SE466121B - SET TO DETECT IONIZING RADIATION AND DEVICE FOR EXERCISE OF THE SET - Google Patents

Description

15 20 25 30 466 121 2 c6s / E> = 2.35. F / Jí '<1) where F is a constant, the so-called The Fanø factor, whose numerical value is generally about 1 or slightly lower. The number of electrons released depends on the type of radiation and energy but also on the medium in question. Approximately, N = E / 6 (2) where E is the energy that the particle loses on average for each released electron, ie for each ionization. The quantity 2 is therefore a constant which depends on the medium. Furthermore, it appears from the expression (2) that materials with a low E-value provide a high energy dissolution capacity. For example, for argon, E = 26 eV, which means that a K particle with an energy of 5 MeV gives rise to about 2,105 released electrons during braking in argon gas.

Most known detectors, which measure the energy of the ionizing radiation, so-called energy dispersive detectors, are based on measuring the released electrons. In this case, an energy resolution capability is obtained in the vicinity of the upper limit, which is determined by the relationship (1) above. In the case of argon as the medium, this intrinsic resolution is about 0.5%. A disadvantage of detection based on the released electrons is that the information about the direction of the ionizing radiation is lost or can only be deduced with difficulty. This is especially troublesome in connection with detector systems for charged particles, where one often wants to detect the "trace" of the ionizing particle in the detector in such a way that the specific energy loss of the particle (dE / dx), ie emitted energy (E) per unit length (x ) in the track, can also be measured. In addition to causing ionization, the ionizing radiation excites neutral atoms and / or molecules in the medium. In many media, these excited atoms are much higher in number than the ion pair, mainly because less energy is needed to excite an atom than to ionize it. In the expression <2) above, therefore, the quantity E has a lower value than that which applies to the detection by the released electrons. The excited atoms therefore constitute according to the relation (1) above a larger number (N) of events, on which the detection can be based. However, the excited atoms are difficult to register. Hitherto, this has been done by the electromagnetic radiation which these may give rise to during their de-excitation, so-called scintillations. These contain light of different wavelengths depending on between which energy states the excitations possess Ium.

If the scintillations are detected, for example with a conventional foot multiplier tube, a matt on the energy of the radiation quantum is obtained.

Since the number of excited atoms is greater than the number of ion pairs, a higher energy resolution would in principle be obtained by detecting the scintillations than detecting the charges. However, only a very small fraction of the excited atoms emit a photon that can be detected by the photomultiplier. The reasons for this are i.a. the following: a) only a part of the de-excitation of the atoms takes place radiatively; b) only a part of the radiative de-excitations gives rise to light of wavelengths, which can be detected by the photomultiplier; and c) a portion of the emitted light is absorbed by the medium. 10 15 20 25 30 35 466 121 4 In addition, losses occur partly as a result of light collection losses in the optical system, and partly by the fact that at most about every fourth photon in the scintillation is converted into a measurable photoelectron in the photomultiplier.

Gases, in particular noble gases, mainly Ar, Kr and Xe, are usually used as the medium for scintillation detection. For all these (and also for Ne) it applies that the lowest excited energy states are four s states, two of which are de-excited radiatively to the basic energy state. The radiation takes place in the so-called vacuum ultraviolet (VUV) wavelength range, which makes detection very difficult. Furthermore, the radiation will be strongly absorbed resonantly (trapping), so that a very small part of it radiates out of the scintillation medium. Therefore, so-called excimer formation, ie an excited atom forms a molecule with another atom. This molecule is then de-excited and emits light. However, this also happens in the VUV area.

It has been shown that most excited noble gas atoms are excited directly to some of the four s states. The majority of the atoms that are excited to higher states are also de-excited, ie they emit energy so that they change to these s states.

For optimal detection, it would be required that each such excited atom could be caused to emit several photons, ie energy quantum of light with wavelengths in the sensitivity range of the photomultiplier.

Against this background, the present invention aims to enable safe and accurate detection of ionizing radiation, in particular by effecting by special means a modification or enhancement of the effects of the interaction of the ionizing radiation with the medium in question, so that the emitted by the medium the energy can be detected with greater certainty and accuracy.

Further, more specific objects are to bring about, by special measures, a higher degree of excitation and concomitant de-excitation of the atoms of the medium resp. molecules, so that the electromagnetic radiation emitted can be detected by simple means, in particular in a wavelength range suitable for conventional detectors; reduce or eliminate the above-mentioned inconveniences associated with scintillation detection; and - enabling the detection of traces of an ionizing particle and specific energy loss during braking in a medium.

