GB2066460A - Method and Apparatus for Detecting or Measuring the Intensity of Microwave or Infrared Radiation - Google Patents
Method and Apparatus for Detecting or Measuring the Intensity of Microwave or Infrared Radiation Download PDFInfo
- Publication number
- GB2066460A GB2066460A GB8040568A GB8040568A GB2066460A GB 2066460 A GB2066460 A GB 2066460A GB 8040568 A GB8040568 A GB 8040568A GB 8040568 A GB8040568 A GB 8040568A GB 2066460 A GB2066460 A GB 2066460A
- Authority
- GB
- United Kingdom
- Prior art keywords
- atoms
- excitation
- radiation
- microwave
- detected
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 230000005855 radiation Effects 0.000 title claims abstract description 43
- 238000000034 method Methods 0.000 title claims description 25
- 230000005284 excitation Effects 0.000 claims abstract description 30
- 230000005684 electric field Effects 0.000 claims abstract description 13
- 230000005670 electromagnetic radiation Effects 0.000 claims description 11
- 239000002800 charge carrier Substances 0.000 claims description 7
- 230000001419 dependent effect Effects 0.000 claims description 4
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 3
- 239000001257 hydrogen Substances 0.000 claims description 3
- 229910052739 hydrogen Inorganic materials 0.000 claims description 3
- 229910052693 Europium Inorganic materials 0.000 claims description 2
- 229910052793 cadmium Inorganic materials 0.000 claims description 2
- 239000011261 inert gas Substances 0.000 claims description 2
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 229910052725 zinc Inorganic materials 0.000 claims description 2
- 239000003513 alkali Substances 0.000 claims 1
- 150000002500 ions Chemical class 0.000 abstract description 7
- 125000004429 atom Chemical group 0.000 description 42
- 238000010521 absorption reaction Methods 0.000 description 8
- 230000005281 excited state Effects 0.000 description 8
- 230000007704 transition Effects 0.000 description 7
- 239000007788 liquid Substances 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 2
- 238000001514 detection method Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000010894 electron beam technology Methods 0.000 description 2
- 125000004435 hydrogen atom Chemical group [H]* 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 229910052708 sodium Inorganic materials 0.000 description 2
- 239000011734 sodium Substances 0.000 description 2
- 150000001340 alkali metals Chemical group 0.000 description 1
- 229910052784 alkaline earth metal Inorganic materials 0.000 description 1
- 150000001342 alkaline earth metals Chemical class 0.000 description 1
- 239000001307 helium Substances 0.000 description 1
- 229910052734 helium Inorganic materials 0.000 description 1
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
- 229910052753 mercury Inorganic materials 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
Abstract
Beam of atoms 12 is produced from source 10 and the atoms are brought to a predetermined state of excitation by a first excitation stage 14a and a second stage 14b including a laser. The excited atoms are then exposed to radiation 16 to be detected and certain atoms absorb radiation to be additionally excited but not ionised. The atoms are subjected by device 18 to an electric field of strength sufficient to ionise only the additionally excited atoms. The ions or electrons produced are detected and amplified by channel multiplier 20 and an output signal at 22 indicates the presence or magnitude of radiation 16. <IMAGE>
Description
SPECIFICATION
Method and Apparatus for Measuring the
Intensity of Electromagnetic Radiation in the
Microwave and Infrared Ranges
The present invention relates to a method of detecting or measuring the intensity of electromagnetic radiation in the microwave and infrared ranges. The invention also relates to an apparatus for carrying out such a method.
Detecting and measuring the intensity of longwave electromagnetic radiation, particularly in the medium and distant infrared and microwave ranges, is difficult because of the low energy of the radiation quanta. In the past it has not been possible in practice to detect individual radiation quanta in the wavelength range above approximately 10 micrometers.
Accordingly it is an object of the present invention to provide a method and apparatus for measuring the intensity of such long-wave electromagnetic radiation which are also suitable for very low radiation intensity and make it possible in practice to detect individual radiation quanta.
According to the present invention there is provided a method of detecting or measuring the intensity of electromagnetic radiation in the microwave or infrared ranches which includes bringing atoms of a predetermined element to a predetermined state of excitation of such energy that they are capable of absorbing the radiation to be measured without being ionised, subjecting the atoms to the radiation to be measured thereby additionally exciting those atoms which absorb the said radiation, subjecting the atoms to an electric field the field strength of which is such that it does ionise the additionally excited atoms but not the atoms in the predetermined state of excitation, detecting the presence of charge carriers produced in the electric field and producing an electrical output signal dependent on the number of charge carriers detected.
