PID: ToF principle. PID: ToF principle

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1 16 16 PID: ToF priniple PID: ToF priniple It requires very good time resolution and a suffiient path L Given partiles m 1 and m with veloities β 1 and β : Sine from p = γmv = γmβ and E = γm Given a beam of partiles with the same momentum from Δt measurement we separate the speies In our example: given one partile rossing two sintillators if we know p and measure Δt and L we get m t 1 t L + + = = = Δ p m p m L L L t t t β β p m p m p p E + = + = = β 1 1 p m L L t t t + = = = Δ β

2 Sintillators in brief Convert energy deposited by a harged partiles or high energy photons into light: atoms or moleules of the sintillating medium are exited and deay emitting photons whih are deteted and onverted into eletri signals (PMTs) Sintillating materials: organi: aromati hydroarbon ompounds, solid rystals, plastis or liquids, typially they are faster but have lower light yield. inorgani: ioni rystals doped with ativator enters or glasses; typially they have larger light yields Two types of light emission: Fluoresene: prompt ns µs in visible wavelength range, temperature independent (omponent useful for partile detetion) Phosphoresene: emission over longer period µs ms, hrs with longer wavelength and temperature dependent 17

3 Properties of sintillators Parameters haraterizing sintillators: Effiieny R s = average n. of emitted photons/energy of inident radiation Sintillation yield = R s hν hν = energy of emitted photons Time response: depends on deay time of fast omponent the sintillator should be transparent to its own sintillation light (Stokes shift: the emission wavelength is longer than absorption one) linearity 18

4 Organi sintillators Organi sintillators (plastis or liquids) are omposed of aromati hydroarbon ompounds. Plasti ones are non-fluid solutions onsisting of fluoresent organi ompounds dissolved in a polymer matrix. Liquid sintillators are fluid solutions with similar fluoresent ompounds. No rystal struture is needed. The emission of light is due to exitation of moleular levels in a primary fluoresent material that emits UV light during de-exitation. This light is absorbed in most organi materials with an absorption length of ~mm. The extration of a light signal beomes possible only by introduing a seond fluoresent material in whih the UV light is onverted into visible light (wavelength shifter). Organi sintillators are typially made of low Z materials and have low density. Hene the main interation >0keV proess is not photoeletri absorption (suh as in the ase of inorgani sintillators) but Compton sattering.typially, beause of the low density more volume is required to obtain a reasonable detetion effiieny, but they have low ost. 19

5 Organi sintillators Energy levels of organi sintillators: at room T a (KT a = 0.05 ev) eletrons on ground state, inident radiation exites eletrons to S 1 states, radiationless deays to base S 1 state, emission of light to S 0. S 1 an deay to a triplet state with lower energy and longer deay time. The fluoresene UV light ( nm) is absorbed by most organi materials, so the light signal an be extrated using a seond fluoresent material (wavelength shifter) that onverts UV in visible light ( nm) 0

6 Time dependene of emitted light Non-radiative transfer of energy from vibrational states to fluoresene state S 1 : ns Deay of fluoresent state: 1-3 ns τ r = rise time onstant Fall with time onstant τ f The deay time depends on the ionization density 1

7 Birk s s law For organi sintillators the relation between emitted light and energy loss is not linear. Deviations from linearity are due to quenhing interations between exited moleules reated along the ionizing partile path absorbing energy or The light output depends on the ionization density Density of ionized and exited moleules along trak Quenhing parameter

8 Inorgani sintillators They are usually made of high Z materials and have a fairly high density. The high Z enhanes the photoeletri interation ontribution and the high density inreases the interation effiieny. They are rystals grown in high temperature furnaes and are made of Alkali Halides (ie. NaI, CsI) or Oxides (eg. BGO). They have sintillation properties by virtue of their rystalline struture that reates the energy bands between whih eletrons an jump. Some rystals need ativators to enable emission in the visible (eg Thallium whih is used in the most frequently used inorgani sintillator NaI(Tl). Ionizing partiles produe free eletrons, holes and ouples of eletron-holes (exitons). These move around the rystal lattie until they meet an ativator enter that they transform into an exited state A* of energy E 1 that an deay emitting light. The deay time depends on the temperature as exp[-e 1 /(KT)] Energy level of a ioni rystal doped with an ativator Condution band Ativator Exitons Valene band 3

