Detecting Particles
Detecting Particles The primary means of detecting a (charged) particle is the detecting the ionization it produces in matter. The ionization can be detected as raw charge (with sufficiently sensitive electronics). High-voltage gas-discharge processes can amplify the charge, allowing cheaper electronics. The ionization (and non-ionizing atomic excitation) also produces photons. This scintillation light can also be detected. Neutral particles are detected only by the (charged) reaction products when they interact with matter. Phys 400 Lecture 4
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Photographic Emulsions Radioactivity was discovered when Becquerel found that uranium caused film to fog. Ionization (and also visible light) causes opaque silver grains to precipitate out of transparent gel loaded with silver-bromide. Charged particles leave tracks of grains in nuclear emulsion. The resolution is at the micron level. Energy (if low enough!) can be measured by range. The ionization, thus βγ, can be estimated by the grain density. This method was important in early cosmic ray work, and is still occasionally used to measure picosecond lifetimes. Phys 400 Lecture 4
2
slow K+ track
Slow pion interacts with emulsion nucleus making 2 charged fragments
fast pion track
K+ stops and decays into 3 charged pions fast pion track
Phys 400 Lecture 4
3
Cloud Chamber Ionization will cause droplets to form in a gas that is supersaturated with vapor. Cooling by rapid decompression will cause fog-tracks.
Cloud chambers let you see longer tracks, so magnetic curvature is visible. Phys 400 Lecture 4
4
Bubble Chamber Ionization will cause bubbles to form in a liquid that is rapidly decompressed to below the vapor pressure at its given temperature. Because liquids are 1000 times denser than gases, the liquid itself can be the target. Usually liquid hydrogen is used because its the simplest target (pure protons). A magnetic field bends the tracks for momentum measurement. Phys 400 Lecture 4
5
Phys 400 Lecture 4
6
Magnetic Deflection Charged particles moving in a magnetic field are deflected by the Lorentz force F = qv × B . In a uniform magnetic field, this causes a particle to travel in a circle (actually a helix if there is a velocity component parallel to the field). There are two complications in using this: most of the particles we care about are relativistic, and we want to use MeV units instead of MKS units. In relativity, it’s no longer true that F = ma , but it turns out dp that F = dt still works. And for circular motion, vdt dp dθ pv R =p =p = dt dt dt R Phys 400 Lecture 4
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Magnetic Deflection (2) dp pv Combining equations gives F = qvB = = dt R
Cancel the v and rearrange to get
qBR = p
Both sides of this are MKS units, with dimensions momentum. Multiply both sides by c to make both sides energy, still in MKS units (Joules). qBRc = pc The number for pc in Joules is the number for pc in eV units, −19 1.602 × 10 times the conversion factor which is by definition the charge of the electron in MKS units. Phys 400 Lecture 4
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Magnetic Deflection (3) Cancel the q on the left with the conversion factor on the right
( BRc )MKS = ( pc )eV This says that if you have a magnetic field of 1 Tesla, and track curvature radius is 1 meter, then the pc of the track is 3x108 electron volts, or 300 MeV. The momentum of such a track is 300 MeV/c. Basically, we absorb the c on the right into the units, but the c on the left is still there. Phys 400 Lecture 4
9
Ionization Chamber Ionization means free electrons and an equal number of positive gas ions. If there is an electric field, the electrons drift toward the + electrode & the ions drift toward the – electrode. The charge that flows through the external circuit is equal to the charge released in the gas by ionization (assuming none of the electron-ion pairs recombine). There is roughly one electron-ion pair per 30 eV energy loss in typical gases. This is 10-300 e-ion pairs per cm at NTP, depending on the gas type. 10 Phys 400 Lecture 4
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28. Detectors at accelerators
Table 28.5: Properties of noble and molecular gases at normal temperature and pressure (NTP: 20◦ C, one atm). EX , EI : first excitation, ionization energy; WI : average energy per ion pair; dE/dx|min, NP , NT : differential energy loss, primary and total number of electron-ion pairs per cm, for unit charge minimum ionizing particles. Gas He Ne Ar Xe CH4 C2 H 6 iC4 H10 CO2 CF4
Density, mg cm−3
Ex eV
EI eV
WI eV
dE/dx|min keV cm−1
NP cm−1
0.179 0.839 1.66 5.495 0.667 1.26 2.49 1.84 3.78
19.8 16.7 11.6 8.4 8.8 8.2 6.5 7.0 10.0
24.6 21.6 15.7 12.1 12.6 11.5 10.6 13.8 16.0
41.3 37 26 22 30 26 26 34 54
0.32 1.45 2.53 6.87 1.61 2.91 5.67 3.35 6.38
3.5 13 25 41 28 48 90 35 63
NT cm−1 8 40 97 312 54 112 220 100 120
When an ionizing particle passes through the gas it creates electron-ion pairs, but often the ejected electrons have sufficient energy to further ionize the medium. As shown
Phys 400 Lecture 4
11
Geiger Tube If one electrode is a cylinder and the other is a thin wire, the E field near the wire is big enough that drifting electrons can ionize other atoms, which can ionize more... A single particle can then cause a large pulse, which can be counted. Geiger tubes were invented in 1908, and are still used in radiation meters. Phys 400 Lecture 4
12
Multi-Wire Proportional Counter You can have many positive wires between parallel ground plates, with each wire being a separate detector. You can also use wires for the ground electrodes instead of plates. Modern drift chambers have tens thousands of wires and and can detect hundreds of simultaneous tracks. Phys 400 Lecture 4
13
1
Photographic Emulsions Radioactivity was discovered when Becquerel found that uranium caused film to fog. Ionization (and also visible light) causes opaque silver grains to precipitate out of transparent gel loaded with silver-bromide. Charged particles leave tracks of grains in nuclear emulsion. The resolution is at the micron level. Energy (if low enough!) can be measured by range. The ionization, thus βγ, can be estimated by the grain density. This method was important in early cosmic ray work, and is still occasionally used to measure picosecond lifetimes. Phys 400 Lecture 4
2
slow K+ track
Slow pion interacts with emulsion nucleus making 2 charged fragments
fast pion track
K+ stops and decays into 3 charged pions fast pion track
Phys 400 Lecture 4
3
Cloud Chamber Ionization will cause droplets to form in a gas that is supersaturated with vapor. Cooling by rapid decompression will cause fog-tracks.