The invention also embraces apparatus for detecting or measuring the intensity of electromagnetic radiation in the microwave or infrared ranges including means for bringing atoms of a predetermined element to a predetermined state of excitation so that they are capable of absorbing the radiation to be measured without being ionised, means for detecting those atoms which have been additionally excited by the radiation to be detected and means for producing an electrical output signal dependent on the number of atoms detected. The apparatus preferably includes means to produce an electric field of such a strength that it will ionise the additionally excited atoms but not the atoms in the predetermined state of excitation and means to detect the presence of charge carriers produced in the electric field.
The method according to the invention makes use of microwave and/or infrared transitions between highly excited atomic energy states which are induced by the radiation to be detected. The probability of such transitions is proportional to the fourth power of the principal quantum number of the energy states between which the relevant transition occurs. Therefore, in atoms which are sufficiently highly excited, it is possible for every incident photon or radiation quantum to cause a transition. The energy change which this causes in the excited atom can be detected because the atom is subsequently ionised by an external electrical field. In highly excited states such a field ionisation can occur with relatively low field intensities which in practice are of the order of some 100 V/cm.Since the ionisation energy depends upon the energy of the excited state in which the relevant atom finds itself, the voltage producing the field can be set so that the atoms which are additionally excited by the radiation to be detected are ionised but not the atoms which are in the excited initial state before absorption of the radiation to be detected.
The field ionisation is quantitative and since every ion or electron resulting from the ionisation can be detected it is possible in the optimal case to detect every incident photon. The absorption of the photons by the atoms, which are in a highly excited initial state, is selective since in the atoms only discrete transitions are possible. However, in highly excited atoms the separations between adjacent energy levels are very small and the apparatus therefore preferably includes means for producing an external additional electric and/or magnetic field which can produce a further shift or splitting of the atomic energy levels so that a
largely continuous frequency or wavelength range can be achieved for the radiation to be detected.
Since the highly excited state of the atoms can be destroyed by collisions, free atoms are desirably used in a high vacuum, preferably in the form of an atomic beam. In addition, the surroundings of the highly excited atoms should be cooled to the lowest temperature possible, preferably less than -1500C, in order to exclude the effect of thermal radiation.
Since the life time of the highly excited states depends upon the third power of the principal quantum number these states have a relatively long duration, so that the regions where the radiation to be detected is incident and where the transitions are detected can be separated from each other.
The excitation to the highly excited state is advantageously produced via a metastable state.
For example, the22S1,2 state of hydrogen is suitable for this. Other atoms which also have suitable metastable states are, for example, the atoms of inert gases, alkaline-earth metals, the elements of the second sub-group (Zn, Cd, Hg) of the periodic table, and the elements Mn and Eu.
The highly excited initial state can be produced in various ways, preferably in a process employing two or even more stages.
In the first stage the atoms are preferably excited by a part-selective excitation process, such as electron bombardment, discharge in a particle beam or charge exchange with ions of other elements or collisions of the second type (i.e.
inelastic collisions in which the colliding particles do not simply rebound but take up some latent or internal energy. In the second stage the highly excited initial state, by which the microwave or infrared radiation to be detected is absorbed, is taken up, preferably with a continually frequencyvariable laser, e.g. a dye laser. The highly excited initial state can, of course, also be produced by a single-stage excitation process for which lasers of appropriate radiation frequency are again suitable.
In order to detect the atoms which have been additionally excited by the absorbed infrared or microwave radiation to be detected, the atoms are subjected to an electric field which can ionise those atoms which have been additionally excited by the absorption of a quantum of the radiation to be detected but not those atoms which are in the highly excited state capable of absorbing the radiation to be detected. In the field ionisation one electron and one ion are produced per atom, and the charge of each of these can be determined.
Preferably the electrons are detected by amplifying them in a secondary electron multiplier, e.g. a Channeltron (channel multiplier).
However, it is also possible to detect the ions produced.
As regards the field ionisation it should be observed that it can be influenced by the magnetic quantum numbers of the states occupied. Thus it is, for example, possible to achieve an additional discrimination of the final state by appropriate selection of the polarisation of the laser radiation producing the highly excited initial state or the radiation to be detected. In addition, atoms with specific final states of excitation (i.e. after absorption of the radiation to be detected) can be focussed by a combination of inhomogeneous electrical and magnetic fields and thus selectively detected in the atomic beam.
Thus additional selection of the final state produced by the radiation to be detected is possible.
Further features and details of the invention will be apparent from the following description of a specific embodiment of an apparatus for carrying out the method according to the invention which is given by way of example with reference to the single diagrammatic drawing.
