A METHOD AND A DEVICE FOR THE DETECTION OF IONIZING RADIATION

Abstract: A method and a device for the detection of ionizing radiation is described. The radiation (from 3) is attenuated in a medium, especially in an inert gas enclosed within a chamber (2), wherein the radiation dissipates at least a part of its radiative energy. In conjuction with this, the medium is irradiated with laser light having at least one wavelength characteristic to the medium, so that the medium interacts with the ionizing radiation and so that the medium is excited by the laser light, whereby the medium emits energy induced by laser light, which is easily detectable by at least one detector (4).

Description: A METHOD AND A DEVICE FOR THE DETECTION OF IONIZING RADIATION The invention relates to a method and a device for the detection of ionizing radiation, wherein the radiation is being attenuated in a medium, so that the ionizing radiation dissipates all or a part of its radiative energy, and the medium thereby emits energy v/hich is detected by at least one detector. Such devices are commonly known. "Ionizing radiation" is in general understood as radiation, either electromagnetic radiation or particle radiation, the energy of which is high enough to cause ionization of the medium while passing therethrough. The ionizing radiation may e.g. originate from a radioactive decay or it may be generated by a particle accelerator, an X-ray tube, or the like. In the former case, the radiation is usually of the type " ζ (electromagnetic) , β (electron, positron) or (X (the nucleus of a He-atom) . In the latter case, the ionizing radiation may be constituted by other elementary particles, such as protons, neutrons, mesons, etc. When ionizing radiation, o.g. an OC-p∑irticl , i F: decelerated in a gas, the atoms of the gas are ionized and excited. In the case of excitation, the atoms and/or the molecules take up a part of the energy of the particle. In the case of ionization, electrons are liberated, which may be collected and detected in the form of a charge pulse, for examp¬ le at an anode, which is utilized in so-called proportional counting. The size or the amplitude of the charge pulse, which corresponds to the number of liberated electrons, is then proportional to the energy (E) of the particular radiation quantum. The number of liberated electrons (N) sets a limit to the resolving power, i.e. the accuracy (&E) with v/hich the energy of the radiation quantum may be determined. In general, for ionization as well as for excitation, one can show that the energy resolving power is determined by the rela¬ tion (&E/E) = 2.35 • F/ / T (1) where F is a constant, the so-called Fano-f ctor, the numerical value of which is normally about 1 or somewhat lower. The number of electrons liberated depends upon the type of radiation and the energy, but also upon the medium referred to, approximately according to the following: N - E/ ζ. (2) where £ is the average energy lost by the particle for each liberated electron, i.e. for every ionization. The quantity £ is therefore a parameter which depends upon the medium. Moreover, it is clear from the expression (2) that materials with a lov; £-value provide a high resolving power. For example, in case of argon £ = 26 eV, and an (X-particle having an energy of 5 MeV will therefore bring about approximately 2 • 10^ liberated electrons when being stopped in an argon gas. Most known detectors for measuring the energy of the ionizing radiation, so-called energy dispersive detectors, are based on the measurement of liberated electrons. As a result, one obtains a resolving power in proximity of an upper limit, being deter¬ mined by the rela.tion (1) above. In the case of argon as a medium, the limit resolving power becomes approximately 0.5%. A disadvantage involved in the detection based upon the liberated electrons, is that information on the direction of the ionizing radiation is lost or may only be derived with difficulty. This is especially troublesome in conjunction with detector systems for charged particles, v/here one often wishes to detect the track of the ionizing particle in the detector in such a way that the specific energy loss (dE/dx) of the particle, i.e. the energy dissipation (E) per unit length (x) of the track, may be measured as well . In addition to causing ionization, the ionizing radiation excites neutral atoms and/or molecules in the medium. In many media these excited atoms may represent a greater number than the ion pairs, mainly due to the fact that less energy is required to excite an atom than to ionize it. In the expression (2) above, the quantity _ therefore has a lovter value than it would have for the detection of liberated electrons. Therefore, according to the relation (1) above, the excited atoms provide a greater number (N> of events, upon which the detection may be based. Hov/ever, the excited atoms are difficult to detect. Hitherto this has been done by way of electromagnetic radiation, possibly emitted during their de-excitation, so-called scintillations. These contain light of different wavelengths dependent