The stated main object is achieved according to the invention in principle by illuminating the medium with laser light with at least one wavelength characteristic of the medium in connection with the deceleration of the ionizing radiation, so that the medium interacts with the ionizing radiation and is excited by the laser light, whereby the medium emits laser light-induced, easily detectable energy.

The invention, its features and advantages are described in more detail below with reference to the accompanying drawings, which partly illustrate the principles underlying the invention and partly show some concrete exemplary embodiments.

Figs. 1a, 1b, 1c and 1d schematically show some different energy transitions used in the practice of the invention; Figs. 2 - 5 show different examples of specific energy transitions according to Fig. Ia in combination with Figs. Ib resp. id with the gas neon as the medium for scintillation detection; and Figs. 6-8 schematically show three different embodiments of the device according to the invention. BU 156 121 Some examples of energy transitions of the medium used in detecting ionizing radiation in accordance with the invention are thus illustrated in Figs. 1a-1d.

In Fig. 1a, the basic energy state of the medium is denoted by G constituting the energy state of the medium before any ionizing radiation quantity hits the same. Under the influence of the ionizing radiation, some of the atoms and / or molecules of the medium, either individually or collectively, are lifted from the basic energy state G to one or more excited states A, which according to the present invention are caused to interact with laser radiation. The state or states A can consist of an individual, a group of or a band of energy states. The state A is so far from the basic energy state G of the medium that the respective state A without the effect of the ionizing radiation is unpopulated, ie practically none of the atoms of the medium can, for example by the thermal movement in the medium, be excited to the state in question.

Under the influence of the laser radiation, however, the atoms in the respective energy state A are excited to another or a band of higher lying energy state B. The wavelengths of the laser radiation have then been chosen so that the transition from state A to state B has the highest probability. The atoms in state B can emit their energy through different processes depending on the medium chosen. These processes are in principle as follows: - the atoms in state B emit, as shown in Fig. 1b, all or part of their energy to a state C during the emission of electromagnetic radiation (indicated by a wavy arrow in the figure), which radiation detected by a detector, of which more below; the atoms in state B emit (Fig. 1c) all or part of their energy upon transition to state C during the emission of an electron (indicated by a minus sign inside a ring in the figure), which is detected by a detector; 10 15 20 25 30 _; PJ _; 7 466 - the atoms in state B emit (Fig. Id) their energy in two or more steps, the atoms in these states radiating or transferring their energy to other atoms or molecules in the medium without radiation or emitting undetectable radiation. in such a way that a state D is populated. State D can then be an energy state in an atom, molecule or material of a different kind than those to which states A, B and C belong. This transition from state B can occur through, for example, collisions, whereby the released energy changes into kinetic energy. Atoms in state D then emit their energy by radiation in a manner similar to the atoms in state B in the processes of Fig. 1b and / or lo. During the release of its energy, the atoms in state D change to a state E; the atoms emit their energy according to one of the processes according to Figs. 1b, 1c, id with the difference that state C or E consists of or is transferred to the initial energy state A, which enables a repetition of the whole process with laser excitation and concomitant detection. This repetition of the process involves a multiplicative amplification, here called multiplier effect.

Figures 2 to 5 illustrate four specific processes corresponding to Figures 1b and 1d for neon gas.

In Fig. 2, the lower, excited states A are the four 3s states, which are occupied under the influence of the ionizing radiation. These states are allowed to interact with the laser light, which in this case contains four wavelengths, namely 7032 Å, 7245 Å, 7439 Å and 8082 Å.

Hereby the atoms are resonantly excited to the state B, which in this example constitutes the lowest of the 3p states. As the atom transitions from state B to state A, electromagnetic radiation (light) with the wavelengths of the laser light will be emitted BU 466 121 8. Since this light emits in all directions, it can be distinguished from the laser light and detected with a detector. For each atom which is excited by the action of the ionizing radiation to some 3s state A, the process can be repeated several times, whereby one photon is obtained for each time.

Virtually every atom in any 3s state A can therefore be measured selectively in the wavelength of the laser with such a high gain that the losses in the collection of the emitted light are compensated. These losses are i.a. the limiting space angle that the detector sees as a result of the incoming laser light having to be shielded from the detector. The invention thus enables a registration of virtually every atom which has been excited by the action of the ionizing radiation to said 3s state A.