The apparatus, which is illustrated only diagrammatically, includes an atomic beam source 10 of known type which emits an atomic beam 12 with a thermal energy distribution, that is to say the energy of the atoms has a distribution corresponding to the energy of the beam. This can be, for example, a beam of hydrogen atoms, alkali metal atoms, such as sodium, whose resonant states can be readily occupied, or any other suitable type of atoms as referred to above. The atoms of the atomic beam 12 are brought to a highly excited state by a device 14 which in this case includes two stages 1 4a and 1 4b, for example in the case of hydrogen atoms they are excited first of all to the state 22S1,2, and then the laser excitation takes place.
Next the radiation to be detected, which is
indicated diagrammatically by an arrow 1 6, is
absorbed by the highly excited atoms. In the
absorption region, or lust behind it in the direction
of radiation, a device 18 symbolised by two
condenser plates is arranged which causes an
electric field to act on the atomic beam. The size
of the electrical field is such that those atoms
which have been additionally excited by
absorption of the radiation 16 to be detected are onised by the field but those highly excited atoms
of the atomic beam which have absorbed no
radiation 1 6 are not ionised.The charge carriers
produced by the ionisation are detected in a
suitable manner, for example by constructing the
positive "condenser plate" as the input electrode
of a secondary electron multiplier 20 which
supplies an output impulse to an output terminai
22 for every electron produced by the ionisation
and absorbed by the input electrode. Alternatively
or in addition the ions produced can also be
detected.
The arrangement described above is located in
a vacuum vessel 24, only shown
diagrammatically, which is evacuated to a high
vacuum. At least the wall regions visible from the
atomic beam 12 or a separate shield (not shown)
surrounding the atomic beam are cooled to the
lowest temperature possibie, in particular the
temperature of liquid nitrogen or advantageously
of liquid hydrogen or liquid helium, in order to
keep the effect of thermal background radiation
on the atoms in their highly excited states as low
as possible.
The band width of the optical-electrical
converter device according to the invention is
provided by the absorption band width of the
highly excited atoms in the initial state. If a well
collimated atomic beam is used the residual
Doppler broadening of the beam is low,
particularly when the frequency corresponding to
the transitions lies in the microwave or infrared
frequency range. The absorption band width then
depends substantially upon the interaction of the
radiation to be traced with the highly excited
atoms and is in practice approximately 100 kHz.
This basically provides the possibility of a narrow
band radiation detection. However, the band
width can also be adapted to the prevailing
requirement by broadening the states by means
of an external inhomogeneous magnetic field.
In one specific embodiment of the apparatus in
accordance with the invention the atomic beam
source 10 included an oven filled with sodium
which produces an atomic beam 12 having an
energy of the order of 0.01 to 0.05 eV. The first excitation stage 14a was an electron beam
producing system including an indirectly heated
cathode and an accelerating electrode between which there was an adjustable positive voltage.
The electron beam producing system produced substantially monoenergetic electrons with an adjustable energy of the order of 10 eV. The second excitation stage 14b included a tunable laser which produced a beam of about 410 nanometer wavelength. The wavelength of the beam was slightly cyclically varied. The voltage between the electrode 1 8 and the input electrode of the secondary electron multiplier 20 which serves to detect ions was about 200 to 500 volts and was so adjusted that only additionally excited atoms which have absorbed a quantum of the radiation 1 6 to be detected were ionised. The apparatus was successfully used for the detection of a beam with a wavelength of 3mm.
Claims (21)
1. A method of detecting or measuring the intensity of electromagnetic radiation in the microwave or infrared ranges which includes bringing atoms of a predetermined element to a predetermined state of excitation of such energy that they are capable of absorbing the radiation to be measured without being ionised, subjecting the atoms to the radiation to be measured thereby additionally exciting those atoms which absorb the said radiation, subjecting the atoms to an electric field the field strength of which is such that it does ionise the additionally excited atoms but not the atoms in the predetermined state of excitation, detecting the presence of charge carriers produced in the electric field and producing an electrical output signal dependent on the number of charge carriers detected.
2. A method as claimed in Claim 1 in which the predetermined state of excitation is produced by at least two successive excitation processes.
3. A method as claimed in Claim 1 or Claim 2 in which the excitation process includes excitation by laser radiation.
4. A method as claimed in Claim 2 or Claim 3 in which the first excitation stage comprises a part-selective excitation process.
5. A method as claimed in any one of Claims 2 to 4 in which the atoms are brought to a metastable state of excitation in the first excitation stage.
6. A method as claimed in any one of the preceding claims in which the electromagnetic radiation is in the microwave or distant infrared ranges.