upon betv/een which energy states the de-excitations occur . When scintillations are detected, for example by means of a conventional photomul iplier tube, a measure of the radiation quantum energy is obtained. Since the number of excited atoms can be greater than the number of ion pairs, a higher resolving power would in principle be obtained by detecting scintillaions rather than charges. Hov/ever, only a very small fraction of the excited atoms emit a photon, which is detectable by the photomultiplier, i.a. for the following reasons: a> only a fraction of the atoms de-excite radiatively; b) only a fraction of the radiative de-excitations give rise to light of wavelengths which can be detected by the photomul¬ tiplier; and c) a part of the emitted light is absorbed by the medium. Furthermore, losses occur partly as a result of light collection losses in the optical system, and partly because, at the most, about every fourth photon in the scintillation is converted into a detectable photo-electron in the photomultiplier . As a medium for scintilla ion detection, gases are commonly used, especially inert gases, principally Ar , Kr and Xe . For all of these (and also for He), the lov/est excited energy states are four s-εtates, two of which are de-excited radiatively to the ground state. The radiation lies in the so-called vacuum-ultra¬ violet (VUV) wavelength range, which makes the detection very difficult. Further, the radiation v/ill be strongly absorbed resonantly (trapping), and consequently only a very small fraction thereof radiates out of the scintillation medium. One therefore makes use of a so-called exci er formation, i.e. an excited atom forming a molecule v/ith another atom. This molecule is subsequently de-excited while emitting light. However, this light lies in the VUV-rangc as we11. It has been shown that most of the excited inert gas atoms are excited directly to any one of the four s-stateε. Likev/ise, the majority of the atoms being excited to higher states are de- excited, i.e. emit energy v/hile making transitions to these s-states . An optimized detection would require that every such excited atom could emit several photons, i.e. energy quanta of light with wavelengths in the sensitivity range of the photo- multiplier . With this background, the present invention aims at enabling reliable and accurate detection of ionizing radiation, especial¬ ly through the use of certain auxilliary means to bring about a modification or an amplification of the effects of the interac¬ tion betv/een the ionizing radiation and the medium, so that the energy emitted by the medium may be detected with greater reli¬ ability and accuracy. Additional, more specific aims are (by way of certain procedures) : - to accomplish a higher degree of excitation and subsequent de-excitation of the atoms or molecules of the medium, so that the emitted electromagnetic radiation may be detected by simple means, especially v/ithin a v/avelength range suitable for conventional detectors; - to reduce or eliminate the above-stated drav/backs involved in conjunction with scintillation detection; and - to enable the detection of the track and the specific energy loss of an ionizing particle during its deceleration in a medium. The stated main object of the invention is basically achieved in that, in conjunction v/ith the deceleration of the ionizing radiation in the medium, the medium is irradiated with laser light having at least one v/avelength being characteristic to the medium, so that the medium interacts with the ionizing radiation and is excited by the laser light, v/hereby the medium emits easily detectable energy induced by the laser light. The invention, its characteristics and advantages v/ill be described in detail below with reference to the accompanying drav/ings, v/hich illustrate the principles behind the invention and some specific embodiments. Figs, la, lb, lc and Id schematically show a number of energy transitions, utilized in carrying out the method according to the invention; Figs.2-5 show various examples of specific energy transitions, according to fig. la in conjunction v/ith figs, lb and Id, v/ith neon gas as a medium for scintillation detection; and Figs. 6 - 8 schematically show three different embodiments of the device according to the invention. Thus, figs. la-Id illustrate a few examples of energy transi¬ tions in the medium being used v/hen detecting ionizing radiation according to the invention. In fig. la the ground state of the medium is denoted by G, which constituee the energy state of the medium before it is hit by any ionizing radiation. Under the influence of the ionizing radiation, some of the atoms and/or molecules of the medium are elevated, either individually or collectively, from the ground state G to one or more excited states A which, according to the present invention, are brought to interact with laser radiation. The state or states A may consist of a single state or a group or a band of energy states. The states A and the ground energy state G of the medium lie so far apart that each state A, with¬ out the influence