Scintillators

1. Scintillators • One of the most widely used particle detection techniques • Ionization -> Excitation -> Photons -> Electronic conversion -> Amplification • Variety of uses in EPP • Calorimetry • Tracking detectors • Time-of-flight measurements • Trigger and veto counters • And other fields • Medical imaging detectors (SPECT, PET, CT, …) • Gamma ray spectroscopy • Homeland security

2. Scintillators • Two types • Organic • Crystal, liquid, plastic (most widely used in particle physics) • Lower light output but faster • Inorganic • Crystal, glass • Higher light output but slower

3. Organic Scintillators In general, +Fast (ns or better time resolution) +Relatively large signal (using PMT or SSPM ) +Simple, machinable, robust +Variety of shapes +Pulse shape discrimination between neutrons and photons (NE213) -Poorer position and energy resolution than other detector types -Sensitive to neutrons

4. Organic Scintillators • Organic scintillators produce light by 4

5. Organic Scintillators • Notes • Some organic substances, such as those containing aromatic rings, release a small fraction of excitation energy as photons • Polystyrene (PS) or polyvinyltoluene (PVT) • With the addition of a fluor to the base plastic (PS or PVT), the Forster mechanism (FRET) becomes the predominant mode of energy transfer 5

6. Organic Scintillators • Notes • The Forster mechanism (FRET) is a non-radiative transfer of energy between two molecules over long distances (10-100 A) • It arises because of an interaction between the electric fields of the dipole moments of donor and acceptor atoms • FRET has a number of applications including photosynthesis and DNA sequencing 6

7. Organic Scintillators • Notes • Base solvent is usually PVT or PS (something with aromatic rings) • The base can produce UV photons itself however the addition of a primary fluor (1% by weight) provides an additional mode of energy transfer from base to fluor • Shorter decay time (2 to 20 ns) • More light • The primary fluor often does not have good emission wavelength or attenuation length characteristics so a second fluor is added (at a fraction of percent by weight) as a wavelength shifter 7

8. Organic Scintillators Organic scintillators produce light by 8

9. Organic Scintillators • Luminescence • Radiation emitted by an atom or molecule after energy absorption • Fluorescence • Radiation emitted from the lowest singlet vibrational level of an excited state • Generally true that a molecule will undergo internal conversion to the lowest vibrational level of its lowest excited state, regardless of the initial excited singlet state • t ~ 10-7 – 10-9 s • Phosphorescence • Radiation emitted from the lowest triplet vibrational level of an excited state, after intersystem crossing • t ~ 10-4 – 10s

10. Organic Scintillators • Energy levels for organic scintillators look like Solvent 10

11. Scintillators 11

12. 12

13. 13

14. Organic Scintillators

15. Organic Scintillators • Crystals • Not used much but anthracene (C14H10) has the highest scintillation efficiency (light output / energy deposited) of all organic scintillators 15

16. Organic Scintillators • Liquids • Base is usually toluene, xylene, benzene • Typical concentration of primary fluor (e.g. PBD) is 3g of solute/liter of solvent • +Arbitrary shapes • +Radiation resistant • +Can be loaded with B, Li or Pb, Sn for n or gamma detection • +Pulse height discrimination • -Toxic • -Messy • -Impurities can render useless 16

17. Organic Scintillators • Plastic • Solvent is usually PVT or PS • Typical concentration of first fluor is 10g of solute / l of solvent • +Fast • +Relatively inexpensive • +Easily machined or extruded into fibers • +Can be loaded • -Ages or crazes with time • -Subject to radiation damage • -Attenuation length (1-3m) can be a problem for large counters • -No pulse height discrimination 17

18. Rules of Thumb • For plastic scintillators • Density is about 1 g/cm3 • Photon yield is about 1 photon / 100 eV of energy deposited • Thus a 1 cm thick scintillator traversed by a mip (e.g. muon) yields about 2 x 104 photons • Collection and transport efficiency will reduce the yield

19. Range 19

20. Birk’s Law • Plastic scintillators do not respond linearly to ionization density • Both in light output and decay time 20

21. Birk’s Law 21

22. Birk’s Law 22

23. Birk’s Law • kB values

24. Pulse Shape Discrimination • In most scintillators, fluorescence is dominated by one time constant (tf ~ 1 ns) • However some scintillators (e.g. NE213) have a substantial slower time component as well (ts~100 ns) • The fraction of light that appears in the slow component often depends on particle type (dE/dx loss rate) • In NE213 there are more long-lived T1 excitations for neutrons compared to photons 24

25. Pulse Shape Discrimination 25

26. Pulse Shape Discrimination ADC value with long digitizing gate 26 ADC (short)/ADC (long)

27. DZero Pixel Counters

28. DZero Pixel Counters

29. Homeland Security • Neutron

30. Homeland Security • Comparison of performance and cost of a few gamma ray detectors