WO1992007283A1

WO1992007283A1 - A method and a device for the detection of ionizing radiation

Info

Publication number: WO1992007283A1
Application number: PCT/SE1991/000691
Authority: WO
Inventors: Peter Lindblom; Torbjörn OLSSON
Original assignee: Peter Lindblom; Olsson Torbjoern
Priority date: 1990-10-15
Filing date: 1991-10-15
Publication date: 1992-04-30
Also published as: SE9003293L; SE466121B; EP0510142A1; SE9003293D0

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 detect ion 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 which 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 γ (electromagnetic), β (electron, positron) or α (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 α-particlo, is 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 example 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 (δE) with which 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 relation

(δE/E) = 2.35 · F/√N (1) where F is a constant, the so-called Fano-factor, 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/ ε (2) where ε is the average energy lost by the particle for each liberated electron, i.e. for every ionization. The quantity ε is therefore a parameter which depends upon the medium. Moreover, it is clear from the expression (2) that materials with a low ε-value provide a high resolving power. For example, in case of argon ε = 26 eV, and an α-particle having an energy of 5 MeV will therefore bring about approximately 2 · 10⁵ 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 determined by the relation (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, where 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 ε 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. However , 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 between which energy states the de-excitations occur.

When scintillations are detected, for example by means of a conventional photomul tiplier 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. However, 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 photomultiplier; 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 scintillation detection, gases are commonly used, especially inert gases, principally Ar, Kr and Xe. For all of these (and also for Ne), the lowest excited energy states are four s-states, two of which are de-excited radiatively to the ground state. The radiation lies in the so-called vacuum-ultraviolet (VUV) wavelength range, which makes the detection very difficult. Further, the radiation will 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 excimer formation, i.e. an excited atom forming a molecule with another atom. This molecule is subsequently de-excited while emitting light. However, this light lies in the VUV-range as well.

It has been shown that most of the excited inert gas atoms are excited directly to any one of the four s-states. Likewise, the ma jor i ty o f the at oms be ing exc i t ed t o higher s tates are deexcited, i.e. emit energy while 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, especially through the use of certain auxilliary means to bring about a modification or an amplification of the effects of the interaction between the ionizing radiation and the medium, so that the energy emitted by the medium may be detected with greater reliability 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 within a wavelength range suitable for conventional detectors;

- to reduce or eliminate the above- s tated drawbacks 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 with the deceleration of the ionizing radiation in the medium, the medium is irradiated with laser light having at least one wavelength being characteristic to the medium, so that the medium interacts with the ionizing radiation and is excited by the laser light, whereby the medium emits easily detectable energy induced by the laser light.

The invention, its characteristics and advantages will be described in detail below with reference to the accompanying drawings, which illustrate the principles behind the invention and some specific embodiments.

Figs, 1a, 1b, 1c and 1d 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. 1a in conjunction with figs, 1b and 1d, with 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. 1a-1d illustrate a few examples of energy transitions in the medium being used when detecting ionizing radiation according to the invention.

In fig. 1a 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, without 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. 1b, 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), which is detected by a detector, as will be explained further below; - the atoms in the state B dissipate (fig. 1c) 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. 1d) their energy in two or more steps, wherein the atoms in these states, without radiating, or while emitting non-detectable radiation, transcend or transfer their energy to other atoms or molecules in the medium in such a way 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 liberated 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. 1b and/or 1c. 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, 1b, 1c, 1d, 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 whole 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, 1b and 1d for neon gas,

In fig. 2 the lower excited states A are composed of the four 3s-states, which are populated under the influence of the ionizing radiation. These states are brought to interact with the laser light, which in this case contains four wavelengths, 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 lowest of the 3p-states. When the atom transcends from the state B into the state A, electromagnetic radiation (light) with the wavelength of the la ser light will be emitted. Since this light propagates in all directions, 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 way 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 wavelengths are likewise used. Like in the example according to fig. 2, these wavelengths will 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-states 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'[1/2](j=0), 3p'[3/2](j=1), 3p[3/2](j=2) and 3p[5/2](j=2). These states have a high probability 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 wavelengths, 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-states, namely 3s[3/2]°(j=2). The upper state B is a 3p-state being denoted 3p[5/2](j=3). The wavelength of the laser light has been selected to 6402 A, which 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, which 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 will undergo fast collisions v/ith other