of the ionizing radiation, is unpopulated, i.e. practically none of the atoms of the medium can be excited into such states, for example through the thermal motion of the medium. Under the influence of laser radiation, however, the atoms of each energy state A are excited to another one or a band of higher energy states B. Then, the wavelengths of the laser radiation have been selected so that the transition from the state A to the state B has the highest probabiity. The atoms of the state B may dissipate their energy through different processes depending on the selected medium. These processes are in principle the following: the atoms in the state B dissipate, as shown in fig. lb, all or a part of their energy in a transition to a state C while emitting electromagnetic radiation (indicated by a wave-like arrow in the figure), v/hich is detected by a detector, as will be explained further below; the atoms in the state B dissipate (fig. lc) all or a part of their energy in a transition to the state C while emitting an electron (indicated by a minus sign inside a circle in the figure), which is registered by a detector; the atoms in the state B dissipate (fig. Id) their energy in two or more steps, wherein the atoms in these states, v/ithout radiating, or while emitting non-detectable radia¬ tion, transcend or transfer their energy to other atoms or molecules in the medium in such a v/ay that a state D is populated. The state D may then be an energy state in an atom, a molecule or a material of different kind than those to which the states A, B and C belong. This transition from the state B may arise e.g. from collisions, where the liber¬ ated energy is transferred into kinetic energy. Atoms in the state D thereupon dissipate their energy by radiation in a similar manner as the atoms in the state B in the processes according to fig. lb and/or lc. While emitting their energy, the atoms in the state D transcend into a state E; the atoms dissipate their energy in any one of the processes according to figs, lb, lc, Id, the only difference being that the state C or E forms or is transferred into the initial energy state A, which enables a repetition of the v/hole process including laser excitation and subsequent detection. This repetition of the process is signified by a multiplied amplification, here called multiplying effect. Figs. 2 - 5 illustrate four specific processes corresponding to figs, lb and Id for neon gas, In fig. 2 the lower excited states A are composed of the four 3s-states, v/hich are populated under the influence of the ioniz¬ ing radiation. These states are brought to interact with the laser light, v/hich in this case contains four v/avelengths , namely 7032 A, 7245 A, 7439 A, and 8082 A. Hereby, the atoms are excited resonantly to the state B, which in this example is composed of the lov/est of the 3p-states. When the atom transcends from the state B into the state A, electro¬ magnetic radiation (light) v/ith the v/avelength of the laser light will be emitted. Since this light propagates in all direc¬ tions, it can be distinguished from the laser light and can be detected by a detector. For each atom being excited to any 3s-state A by v/ay of the ionizing radiation, the process may be repeated a number of times, a photon being obtained each time. Almost every atom in any 3s-state A may therefore be measured selectively in the wavelength of the laser with such a high amplification that the losses in collecting the emitted light are compensated. These losses involve e.g. the limited solid angle covered by the detector because the incoming laser light must be screened from the detector. Thus, the invention enables the detection of approximately every atom being excited to said 3s-state through the influence of the ionizing radiation. In the example according to fig. 3, four laser v/avelengths are likewise used. Like in the example according to fig. 2, these wavelengths v/ill excite each of the four 3s-states into a 3p- state. However, these excited 3p-states are mutually different and have been selected so as to provide strong transitions to the 3s-εtates other than those used for the laser excitation. For example, the laser wavelengths may be selected to 5401 A, 6266 A, 6334 A, and 6929 A, resulting in excitation to the conventionally denoted states 3p ' [ 1/23 ( j=0 ) , 3p' C3/23 ( =l ) , 3pC3/23(j=2) and 3pE5/23 ( =2 ) . These states have a high prob¬ ability of emitting light of the wavelengths 5852 A, 6717 A, 6507 A, and 6143 A, whereupon the 3s-states A are populated again and the process may be repeated in a way corresponding to the example above. Since the emitted v/avelengths, detectable by the detector, are different from the wavelengths of the laser light, they may be filtered e.g. by means of a so-called interference filter. In the process according to fig. 4, the lower excited state A is one of the 3s-βtates, namely 3sC3/23°( j=2 ) . The upper state B is a 3p-state being denoted 3pC5/23 ( j=3 ) . The wavelength of the laser light has been selected to 6402 A, v/hich is tuned such that an atom in the lower, excited state A