9 Sintillators oupled to PMTs A plasti light guide ouples the large ross setion sintillator to the small area PMT. The light emitted by the sintillator is transported to the photoathode through the light guide through total internal refletions (when the inident angle is larger than the ritial one θ = sin -1 (n med /n) n med =1 for air) Typially frations of trapped light are around 0%, the sintillator and light guide may be wrapped of Aluminium foil to prevent leaks of light esaping internal refletions. They are wrapped of blak layers to prevent outside light to enter. The first PMT was built in 1913 by Elster and Geiter RCA made them ommerial lin

10 Spetral regions and Units Wavelength (nm) Spetral range Frequeny (Hz) Energy (ev) X-ray Soft X-ray Extreme UV UV Visible Near IR IR Far IR

11 Phototubes Transform a light signal into an eletri one: photons hitting the thin photoathode layer of an evaquated glass or quartz tube extrat photoeletrons (photoeleti effet) The energy of the emitted eletron is E = hν - Φ, ν = frequeny of photon and Φ = work funtion of the photoathode Photoathodes are semionduting alloys ontaining 1 or more metals from the alkali group (Na, K, Cs) and materials from group V (eg Sb - antimony). Bialkali athodes are made of alkali omponents. QE(ν) = #pe emitted by the photoathode/inident photons ~ 0-5% at maximum (λ ~ nm) ANTARES 10 inh Hamamtsu 14 stages 6

12 Dynode strutures The olletion effiieny of the 1st dynode = # of eletrons hitting it/#of emitted photoeletrons ~ 60-90% Seondary emissive materials used for dynodes are alkali Venetian blind antimonide, Be or Mg oxyde (BeO). Gallium phosphide (GaP) and gallium arsenide phospied. Box and grid δ=seondary emission ratio= #of seondary e - per primary e - Varies for various materials as a funtion of the Linear-foused aelerating voltage of primary e - Seondary emissive surfae Cirular age 7

13 IeCube 10 inh Hamatsu 10 stages Gain V KN N = # dynodes K = depends on struture and material of eletrodes Photosensors pe s undergo an amplifiation hitting the dynodes Dynodes are made of materials with high seondary eletron emission (BeO, Mg-O-Cs) and are kept at potential differenes ( V) that aelerate eletrons. Multipliation of the number of eletrons an reah up to This harge of 10 8 x C ~ C arrives at the anode in a time interval ~ 5 ns I = 4 ma. If the anode is onneted to ground via a R = 50 W a pulse of 00 mv is formed. While the rise time of the pulse is ~ ns the transit time in the tube is ~ 40 ns and the Transit Time Spread (spread in times of propagation of eletrons from the photoathode to the first dynode due to the distribution of the veloity of emitted pe and to the different pathlengths Tyially < 3 ns). 8

14 Photosensors for Neutrino Telesopes Semiondutor (Bialkali( Bialkali) ) photoathode Sb-Rb Rb-Cs, Sb-K-Cs Time response: depends on eletrode struture and supply voltage Linearity: linear response of anode urrent vs inident light intensity Glass sphere Optial gel Smart ideas: Baikal QUASAR Short wave length limit Determined by window material Long wave length limits: determined by photoathode material hν too small to extrat pe hamamatsu.om/assets/appliations/etd/pmt_handbook/pmt_handbook_omplete.pdf 9

15 Parameters for seleting PMTs Inident light onditions Light wavelength Light intensity Light beam size Speed of optial phenomenon Window material Photoathode spetral response Number dynodes Dynode type Voltage applied to dynodes Effetive diameter (size) Viewing onfiguration Time response To whih follows the seletion of the iruit onditions: signal proessing method (analog or digital method) and of the bandwidth of assoiated iruit 30