In the example according to Fig. 3, four laser wavelengths are also used which, as in the example according to Fig. 2, excite 3p states from each of the four 3s states.

However, the excited 3p states are not the same state, but have been chosen so that they have other strong transitions to the 3s states than those used by the laser excitation. For example, the laser wavelengths can be selected to 5401 Å, 6266 Å, 6334 Å and 6929 Å, the states usually denoted 3p '[1 / 2J (j = 0), 3p'E3 / 2J (j = 1), 3p [3 / 2J (j = 2) and 3p [5 / 2J (j = 2) are excited. These states have a high probability of emitting light of the wavelengths 5852 Å, 6717 Å, 6507 Å and 6143 Å, whereby the 3s states A are again occupied and the process can be repeated in a corresponding manner as in the example above. Since the emitted wavelengths, which are registered by the detector, have other wavelengths than the laser light, these can be filtered, for example, with so-called interference filter.

In the process according to Fig. 4, the lower, excited state A consists of one of the 3s states, namely 3s [3/21 ° (j = 2). The upper state B consists of a 3p state, the designation of which is 3pE5 / 2] (j = 3). The wavelength of the laser light has been selected to 6462 A, which is tuned so that an atom in the lower, excited state S 466 121 state A is resonantly resonated to state B. This state has no other "allowed" transition to 3s-to stalls other than those corresponding to laser excitation. Atoms excited by the ionizing radiation to state A will therefore, if the intensity of the laser light is high enough, be in state B for a large part of the time.

If the pressure in the neon gas is high enough, about 1 bar, these atoms undergo rapid collisions with other Ne atoms. In such a collision, part of the excitation energy is emitted in such a way that either atom after the collision is in a lower state D, which in this case consists of 3pE1 / 2J (j = 1).

The difference in excitation energy between states B and D is usually given to the kinetic energy of the atoms. The atoms in said state have only a small possibility that by such, so-called transfer reaction, emit additional portions of its excitation energy, as this slightly lower state is the lowest of the 3p states. Consequently, a radiative transition to a 3s state takes place. The most probable transition is to state A, so the process can be repeated. At this transition, light with a wavelength of 7032 Å is emitted, which is registered by the detector. Since this wavelength is longer than that of the laser light, the detector can be effectively isolated from the laser light, for example with so-called "Cut-off" filter. This enables a more favorable illumination of the medium with the laser light, and a more favorable collection geometry than in the example according to Fig. 3.

State D also has so-called permissible radiative transitions to the other 3s states, whereby light with the wavelengths 7245 Å, 7439 Å and 8082 Å is also emitted. These wavelengths are also distinguished by said filter and registered by the detector.

The process cannot be directly repeated when the latter transitions occur. However, it is known that similar transfer reactions such as those described above also occur between the 3s states, with occupation of the lowest 3s state A being preferred. These reactions also enable a repeated excitation and multiplier effect in this case.

Another example of laser light-induced detection of ionizing radiation using neon gas is shown in Fig. 5.

As in Fig. 2, laser light containing four wavelengths was used.

These wavelengths are so selected that atoms excited by the action of the ionizing radiation to any of the four 3s states are resonantly excited from each of them to one or more preselected 3p states.

The choice of 3p state is made by optimizing the process with respect to high speed and low impact of loss mechanisms.

For example, the wavelengths of the lasers may be 5852 Å corresponding to the transition to 3p'E1 / 2l (j = 0) from 3s'E1 / 2J ° (j = 1), 6402 Å corresponding to the transition to 3pE5 / 2l (j = 3) from 3sE3 / 2] ° (j = 2), 6164 Å corresponding to the transition to 3p '[1 / 2l (j = 1) from 3s' [1/2] ° (j = 0) and 6383 Å corresponding to the transition to 3p [ 3 / 2J (j = 1) from 3sE3 / 2J ° (j = 1). Atoms in the 3p states, which are popularized by the laser light excitation, undergo similar collisions with other Ne atoms as those described above, whereby the atoms tend to occupy lower states and especially the lowest 3p state, namely 3p [1/2] ( j = 1), from which the atoms transfer to the different 3s states in the same way as in the previous example.

In the example according to Fig. 5, the excitation process is repeated in the same way as in the examples above, whereby, however, all 3s conditions, which have been populated by the ionizing radiation, can be detected, which results in high energy resolution.