is excited resonantly to the state B. This state has no so-called "allowed" transition to the 3s-states other than the one corresponding to the laser excitation. Atoms, v/hich have been excited to the state A by the ionizing radiation, will therefore, if the intensity of the laser light is sufficiently high, occupy the state B during a large part of the time. If the pressure of the neon gas is sufficiently high, about 1 bar, these atoms v/ill undergo fast collisions v/ith other Ne-atoms . In such a collision, a portion of the excitation energy ie dissipated in such a v/ay that one of the atoms, after the collision, will occupy a lower state D, which in this case is 3pC 1/2 ( j=1 ) . The difference in excitation energy between the states B and D is normally normally converted to kinetic energy that the atoms receive. The atoms in said state have only a small possibility to dissipate additional parts of their excitation energy in such a so-called transfer reaction, since this somewhat lower state is the lowest αf the 3ρ-ntates. Thus, a radiative transition occurs to one of the 3s-states. Since the most probable transition is the one to the state A, the process may be repeated. In this tran¬ sition, light is emitted with the wavelength 7032 A, which is detected by the detector. Since this v/avelength is longer than the wavelength of the laser light, the detector may be effect¬ ively isolated from the laser light, e.g. through a so-called "cut-off" filter. Hereby, a more favourable irradiation of the medium by the laser light is enabled, as v/ell as a more favour¬ able collection geometry than in the. example according to fig, 3. The state D also has "allowed" radiative transitions to the other 3s-states, v/hile emitting light v/ith v/avelengths 7245 A, 7439 A and 8002 A. Thesα waveleng hs are also selected by said filter and are detected by the detector. The process cannot be repeated directly, when the transitions last mentioned occur. Hov/ever, it is knov/n that transfer reactions similar to those described above also occur between the 3s-states, the occupancy of the lov/est 3s-state A being preferred. Through these reactions, a repeated excitation and a multiplying effect are enabled in this case as v/ell. Another example of laser light induced detection of ionizing radiation, through the use of neon gas, is shov/n in fig. 5. Like in fig. 2, laser light containing four wavelengths is used. These v/avelengths are selected so that atoms, v/hich have been excited to one of the four 3s-εtates by the influence of the ionizing radiation, are excited resonantly from each one of these 3s-states to one or more preselected 3p-stateε . The selection of the 3p-states is carried out by optimizing the process v/ith regard to high speed and low influence of loss mechanisms. For example, the wavelengths of the lasers may be 5852 A corresponding to the transition to 3p' C 1/23 ( j=0) from 3s ' C 1/23°( j=2) , 6402 A corresponding to the transition to 3pC5/23(j=3) from 3sC3/23°( j=2) , 6164 A corresponding to the transition to 3p' C 1/2 ( =1 ) from 3s ' C 1/2 °( j=0) and 6383 A corresponding to the transition to 3pC3/23(j=l) from 3s.C3/23°(j=l) , Atoms in the 3p-states, being populated by laser light excitation, undergo v/ith other He-atoms, collisions similar to the ones described above, whereby the atoms tend to occupy the lov/er state and especially the lov/est 3p-state, namely 3pC1/23 (j=l ) , from which the atoms decay to the various 3s-states in a v/ay corresponding to the previous example. In the example according to fig. 5 the excitation process is repeated in a v/ay corresponding to the examples above. However, all 3s-states being populated through the ionizing radiation may be detected, v/hich results in high energy resolution power . Moreover, the process may in principle be continued for a long time and be stopped v/hen desired, as v/ill be described below. The process according to fig. 5 is effective enough to enable imaging of the path of an ionizing radiation quantum in the gas volume. The imaging can easily be accomplished through the use of conventional optics, such as e.g. lenses. Similar detection according to the invention may be accomplished v/ith the other inert gases, the transitions between the lowest s- and p-states being utilized in a similar way as described in the examples according to figs. 2 - 5. The inert gases may also be present in a liquid state or in a solid state. In such a case, the mechanisms shov/n in the examples according to figs. 2 - 5 may in principle be applied, taking into consideration the changes in structure of the energy states implied by the liquid and the solid state, respectively. Hov/ever, the invention is not limited to inert gases as a medium. Accordingly, other gases as well as other liquid and solid materials may be used v/ithin the concept of the invention. Nor is the invention limited to the feature that the state (B) , being populated by laser excitation, dissipates its energy through detectable electromagnetic radiation. This energy may also be dissipated through liberation of charges, especially electrons