7. A method as claimed in any one of the preceding claims in which the atoms are present in a high vacuum.
8. A method as claimed in any one of the preceding claims in which the atoms are present in the form of an atomic beam.
9. A method as claimed in any one of the preceding claims in which the atoms and at least a part of their surroundings are cooled to a temperature below -1 500C.
10. A method as claimed in anyone of the preceding claims in which the atoms are inert gas atoms, alkaline-earth atoms or atoms of one of the elements Zn, Cd, Mn, Eu and hydrogen or alkali atoms whose resonant states can be readily populated.
11. Apparatus for detecting or measuring the intensity of electromagnetic radiation in the microwave or infrared ranges including means for bringing atoms of a predetermined element to a predetermined state of excitation that they are capable of absorbing the radiation to be measured without being ionised, means for detecting those atoms which have been additionally excited by the radiation be detected and means for producing an electrical output signal dependent on the number of atoms detected.
12. Apparatus as claimed in Claim 11 including means to produce an electric field of such a strength that it will ionise the additionally excited atoms but not the atoms in the predetermined state of excitation and means to detect the presence of charge carriers produced in the electric field.
13. Apparatus as claimed in Claim 11 or Claim 12 including an atomic beam source.
14. Apparatus as claimed in any one of Claims 11 to 13 in which the means for bringing the atoms to the predetermined state of excitation includes at least one tunable laser.
1 5. Apparatus as claimed in any one of Claims 11 to 14 in which the means for bringing the atoms to the predetermined state of excitations includes at least two excitation stages.
1 6. Apparatus as claimed in Claim 1 5 in which the first excitation stage comprises a partselective excitation stage.
1 7. Apparatus as claimed in any one of Claims 11 to 16 including a vacuum chamber.
18. Apparatus as claimed in Claim 17 in which the walls of the vacuum chamber are adapted to be cooled to a temperature below --1 500C.
19. Apparatus as claimed in any one of Claims 11 to 18 including means for the production of an additional electrical and/or magnetic field to shift the energy level of the atoms.
20. A method of measuring the intensity of electromagnetic radiation in the microwave or infrared ranges substantially as specifically herein described with reference to the accompanying drawing.
21. Apparatus for measuring the intensity of electromagnetic radiation in the microwave or infrared ranges substantially as specifically herein described with reference to the accompanying drawing.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19792950996 DE2950996C2 (en) | 1979-12-18 | 1979-12-18 | Method and device for measuring the intensity of electromagnetic radiation with a wavelength in the microwave and infrared spectral range |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2066460A true GB2066460A (en) | 1981-07-08 |
| GB2066460B GB2066460B (en) | 1984-06-20 |
Family
ID=6088845
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8040568A Expired GB2066460B (en) | 1979-12-18 | 1980-12-18 | Method and apparatus for detecting or measuring the intensiy of microwave or infrared radiation |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS5694225A (en) |
| DE (1) | DE2950996C2 (en) |
| FR (1) | FR2472189B1 (en) |
| GB (1) | GB2066460B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4600840A (en) * | 1985-02-21 | 1986-07-15 | The United States Of America As Represented By The Administrator, National Aeronautics And Space Administration | Double photon excitation of high-rydberg atoms as a long-lived submillimeter detector |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR811164A (en) * | 1935-12-27 | 1937-04-08 | Radiologie Cie Gle | Device for measuring the quantities and intensities of electromagnetic radiation |
| US2877417A (en) * | 1955-11-28 | 1959-03-10 | Itt | Gas tube microwave detector |
| US3423679A (en) * | 1963-09-30 | 1969-01-21 | Lyons & Co Ltd J | Detector means for indicating a predetermined intensity of radiation |
| GB1523003A (en) * | 1976-11-19 | 1978-08-31 | Hartnagel H L | Microwave monitor |
-
1979
- 1979-12-18 DE DE19792950996 patent/DE2950996C2/en not_active Expired
-
1980
- 1980-12-17 FR FR8026759A patent/FR2472189B1/en not_active Expired
- 1980-12-18 GB GB8040568A patent/GB2066460B/en not_active Expired
- 1980-12-18 JP JP17812680A patent/JPS5694225A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4600840A (en) * | 1985-02-21 | 1986-07-15 | The United States Of America As Represented By The Administrator, National Aeronautics And Space Administration | Double photon excitation of high-rydberg atoms as a long-lived submillimeter detector |
Also Published As
| Publication number | Publication date |
|---|---|
| DE2950996C2 (en) | 1986-03-20 |
| DE2950996A1 (en) | 1981-06-25 |
| JPS5694225A (en) | 1981-07-30 |
| FR2472189B1 (en) | 1985-11-15 |
| GB2066460B (en) | 1984-06-20 |
| FR2472189A1 (en) | 1981-06-26 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |