Presentation on theme: "880.P20 Winter 2006 Richard Kass 1 Scintillation Devices As a charged particle traverses a medium it excites the atoms (or molecules) in the the medium."— Presentation transcript:
880.P20 Winter 2006 Richard Kass 1 Scintillation Devices As a charged particle traverses a medium it excites the atoms (or molecules) in the the medium. In certain materials called scintillators a small fraction of the energy released when the atoms or molecules de-excite goes into light. ENERGY IN LIGHT OUT The use of materials that scintillate is one of the most common experimental techniques in physics. Used by Rutherford in his scattering experiments Scintillation light can be used to: Signal the presence of a charged particle Measure the time it takes for a charged particle to travel a known distance (“time of flight technique”) Measure energy since the amount of light is proportional to energy deposition There are lots of different types of materials that scintillate: non-organic crystals (NaI, CsI, BGO) organic crystals (Anthracene) Organic plastics (see table on next page) Organic liquids (toluene, xylene) Our atmosphere (nitrogen)
880.P20 Winter 2006 Richard Kass 2 Scintillators A typical plastic Scintillator system violet blue Emission spectrum of NE102A Plastic scintillator Properties of common plastic scintillators Typical cost 1$/in 2
880.P20 Winter 2006 Richard Kass 3 Photomultiplier Tubes We need a way to convert the scintillation photons into an electrical signal. Photons photoelectric effect electrons Use a photomultiplier tube to convert scintillation light into electrical current Properties of phototubes: very high gain, low noise current amplifier gains 10 6 possible possible to count single photons Off the shelf item, buy from a company wide variety to choose from (size, gain, sensitivity) tube costs range from $10 2 -$10 3 Sensitive to magnetic fields (shield against earth’s): use “mu-metal” In situations where a lot of light is produced (>10 3 photons) a photodiode can be used in place of a phototube, e.g. BaBar’s calorimeter Quantum efficiency of bialkali cathode vs wavelength violet blue green Electric field accelerates electrons Electrons crash into dynodes create more electrons light e’s
880.P20 Winter 2006 Richard Kass 4 Scintillation Counter Example Some typical parameters for a plastic scintillation counter are: energy loss in plastic scintillator:2MeV/cm scintillation efficiency of plastic:1 photon/100 eV collection efficiency (# photons reaching PMT):0.1 quantum efficiency of PMT0.25 What size electrical signal can we get from a plastic scintillator 1 cm thick? A charged particle passing perpendicular through this counter: deposits 2MeV which produces 2x10 4 ’s of which 2x10 3 ’s reach PMT which produce 500 photo-electrons Assume the PMT and related electronics have the following properties: PMT gain=10 6 so 500 photo-electrons produces 5x10 8 electrons =8x C Assume charge is collected in 50nsec (5x10 -8 s) current=dq/dt=(8x coulombs)/(5x10 -8 s)=1.6x10 -3 A Assume this current goes through a 50 resistor V=IR=(50 )(1.6x10 -3 A)=80mV (big enough to see with O’scope) So a minimum ionizing particle produces an 80mV signal. What is the efficiency of the counter? How often do we get no signal (zero PE’s)? The prob. of getting n PE’s when on average expect is a Poisson process: The prob. of getting 0 photons is e - =e -500 0. So this counter is 100% efficient. Note: a counter that is 90% efficient has =2.3 PE’s
880.P20 Winter 2006 Richard Kass 5 Time of flight with Scintillators Time of Flight (TOF) is a particle identification technique. measure particle speed and momentum determine mass t=x/v=x/( c) with =pc/E=pc/[(mc 2 ) 2 +(pc) 2 ] 1/2 Consider two particles with different masses but same momentum: For high momentum (e.g. p>1 GeV/c for ’s): t 1 +t 2 =2t and x/t c Actually, we measure the time it takes for the particle to travel a known distance. x
880.P20 Winter 2006 Richard Kass 6 Time of Flight with Scintillators As an example, assume m 1 =m (140MeV), m 2 =m k (494MeV), and x=10m t=3.8 nsec for p=1 GeV t=0.95 nsec for p=2GeV Time resolution of a “good” TOF system is 150ps (0.15 ns) Scintillator+phototubes are capable of measuring such small time differences In colliding beam experiments, 0.5
880.P20 Winter 2006 Richard Kass 7 Photoelectric Effect absorbed by material, electron ejected Compton Scattering e - → e - “elastic scattering” Pair Production →e + e - creates anti-matter Basic Physics Processes in a Sodium Iodide (NaI) Calorimeter e-e- e-e- e-e- e+e+ hv < 0.05 MeV 0.05 < hv < 10 MeV hv > 10 MeV -ray must have E>2m e The amount of light given off by NaI is proportional to the amount energy absorbed. The light yield is ~ 1 photon per 25 eV deposited in NaI, max =415 nm, decay time ~250nsec NaI radiation length of NaI ~2.5 cm but only useful for E > few MeV NaI is often used to measure the energy low gamma rays Attenuation of the gamma rays is energy dependent
880.P20 Winter 2006 Richard Kass 8 NaI & Homeland Security
880.P20 Winter 2006 Richard Kass 9 Example: Cs137 -ray Spectrum in NaI E =662keV photopeak backscatter E =184keV K-shell x-rays E ~35 keV Compton scatterings Compton Edge energy decay E =662keV decay gives off electrons with a range of energies Emax = 514 keV, 1170 keV decay gives off a monchromatic photon E = 662 keV decay Cs137 e-e backscatter forward scattered electron energy resolution: E 662KeV NaI crystal ~ 5cm X 5cm
880.P20 Winter 2006 Richard Kass 10 NaI is a Dirty Bomb Detector
880.P20 Winter 2006 Richard Kass 11 What’s in Your Air? I set up a NaI counter in PRB3153 and took data for 24 hours. Find lots of -ray peaks Use ROOT to fit the -ray peaks to a Gaussian (signal) + linear background Pb214, Bi214 are Radon (Rn222) by-products (~1pc/L in PRB3153) K40 is common in many building materials (and bananas) TL208 (Thallium 208) is from Rn220 Energy (keV) Pb214 Bi214 K40 Bi214 TL208 Bi214
880.P20 Winter 2006 Richard Kass 12 Cerenkov Light The Cerenkov effect occurs when the velocity of a charged particle traveling through a dielectric medium exceeds the speed of light in the medium. Index of refraction (n) = (speed of light in vacuum)/(speed of light in medium) Will get Cerenkov light when: Angle of Cerenkov Radiation: For water n=1.33, will get Cerenkov light if v > 2.25x10 10 cm/s No radiation radiation ct (c/n)t In a time t wavefront moves (c/n)t but particle moves ct. Huyghen’s wavefronts speed of particle > speed of light in medium
880.P20 Winter 2006 Richard Kass 13 Threshold Momentum for Cerenkov Radiation Example: Threshold momentum for Cerenkov light: For gases it is convenient to let =n-1. Then we have: The momentum (p t ) at which we get Cerenkov radiation is: For a gas +2 so the threshold momentum can be approximated by: For helium =3.3x10 -5 so we find the following thresholds: electrons63 MeV/ckaons61 GeV/c pions17 GeV/cprotons115GeV/c Medium =n-1 t helium3.3x CO 2 4.3x H 2 O glass
880.P20 Winter 2006 Richard Kass 14 Number of photons from Cerenkov Radiation From classical electrodynamics (Frank&Tamm 1937, Nobel Prize 1958) we find the following for the energy loss per wavelength ( ) per dx for charge=1, n>1: With =fine structure constant, n( ) the index of refraction which in general depends on the wavelength ( ) of light. We can re-write the above in terms of the number of photons (N) using: dN=dE/E For example see Jackson section 13.5 We can simplify the above by considering a region were n( ) is a constant=n: We can calculate the number of photons/dx by integrating over the wavelengths that can be detected by our phototube ( 1, 2 ): Note: if we are using a phototube with a photocathode efficiency that varies as a function of then we have:
880.P20 Winter 2006 Richard Kass 15 Number of photons from Cerenkov Radiation For a typical phototube the range of wavelengths ( 1, 2 ) is (350nm, 500nm). We can simplify using: For a highly relativistic particle going through a gas the above reduces to: For He we find:2-3 photons/meter (not a lot!) For CO 2 we find:~33 photons/meter For H 2 Owe find:~34000 photons/meter GAS Photons are preferentially emitted at small ’s (blue) For most Cerenkov counters the photon yield is limited (small) due to space limitations, the index of refraction of the medium, and the phototube quantum efficiency.
880.P20 Winter 2006 Richard Kass 16 Types of Cerenkov Counters There are three different types of Cerenkov counters used to identify particles. Listed in order of their sophistication they are: Threshold counter (on/off device) Differential counter (makes use of the angle of the Cerenkov radiation) Ring imaging counter (makes use of the “cone” of light) Each of the above counter is designed to work in a certain momentum range. Threshold counter: Identify the particle(s) which give off light. Can use to separate electrons from heavier particles ( , K, p) since electrons will give off light at a much lower momentum (e.g. 68 MeV/c vs 17 GeV/c for He) Problems with device: above a certain momentum several particles will give light. usually threshold counters use gas which implies low light levels (n-1 small) low light levels leads to inefficiency, e.g. =3, the prob. of zero photons: P(0)=e -3 =5%! Phototubes must be shielded from magnetic fields above a few tenths of a gauss.
880.P20 Winter 2006 Richard Kass 17 Types of Cerenkov Counters Differential Cerenkov Counter: Makes use of the angle of Cerenkov radiation and only samples light at certain angles. For fixed momentum cos is a function of mass: Not all light will make it to phototube Differential cerenkov counters typically on work over a fixed momentum range (good for beam monitors, e.g. measure or K content of beam). Problems with differential Cerenkov counters: Optics are usually complicated. Have problems in magnetic fields since phototubes must be shielded from B-fields above a few tenths of a gauss.