16 New appliations Main drawbaks of PMTs are the bulky shape, the high prize and the sensitivity to magneti fields. Photodiodes are semiondutors light sensors that generate a urrent or voltage when light illuminates the p-n juntion. Allow detetion of light in nm) PIN photodiodes are Near IR detetors (InGaAs( or Si) ) have exellent linearity with light, high speed, low noise and dark urrent but major drawbak: low gain. Avalanhe Si photodiodes (APDs) have internal gain so are more sensitive and have muh higher QE > 70%. A = absorption region where there is an eletri field that separates the generated eletron-hole pairs and sweeps one arrier towards M = multipliation region. Here there is a high field that guarantees a high gain through impat ionization G = 100 in Si APDs,, in Ge or InGaAs G is limited by statistial flutuations of 31 avalanhe multipliation

17 Avalanhe PhotoDiodes 3

18 Silion PMTs (Dolgoshein et al) Main problem inter-pixel ross-talk Fast Geiger disharge development < 500 ps Pixel reovery time ~ 0 ns 33

19 The Cherenkov effet A harged partile traveling in a dieletri medium with n>1 radiates Cherenkov radiation if its veloity is larger than the phase veloity of light v>/n or β > 1/n (threshold) A B Wave front C Charged partile The emission is due to an asymmetri polarization of the medium in front and at the rear of the partile, dn γ = 491 giving rise to a varying eletri dipole momentum. dx Some of the partile energy is onverted into light. A oherent wave front is generated moving at veloity v at an angle Θ If the media is transparent the Cherenkov light an be deteted. If the partile is ultra-relativisti β~1 Θ = onst and has max value t AB os θ n = = = AC βt 1 βn In water Θ = 43,, in ie 41 34

20 The Cherenkov effet The intensity of the Cherenkov radiation (number of photons per unit length of partile path and per unit of wave length) d N dxdλ = 4π z e 1 1 hλ n β α = πe h = πz α sin Θ λ C Number of photons/l and radiation length depends on harge and veloity of partile Using light detetors (photomultipliers) dn γ = sensitive 491 in nm for an ideally dx 100% effiient detetor in the visible dn γ dx = λ dλ d N γ λ 1 dxdλ = πz α sin λ dλ 1 Θ C = πz α sin Θ λ λ C 1 λ 1 = zz sinθ α Θ C photons / m 1 λ d N d N dλ λ = = dxde dxdλ de πh h πh E = hν = = λ λ d N dxdλ Energy loss is about 10 4 less than MeV/m in water from ionization but diretional effet 35

21 Example of radiators Medium n-1 γ th Photons/m He (STP) CO (STP) Silia aerogel water Glass

22 PID: Cherenkov detetors Threshold Cherenkov detetors (ommon partiles an be distinguished up to ~30 GeV/): The Cherenkov threshold an be used to separate partiles of momentum p and masses m 1 and m > m 1. The radiator medium an be hosen suh that the heavier partile is just below threshold: β 1/n sin Θ C =1 1 β 1 n 1 β β 1 β 1 β =1 E 1 1 E (m m 1 ) p p >> m Hene for a PMT with QE = 0% N / L =100sin Θ C =100 (m m 1 ) p And to detet 10 photoeletrons one needs a radiator of length L =10 p (m m 1 ) in m Eg for K/π separation at 1 GeV Npe/L 16/m for π and by design 0 for K 37

23 PID: Cherenkov detetors Differential or fousing Cherenkov detetors: Angle of emission is measured with mirrors. There are variations of the method: 1) If partiles travel in the same diretion the one an be foused on to a slit diaphragm arranged to transmit light to a PMT. Sanning over a veloity range an be done adjusting the diaphragm to selet different angles or modifying the refrative index by adjusting the pressure or omposition of the gas ) If partiles do not travel along the same axis, the radiator an be ontained between spherial surfaes or radii R and R entered on the target or interation region. The outer surfae is lined with a mirror whih fouses the Cherenkov radiation into a ring at the inner detetor surfae. The radius of the ring depends on q and hene on the partile veloity. An eletroni image of the ring is onstruted eg passing photons through a suitable gas, and drifting the liberated photoeletrons on the wires of a planar MWPC or drift hamber (ring image Cherenkov detetor) 38