Furthermore, the process can in principle continue for a long time and be interrupted at the desired time, in accordance with what is described below. 10 15 20 25 3U _; ND _; The process according to Fig. 5 is sufficiently efficient to enable imaging of the path of an ionizing radiation quantity in the gas volume. The image can be easily achieved by means of conventional optics, such as for example lenses.

Similar detection according to the invention can be achieved with other noble gases, the transitions between the lowest s- and p-states being used in the same way as in the described examples according to Figs. 2 - 5. The noble gases can also be present in liquid phase as well as in solid phase.

For these, mainly the mechanisms shown in the examples according to Figs. 2 - 5 can be applied taking into account the changes in the energy states that the liquid phase resp. the solid phase means.

However, the invention is not limited to noble gases as a medium.

Thus, other gases as well as other liquids and solids can be used within the scope of the invention.

The invention is also not limited to the state (B) occupied by laser excitation emitting its energy by detectable electromagnetic radiation. The energy can also be released by releasing charges from another atom or molecule in the medium, in particular electrons, which are detected with, for example, an anode in the medium. A process that can be used for this release is the so-called Money ionization.

The laser illumination can also be made broadband, ie instead of discrete wavelengths, the laser light can contain one or more bands of wavelengths. Furthermore, the laser light can be interrupted, for example, by a so-called optical modulator, whereby the repeated excitation process described according to Figs. 1-4 can be interrupted as desired in order for the medium to be able to be restored, meaning that the excited atoms resp. the molecules return to their basic energy states. When this has occurred, the medium is prepared for new detection of ionizing radiation. Laser beams with a pulsating character can also be used in the application of the invention. Likewise, other laser technology can be used to achieve optimal results in each individual case.

An apparatus embodiment according to the invention is shown schematically in Fig. 6. The laser light from one or more lasers (not shown) is expanded and is allowed to illuminate via a lens 1 a chamber 2 containing a medium, for example neon gas. Radioactive radiation, for example G-radiation, from a preparation 3 or other radiation source located inside or outside the chamber, is detected in a detector 4 by the light generated according to the invention.

If the apparatus of Fig. 6 utilizes the process of Fig. 2, the chamber must be designed so that the laser light does not generate scattered light which interferes with the emission generated according to the invention from the medium. If instead the processes according to Figs. 3, 4 and 5 are used, the laser light can be eliminated from the detector in a known manner by means of, for example, optical filters 5 placed between the chamber 2 and the detector 4. These filters are transparent to the emission generated from the medium.

In another embodiment according to the invention (Fig. 7), the device has two detector units 4a, 4b, each of which consists of a lens Sa and 6b and an electronic image detector 7a resp. 7b.

The detector units are positioned so that the images in the two units take place in two mutually perpendicular directions. As in the previous example, filters Sa, Eb are inserted between the chamber 2 and resp. detector unit 4a, 4b. Through the lenses 6a, Bb, the chamber 2 is imaged in resp. detector unit which thereby registers the path of movement of the ü-particles in the detector medium. In this embodiment, the invention is thus used for trace detection of charged particles.

The path of motion of the particles is then imaged by the light emitted by the excited atoms using one of the processes according to Figs. 2 - 5. The intensity of this light along the groove thereby constitutes a mat on the specific energy loss dE / dx, (if consideration taken to the intensity of the laser light). 10 15 20 25 wm FJ ... s 466 1 13 A further apparatus embodiment is shown in Fig. 8. A cylindrical chamber 2 ', the walls of which consist of an electrically conductive material and which contains a gaseous medium, for example a noble gas, such as Ne, is provided with a thin wire 8 arranged along the cylinder axis, which serves as an anode and thus a positive electrical voltage is applied in relation to the walls of the chamber 2 '. The laser light, which in this case is collimated more closely via the lens 1 ', is caused to fall along the wire 8, so that the wire is in the middle of the laser light beam. Under the influence of the G-radiation from the preparation 3 ', the noble gas is ionized in the chamber 2', whereby electrons, so-called secondary electrons, are released and collected by the anode 8. In its vicinity, the electrons receive such high energy that in the event of a collision with the noble gas atoms they are able to excite them to the s state.

By subsequent laser excitation, light emission is obtained using one of the processes described above.

