JPH031832Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a proportional counter tuned to obtain discharge pulses of magnitude proportional to the energy loss of radiation (incident particles).

In conventional proportional counters, when the gas amplification degree and counting rate increase, positive ions form a space charge that weakens the electric field around the center electrode (anode), resulting in a so-called wave height shift in which the output pulse height decreases. This phenomenon caused an upper limit to the possible counting rate of proportional counting.

This invention aims to realize a proportional counter tube in which the upper limit at which the output pulse height is saturated is raised, and one embodiment of this invention will be explained below with reference to the drawings.

Figure 1 is a schematic explanatory diagram of a proportional counter according to an embodiment of this invention, in which 1 is a thin linear anode made of metal wire (hereinafter referred to as anode), and 2 is a metal wire wound into a coil. A grid electrode concentrically surrounding the anode 1, 3 a planar cathode made of a conductive material (hereinafter referred to as cathode) constituting the ionization chamber 4, 5 a window (incident window) 6 made of a radiation transparent material,
7 are the first and second power supplies, respectively.

As shown in FIG. 1, the grid electrode 2 surrounds the anode 1 in close proximity.

If the radius of the anode is r ₁ , the radius of the cathode is r ₂ , the radius of the grid electrode is r ₀ , the voltage between the anode and cathode is V, and the voltage between the grid electrode and the cathode is V ₀ , then at radius r from the center, The potential gradient E is: When r≦ _r0 , E=V− _V0 /rln( _r0 / _r1 ) When r≦ _r0 , E= _V0 /rln( _r2 / _r0 ) In the present invention, The potential gradient is set to be larger on the anode side than on the cathode side with the grid electrode as the boundary. In other words, in the immediate vicinity of the grid electrode, from the above equation (assuming r≒r ₀ ), V-V ₀ /r ₀ ln (r ₀ /r ₁ )>V ₀ /r ₀ ln (r ₂ /r ₀ ). Do it like this. Therefore, V ₀ <ln(r ₂ /r ₀ )/ln(r ₂
/r ₁ )゜V The right side of the above equation is equal to the potential formed only by the anode and cathode (assumed to be zero potential) at the position of the grid electrode. ``applying a voltage that gives a potential lower than the potential created at the grid electrode location by the lattice electrode.''

Also, 8 is an output resistor, 9 is an output capacitor,
Reference numeral 10 denotes an amplifier, by means of which a discharge pulse signal within the ionization chamber 4 is obtained at an output terminal 11.

When radiation enters through the entrance window 5, discharge gas molecules within the ionization chamber 4 are ionized. This is called initial ionization, and ion pairs proportional to the incident photon energy [λν] are generated, and negatively charged electrons move toward the grid electrode 2. At this time, as mentioned above, the electric field inside the grid electrode is larger than the outside.
Since the potentials of the two electrodes are set, the electrons are not captured by the grid electrode 2 very much and move toward the anode to which a more positive potential is applied. Since the anode 1 is made of a thin metal wire, ions are multiplied (gas amplification effect) by the action of a strong electric field around it, just like in a conventional proportional counter tube. The electrons generated here reach the anode 1 almost instantaneously. On the other hand, cations move toward the cathode 3 due to the electric field. What is taken out as the counter output is the amount of induced charge on the anode surface induced by the positive ions. In the case of the counter of this invention, when positive ions pass through the grid electrode 2, the induced charge due to the positive ions disappears, and the space between the grid electrode 2 and the anode 1 returns to the initial state. After the cations pass through the grid electrode 2, it usually takes about 10 ^-3 seconds for them to reach the wall (cathode 3) of the ionization chamber 4. In the case of a conventional proportional counter without a grid electrode 2, moving cations act to weaken the electric field around the anode, and its size depends on the cross-sectional area of the cathode (or the base area if the shape is cylindrical). Calculation confirms that it is proportional.

The strength of the electric field around the anode has an exponential effect on the degree of gas amplification, so the concentration of moving cations changes depending on the magnitude of the incident radiation energy, so the degree of gas amplification changes. In the case of the proportional counter of this invention, among the moving cations, only those inside the grid electrode 2 act on the electric field, so the magnitude of the incident particle energy has an extremely small effect on the degree of gas amplification.

The above principle of the proportional counter with grid electrodes applies even when it is a gas flow type, when it is used for radioactive materials without a window, and when the ionization chamber is an end-on type as shown in Figure 2. It can be used in the same way. In FIG. 2, parts common to those in FIG. 1 are given the same reference numerals. Even if the grid electrode 2 and the anode 1 are arranged in this way with respect to the shape of the ionization chamber 4, the degree of gas amplification is kept almost constant, so a counter tube with a high selectivity in the overall structure can be used. For example, a proportional counter with high energy proportionality can be realized by using a counter as shown in FIG. 2, which avoids a dead zone near the window and increases absorption efficiency.

As detailed above, the effect of this invention is that it can be used for fluorescent X-ray analysis, etc. by adding an extremely small configuration to the conventional device, and maintains a proportional relationship at a counting rate that is 10 to 100 times higher than conventional devices. It is possible to realize a proportional counter tube with high accuracy.

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram of a proportional counter according to one embodiment of this invention, and FIG. 2 is a sectional view of a main part of a proportional counter according to another embodiment of this invention. DESCRIPTION OF SYMBOLS 1... Linear anode, 2... Grid electrode, 3... Planar cathode, 4... Ionization chamber, 5... Entrance window, 6... First power source, 7... Second power source.

Claims (1)

[Scope of utility model registration request] A grid electrode is arranged near the anode to surround the anode, and a voltage is applied to the grid electrode to give a potential lower than the potential formed at the grid electrode only by the anode and cathode. proportional counter.