880.P20 Winter 2006 Richard Kass 18 Ring Imaging Cerenkov Counters (RICH) RICH counters use the cone of the Cerenkov light. The ½ angle ( ) of the cone is given by: The radius of the cone is: r=Ltan , with L the distance to the where the ring is imaged. L r For a particle with p=1GeV/c, L=1 m, and LiF as the medium (n=1.392) we find: deg r(m) K P Thus by measuring p and r we can identify what type of particle we have. Problems with RICH: optics very complicated (projections are not usually circles) readout system very complicated (e.g. wire chamber readout, channels) elaborate gas system photon yield usually small (10-20), only a few points on “circle” Great /K/p separation!
880.P20 Winter 2006 Richard Kass 19 CLEO’s Ring imaging Cerenkov Counter The figures below show the CLEO III RICH structure. The radiator is LiF, 1 cm thick, followed by a 15.7 cm expansion volume and photon detector consisting of a wire chamber filled with a mixture of TEA and CH4 gas. TEA is photosensitive. The resulting photoelectrons are multiplied by the HV on the wires and the resulting signals are sensed by a rectangular array of pads coupled with highly sensitive electronics. Challenge is to separate ’s from K’s in the range 1.5
880.P20 Winter 2006 Richard Kass 20 CLEO’s Ring imaging Cerenkov Counter Lithium Floride (LiF) radiator Assembled radiators. They are guarded by Ray Mountain. Without Ray “living”at the factory that produced the LiF radiators we would still be waiting for the order to be completed. A photodetector: CaF 2 window+cathode pads Assembled photodetectors
880.P20 Winter 2006 Richard Kass 21 Performance of CLEO’s RICH Number of detected photons on 5 GeV electrons A track in the RICH D*’s without/with RICH information Preliminary data on /K separation
880.P20 Winter 2006 Richard Kass 22 The BaBar DIRC Here the challenge is to separate ’s and K’s in the range: 1.7
880.P20 Winter 2006 Richard Kass 23 The BaBar DIRC 1.5 T Solenoid Electromagnetic Calorimeter (EMC) Detector of Internally Recflected Cherenkov Light (DIRC) Instrumented Flux Return (IFR) Silicon Vertex Tracker (SVT) Drift Chamber (DCH) e - (9 GeV) e + (3.1 GeV) phototube array
880.P20 Winter 2006 Richard Kass 24 Performance of the BaBar DIRC Timing information very useful to eliminate photons not associated with a track 300 nsec window background hits 8 nsec window 1-2 background hits Note: the pattern of phototubes with signals is very complicated. The detection surface is toroidal and therefore the cerenkov rings are disjoint and distorted. Use a maximum likelihood analysis to separate /K/p: L=L( c, t, n ) DIRC works very well!
880.P20 Winter 2006 Richard Kass 25 SuperK 481 MeV muon neutrino produces 394 MeV muon which later decays at rest into 52 MeV electron. The ring fit to the muon is outlined. Electron ring is seen in yellow-green in lower right corner. This is perspective projection with 110 degrees opening angle, looking from a corner of the Super-K detector (not from the event vertex). Color corresponds to time PMT was hit by Cerenkov photon from the ring. Color scale is time from 830 to 1816 ns with 15.9 ns step. In the charge weighted time histogram to the right two peaks are clearly seen, one from the muon, and second one from the delayed electron from the muon decay. Size of PMT corresponds to amount of light seen by the PMT. From: SuperK is a water RICH. It uses phototubes to measure the Cerenkov ring. Phototubes give time and pulse height information From SuperK site SuperK has: 50 ktons of H 2 O Inner PMTS: 1748 (top and bottom) and 7650 (barrel) outer PMTs: 302 (top), 308 (bottom) and 1275(barrel) For water n=1.33 For =1 particle cos =1/1.33, =41 o
880.P20 Winter 2006 Richard Kass 26 Askaryan Effect Radio Frequency Cerenkov Radiation Askaryan Effect: EM showers in a dielectric medium generate coherent radio cerenkov emission From: D. Saltzberg, Orion Workshop An EM shower propagating in air+pb In EM shower there will be more e - ’s than e + ’s ( ~20% ), a net current which can radiate. No radiation if exactly same amount of + and - charges Excess charge moving faster than speed of light will emit cerenkov radiation. In ice the peak frequency of radiation ~ 2 GHz ( ~15 cm). The radiation is coherent ( rad lateral shower size) and power ~ E 2 Possible to observe very high interactions in ice (or salt) Radiation is linearly polarized predicted 1962, observed 2000 Saltzberg, et al, Phys.Rev.Lett. 86 (2001)
880.P20 Winter 2006 Richard Kass 27 Radio Frequency Cerenkov Radiation from Ice From: Andrea Silvestri, UCI, International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily
880.P20 Winter 2006 Richard Kass 28 ANITA Experiment Antarctic Impulsive Transient Antenna From: Andrea Silvestri, UCI, presented at International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily ANITA is an experiment designed to detector ultra high energy neutrino interactions