are stored quantitatively on a The activity distribution can then be displayed and evaluated directly on screen 21. With the described Tritium radiation sources can be arranged with an appropriate structural design of the counter tube triggered flawlessly from a distance of 05 mm and carbon-14 sources from a distance of 1 mm will.

1 sheet of drawings

Claims (1)

Patent claims: 1. Procedure for operating a position-sensitive proportional counter tube to measure the area distribution ionizing radiation, the continuous counting gas with an open entry window is flowed through or is provided with an entry window closed with a thin film, at As a result of the unavoidable height of the slats the distance between the counting wire and the object to be measured (thin-layer plate) increases. With increasing At the height of these lamellas, a greater improvement in co-ordination can be achieved, but must be correspondingly increased Accept a reduction in the sensitivity of the arrangement. Scattered radiation also determines the ratio which deteriorates the anode from one or more counting wires from the signal background, in particular