1. 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. 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. 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. 4 Organic Scintillators  Organic scintillators produce light by 4

5. 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. 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. 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. 8 Organic Scintillators  Organic scintillators produce light by 8

9. 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  ~ 10 -7 – 10 -9 s  Phosphorescence Radiation emitted from the lowest triplet vibrational level of an excited state, after intersystem crossing  ~ 10 -4 – 10 s

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

11. 11 Scintillators 11

12. 12

13. 13

14. 14 Organic Scintillators

15. 15 Organic Scintillators  Crystals Not used much but anthracene (C 14 H 10 ) has the highest scintillation efficiency (light output / energy deposited) of all organic scintillators 15

16. 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. 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. 18 Rules of Thumb  For plastic scintillators Density is about 1 g/cm 3 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 10 4 photons  Collection and transport efficiency will reduce the yield

19. 19 Range 19

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

21. 21 Birk’s Law 21

22. 22 Birk’s Law 22

23. 23 Birk’s Law  kB values

24. 24 Pulse Shape Discrimination  In most scintillators, fluorescence is dominated by one time constant (t f ~ 1 ns)  However some scintillators (e.g. NE213) have a substantial slower time component as well (t s ~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 T 1 excitations for neutrons compared to photons 24

25. 25 Pulse Shape Discrimination 25

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

27. 27 DZero Pixel Counters

28. 28 DZero Pixel Counters

29. 29 Homeland Security  Neutron

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