from another atom or molecule in the medium, which charges are detected e.g. v/ith an anode in the medium. A feasible process for this liberation is the so-called Penning-ionization . The laser irradiation can also be made in a broad band, i.e. instead of discrete v/avelengths, the laser light may contain one or several bands of wavelengths. Further, the laser light may be interrupted for example by a so-called optical modulator, where¬ in the repeated excitation process, described in figs. 1 - 4, may be stopped v/hen desired so as to restore the medium in the sense that the excited atoms and molecules return to their ground energy states. Thereafter, the medium is ready again for the detection of ionizing radiation. Also, pulsed laser light can be used v/ithin the scope of the invention. Likewise, other laser techniques may be used to achieve an optimal result in each individual case. One realization of the device according to the invention is shov/n schematically in fig. 6. The laser light from one or several lasers (not shown) is expanded so as to irradiate, via a lens 1, a chamber 2 containing a medium, for example neon gas. Ionizing radiation, for example K-radiation, from a sample 3 or some other radiation source located v/ithin or outside the chamber, is detected in a detector 4 by way of the light generated according to the invention. If the device according to fig. 6 makes uεe of the process according to fig. 2, the chamber must be designed in such a v/ay, that the laser light does not generate scattered light which disturbs the detection of the emission from the medium as generated according to the invention. On the other hand, if the processes according to figs. 3, 4 and 5 are used, the laser light can be eliminated from the detector in a familiar v/ay, with the aid of e.g. optical filters 5 located between the chamber 2 and the detector 4, These filters are transparent to the emission from the medium as generated according to the invention . In another embodiment according to the invention (fig. 7), the device includes tv/o detector units 4a, 4b, each comprising a lens 6a, 6b and an electronic image detector 7a, 7b. The detector units are oriented in such a v/ay that the image recordings in the two units are carried out. in two mutually perpendicular directions. As in the previous example, the filters 5a, 5b are inserted between the chamber 2 and each corresponding detector unit 4a, 4b. Through the lenses 6a, 6b the image of the chamber 2 is reproduced in each detector unit, which detects the path of the CX-particles in the detector medium. In this embodiment, the invention is thus used for track detection of charged particles. The paths of the particles are then imaged in the light, v/hich is emitted by the excited atoms while using any of the processes according to figs. 2-5. The intensity of this light along the track is then a measure of the specific energy loss dE/dx, (if the intensity of the laser light is taken into consideration) . Another realization of the device is shown in fig. 8. A cylindr¬ ical chamber 2', the walls of v/hich are made of an electrically conducting material and which contains a gas medium, for example an inert gas, such as He, is provided v/ith a thin v/ire 8 arranged along the cylinder axis, the wire serving as an anode or v/hich, accordingly, a positive electrical voltage is applied in relation to the walls of the chamber 2', The laser light, v/hich in this case is further focused through the lens 1', is brought to graze along the wire 8, which is thus located in the center of the beam of the laser light. Under the influence of the O-radiation from the sample 3', the inert gas in the chamber 2' is ionized, and electrons, so-called secondary electrons, are liberated and collected by the anode 8. In proximity to the anode 8 the electrons reach such a high energy that, v/hen colliding with the inert gas atoms, they are able to excite the latter into their s-states. Through subsequent laser excitation, light emission is then obtained v/hen using any of the processes described above. This light emission, upon passing through a filter 5' , is registered by 'the detector 4' . In this case, the light is constituted by laser induced, proportional scintillations, which also have v/avelengths deviating from the v/avelengths v/hich the scintillations would have without laser light excitation. Thus, the invention is used in amplifying the scintillations and simultaneously change their wavelengths. The amplified signal then provides better energy resolving power, as discussed above, and the change in wavelength of the scintillations may be optimized for reliable and accurate detection of the same. Experiments have shown that the amplification factor at certain energy transitions in inert gases may amount to about 1.5-5 and in exceptionally cases yet higher, namely about 9 for the transition mentioned in connection v/ith fig. 3, the transition corresponding to a laser light wavelength of 6402 A in neon. Those skilled in the art may apply the invention in a variety of v/ays v/ithin the scope of the appended claims.