Ne-atoms. In such a collision, a portion of the excitation energy is dissipated in such a way that one of the atoms, after the collision, will occupy a lower state D, which in this case is 3p[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 i s the l owe s t o f th e 3 p- s ta t e s . Thu s , a radi a t i ve t ran s i t i on 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 transition, light is emitted with the wavelength 7032 A, which is detected by the detector. Since this wavelength is longer than the wavelength of the laser light, the detector may be effectively 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 well as a more favourable collection geometry than in the example according to fig. 3.

The state D also has "allowed" radiative transitions to the other 3s-states, while emitting light with wavelengths 7245 A, 7439 A and 8002 A. These wavelengths are also selected by said filter and are detected by the detector. The process cannot be repeated directly, when the transitions last mentioned occur. However, it is known that transfer reactions similar to those described above also occur between the 3s-states, the occupancy of the lowest 3s-state A being preferred. Through these

reactions, a repeated excitation and a multiplying effect are enabled in this case as well.

Another example of laser light induced detection of ionizing radiation, through the use of neon gas, is shown in fig. 5. Like in fig. 2, laser light containing four wavelengths is used.

These wavelengths are selected so that atoms, which have been excited to one of the four 3s-states by the influence of the ionizing radiation, are excited resonantly from each one of these 3s-states to one or more preselected 3p-states.

The selection of the 3p-states is carried out by optimizing the process with 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'[1/23(j=0) from 3s'[1/23°(j=2), 6402 A corresponding to the transition to 3p[5/2](j=3) from 3s [3/23°(j=2), 6164 A corresponding to the transition to 3p'[1/2](j=1) from 3s'[1/2]°(j=0) and 6383 A corresponding to the transition to 3p[3/2](j=1) from

3s.[3/2]°(j=1), Atoms in the 3p-states, being populated by laser light excitation, undergo with other Ne-atoms, collisions similar to the ones described above, whereby the atoms tend to occupy the lower state and especially the lowest 3p-state, namely 3p[1/2](j=1), from which the atoms decay to the various 3s-states in a way corresponding to the previous example.

In the example according to fig. 5 the excitation process is repeated in a way corresponding to the examples above. However, all 3s-states being populated through the ionizing radiation may be detected, which results in high energy resolution power.

Moreover, the process may in principle be continued for a long time and be stopped when desired, as will 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 with 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 shown 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.

However, 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 within 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. with 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 wavelengths, 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, wherein the repeated excitation process, described in figs. 1 - 4, may be stopped when 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 within 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 shown 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 within 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 use of the process according to fig. 2, the chamber must be designed in such a way, 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 way, 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 two 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 way 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 α-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, which 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 cylindrical chamber 2', the walls of which are made of an electrically conducting material and which contains a gas medium, for example an inert gas, such as Ne, is provided with a thin wire 8

arranged along the cylinder axis, the wire serving as an anode or which, accordingly, a positive electrical voltage is applied in relation to the walls of the chamber 2', The laser light, which 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 α-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, when 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 when 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 wavelengths deviating from the wavelengths which 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 with 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 ways within 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 which 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 with the deceleration of the ionizing radiation in the medium, the medium is irradiated with laser light having at least one wavelength 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, whereupon 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 with the emission of said energy induced by the laser light.

3. A method according to claim 1 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 which 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 t 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 first 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 a r a c te 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 wavelength 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 wavelength (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, which are detected by said detector(s).

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 within 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 lowest 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 which the ionizing radiation is being attenuated while dissipating all or a part of its radiative energy, so that the medium itself emits energy, and at least one detector (4;4a,4b;4') for direct or indirect detection of the enorgy omittod by the medium, c h a r a c t e r i z e d i n that the device also includes at least one laser apparatus adapted to irradiate the medium with laser light having at least one wavelength characteristic to the medium, so that the medium interacts with 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.