This light emission, which after passage through a filter 5 'is detected by the detector 4', in this case constitutes laser light-induced, proportional scintillations, which in addition have wavelengths deviating from the wavelengths the scintillations would have without laser light excitation. Thus, the invention is used to amplify the scintillations and at the same time change their wavelengths. The amplified signal thereby provides better energy resolution capability, as discussed above, while the change in the wavelength of the scintillations can be optimized for safe and accurate detection thereof. Experiments have shown that the gain factor at certain energy transitions of noble gases can amount to about 1.5-5 and in exceptional cases even higher, namely about 9 for the transition mentioned in connection with Fig. 3 corresponding to a laser light wavelength of 6402 Å for neon.

Those skilled in the art can apply the invention in a variety of ways within the scope of the appended claims.

Claims (16)

10 15 20 466 121 1-4 PATENT REQUIREMENTS A method of detecting ionizing radiation by allowing the radiation to decelerate in a medium, so that the ionizing radiation emits all or part of its radiant energy and the medium thereby emits energy which is detected in at least one detector, characterized in that in connection with the deceleration of the ionizing radiation in the medium illuminates the medium with laser light with at least one wavelength characteristic of the medium, so that the medium partly interacts with the ionizing radiation and partly is excited by the laser light, whereby the medium emits laser light-induced, easily detectable energy. 2. A method according to claim 1, characterized in that the atoms or molecules of the medium also interact with each other, for example by collision, in connection with said laser light-induced energy being emitted. A method according to claim 1 or 2, that a part of said atoms or molecules of said medium is caused to change from its basic energy state (G) to at least a first, higher lying energy state (A), wherein said atoms resp. molecules in said first energy state (A) interact with the laser light and thereby transition to at least one second energy state (B) from which said atoms resp. molecules are de-excited and directly or indirectly emit said laser light-induced energy to said detector (s). A method according to claim 3, said second energy state (B) being higher than said first characteristic of that energy state (A). A method according to claim 3 or 4, that the atoms resp. molecules in said second energy state of state (B) during the release of their energy cause that someone 6! 10 15 20 25 (I) (f: .An Ö \ O \ ... a B) ... x 15 another or the same atom resp. molecule transitions to said first energy state (A), whereby the process can be repeated and a multiplier effect is obtained. 6. A method according to any one of claims 1-5, characterized in that an electromagnetic, in particular electrostatic, field is additionally applied to at least a part of the medium. Method according to claims 3 and 6, characterized in that the transition from the basic energy state (G) to said first energy state (A) is effected by the ionizing radiation during braking in the medium releasing charged particles from the medium, in particular electrons, which charged particles are affected by the applied electromagnetic field and again interact with the medium under the influence of the laser light. 8. A method according to any one of claims 1 - 7, characterized in that said laser light-induced energy consists at least in part of electromagnetic radiation, which is detected by means of said detector (s). 9. A method according to claim 8, characterized in that said electromagnetic radiation has a different wavelength than the laser light. 10. A method according to claim B or 9, characterized in that said characteristic wavelength (s) of the laser light is filtered out of the electromagnetic radiation before the latter reaches said detector (s). 11. A method according to any one of claims 1 to 10, characterized in that said laser light-induced energy consists at least in part of charged particles, which are detected by means of said detector (s). 10 15 20 25 BU 466 121 15 12. A method according to claim 11, characterized in that the motion path of the ionizing radiation is registered in the medium, wherein said detector (s) consists of at least one imaging detector (4a, 4b). 13. A method according to any one of the preceding claims, characterized in that a gas enclosed in a chamber (2; 2 '), in particular a noble gas, is used as said medium. A method according to claims 3 and 13, characterized in that said first higher state (A) consists of at least one of the four lowest s states and that said second energy state (B) consists of at least one of the lowest excited p states. conditions. An apparatus for detecting ionizing radiation, comprising a medium (in 2; 2 '), in which the ionizing radiation is allowed to decelerate while emitting all or part of its radiant energy, the medium in turn emitting energy, and at least a detector (4; 4a, 4b; 4 ') for direct or indirect detection of the energy emitted by the medium, characterized in that the device further comprises at least one laser apparatus arranged to illuminate the medium with laser light with at least one wavelength characteristic of the medium, so that the medium partly interacts with the ionizing radiation and partly is excited by the laser light, whereby the medium emits laser light-induced, easily detectable energy. Device according to claim 15, characterized by a chamber (2; 2 ') arranged for enclosing said medium in the form of a gas, in particular a noble gas. 1, / n: