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 ( 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  ~ – s  Phosphorescence Radiation emitted from the lowest triplet vibrational level of an excited state, after intersystem crossing  ~ – 10 s

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

11 Scintillators 11

12

13

14 Organic Scintillators

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 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/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 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 (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 Pulse Shape Discrimination 25

26 Pulse Shape Discrimination 26 ADC value with long digitizing gate 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