Claims: C L A I M S 1. A method for the detection of ionizing radiation, wherein the radiation is being attenuated in the medium, so that the ionizing radiation dissipates all or a part of its radiative energy, and the medium thereby emits energy v/hich is detected by at least one detector, c h a r a c t e r i z e d i n that, in conjunction v/ith the deceleration of the ionizing radiation in the medium, the medium is irradiated with laser light having at least one v/avelength being characteristic to the medium, so that the medium interacts with the ionizing radiation and so that the medium is excited by the laser light, v/hereupon the medium emits easily detectable energy induced by the laser light. 2. A method according to claim 1, c h a r a c t e r i z e d i n that the atoms or the molecules of the medium also interact mutually, for example through collision, in connection v/ith the emission of said energy induced by the laser light. 3. A method according to claim l or 2, c h a r a c t e r ¬ i z e d i n that a part of the atoms or molecules of said medium is transferred from the ground states (G) to at least one first, higher energy state (A), wherein said atoms and molecules of said first energy state (A) interact v/ith the laser light and are thereby transferred to at least one second energy state (B), from v/hich said atoms or molecules are de-excited, directly or indirectly, while emitting said energy induced by the laser light to said detector(s) . 4. A method according to claim 3, c h a r a c t e r i z e d i n that said second energy state (B) is higher than said first energy state (A) . 5. A method according to claim 3 or 4, c h a r a c e r ¬ i z e d i n that the atoms or the molecules of the medium in said second energy state (B) , while emitting their energy, cause another one or the same atom or molecule to be transferred to said firεt energy state (A), whereby the process may be repeated and a multipliying effect may be obtained. 6. A method according to any one of the claims 1 - 5, c h a r a c t e r i z e d i n that one further applies an electromagnetic field, especially an electrostatic field, on at least a part of the medium. 7. A method according to the claims 3 and 6, c h r a c t¬ e r i z e d i n that the transition from the ground state (G) to said first energy state (A) is accomplished in that the ionizing radiation, during the atternuation in the medium, liberates charged particles from the medium, especially electrons, said charged particles being affected by the applied electromagnetic field and again interacting with the medium under the influence of the laser light. 8. A method according to any one of the claims 1 - 7, c h a r a c t e r i z e d i n that said laser induced energy at least partly comprises electromagnetic radiation, which is detected by said detector(s) . 9. A method according to claim 8, c h a r a c t e r i z e d i n that said electromagnetic radiation has a v/avelength being different from the one of the laser light. 10. A method according to claim 8 or 9, c h a r a c t e r - i z e d i n that said characteristic v/avelength( s ) is filtered away from the electromagnetic radiation before the latter reaches said detector(s) . 11. A method according to any one of the claims 1 - 10, c h a r a c t e r i z e d i n that said energy induced by the laser light at least partly comprises charged particles, v/hich are detected by εaid detector(ε) . 12. A method according to any one of the claims 1 - 11, c h a r a c t e r i z e d b y detecting the path of the ionizing radiation through the medium, said detector(s) comprising at least one image detector (4a, 4b) . 13. A method according to any one of the preceding claims, c h a r a c t e r i z e d b y using a gas enclosed v/ithin a chamber (2;2'), especially an inert gas, as said medium, 14. A method according to the claims 3 and 13, c h a r a c ¬ t e r i z e d i n that said first higher state (A) includes at least one of the four lov/est s-states and that said second energy state (B) includes at least one of the lowest excited p-states . 15. A device for the detection of ionizing radiation, including a medium (in 2;2'), in v/hich the ionizing radiation is being attenuated while dissipating all or a part of its radiative energy, βo that the medium itself emits energy, and at least one detector (4;4a,4b;4') for direct or indirect detection of the ♦snoryy omittod by Urn medium, c li « i n r I o r i • . « el i n that the device also includes at least one laser apparatus adapted to irradiate the medium v/ith laser light having at least one wavelength characteristic to the medium, so that the medium interacts v/ith the ionizing radiation and is excited by the laser light, whereby the medium emits easily detectable laser light induced energy. 16. A device according to claim 15, c h a r a c t e r i z e d b y a chamber (2;2') for enclosing said medium in the form of a gas, especially an inert gas.

Inventors: LINDBLOM PETER, OLSSON TORBJOERN, LINDBLOM, PETER, OLSSON, TORBJOERN
Applicants: LINDBLOM PETER, OLSSON TORBJOERN, LINDBLOM, PETER, OLSSON, TORBJOERN
Application year: 1991
Main classification: G01T1
IPC codes: G01T1