Cloud chambers let you see longer tracks, so magnetic curvature is visible. Phys 400 Lecture 4
4
Bubble Chamber Ionization will cause bubbles to form in a liquid that is rapidly decompressed to below the vapor pressure at its given temperature. Because liquids are 1000 times denser than gases, the liquid itself can be the target. Usually liquid hydrogen is used because its the simplest target (pure protons). A magnetic field bends the tracks for momentum measurement. Phys 400 Lecture 4
5
Phys 400 Lecture 4
6
Magnetic Deflection Charged particles moving in a magnetic field are deflected by the Lorentz force F = qv × B . In a uniform magnetic field, this causes a particle to travel in a circle (actually a helix if there is a velocity component parallel to the field). There are two complications in using this: most of the particles we care about are relativistic, and we want to use MeV units instead of MKS units. In relativity, it’s no longer true that F = ma , but it turns out dp that F = dt still works. And for circular motion, vdt dp dθ pv R =p =p = dt dt dt R Phys 400 Lecture 4
7
Magnetic Deflection (2) dp pv Combining equations gives F = qvB = = dt R
Cancel the v and rearrange to get
qBR = p
Both sides of this are MKS units, with dimensions momentum. Multiply both sides by c to make both sides energy, still in MKS units (Joules). qBRc = pc The number for pc in Joules is the number for pc in eV units, −19 1.602 × 10 times the conversion factor which is by definition the charge of the electron in MKS units. Phys 400 Lecture 4
8
Magnetic Deflection (3) Cancel the q on the left with the conversion factor on the right
( BRc )MKS = ( pc )eV This says that if you have a magnetic field of 1 Tesla, and track curvature radius is 1 meter, then the pc of the track is 3x108 electron volts, or 300 MeV. The momentum of such a track is 300 MeV/c. Basically, we absorb the c on the right into the units, but the c on the left is still there. Phys 400 Lecture 4
9
Ionization Chamber Ionization means free electrons and an equal number of positive gas ions. If there is an electric field, the electrons drift toward the + electrode & the ions drift toward the – electrode. The charge that flows through the external circuit is equal to the charge released in the gas by ionization (assuming none of the electron-ion pairs recombine). There is roughly one electron-ion pair per 30 eV energy loss in typical gases. This is 10-300 e-ion pairs per cm at NTP, depending on the gas type. 10 Phys 400 Lecture 4
24
28. Detectors at accelerators
Table 28.5: Properties of noble and molecular gases at normal temperature and pressure (NTP: 20◦ C, one atm). EX , EI : first excitation, ionization energy; WI : average energy per ion pair; dE/dx|min, NP , NT : differential energy loss, primary and total number of electron-ion pairs per cm, for unit charge minimum ionizing particles. Gas He Ne Ar Xe CH4 C2 H 6 iC4 H10 CO2 CF4
Density, mg cm−3
Ex eV
EI eV
WI eV
dE/dx|min keV cm−1
NP cm−1
0.179 0.839 1.66 5.495 0.667 1.26 2.49 1.84 3.78
19.8 16.7 11.6 8.4 8.8 8.2 6.5 7.0 10.0
24.6 21.6 15.7 12.1 12.6 11.5 10.6 13.8 16.0
41.3 37 26 22 30 26 26 34 54
0.32 1.45 2.53 6.87 1.61 2.91 5.67 3.35 6.38
3.5 13 25 41 28 48 90 35 63
NT cm−1 8 40 97 312 54 112 220 100 120
When an ionizing particle passes through the gas it creates electron-ion pairs, but often the ejected electrons have sufficient energy to further ionize the medium. As shown
Phys 400 Lecture 4
11
Geiger Tube If one electrode is a cylinder and the other is a thin wire, the E field near the wire is big enough that drifting electrons can ionize other atoms, which can ionize more... A single particle can then cause a large pulse, which can be counted. Geiger tubes were invented in 1908, and are still used in radiation meters. Phys 400 Lecture 4
12
Multi-Wire Proportional Counter You can have many positive wires between parallel ground plates, with each wire being a separate detector. You can also use wires for the ground electrodes instead of plates. Modern drift chambers have tens thousands of wires and and can detect hundreds of simultaneous tracks. Phys 400 Lecture 4
13
