Energy Resolution of Scintillation Detectors

1
Energy Resolution of Scintillation Detectors

• M. Moszynski
• Soltan Institute for Nuclear Studies
• PL 05-400 Otwock-Swierk, Poland

2
Why scintillation detection?

• High detection efficiency for different type of
  radiations, including neutrons,
• capability to measure energy spectra,
• very good timing,
• high counting rate capabilities,
• great variety in size and constitution.

3
Scintillators in ?-spectrometry

• High detection efficiency large Z and large
  density of the crystal,
• High light output
• Fast decay time a short life time of
  fluorescence,
• Energy resolution light output and internal
  properties of scintillators.

4
Energy resolution

• Detection process of ?-rays
• ?-ray absorption and light emission,
• Light collection at the photocathode,
• Production of photoelectrons,
• Collection of photoelectrons,
• Multiplication by PMT dynodes.

5
Energy resolution

• Energy resolution
• (?E/E)2 (?sc)2 (?p)2 (?st)2,
• where ?sc intrinsic resolution of the
  crystal,
• ?p transfer resolution,
• ?st PMT contribution
• (?sc)2 (?E/E)2 - (?st)2

6
Energy resolution

• ?st 2.35 ? 1/N1/2 ? (1 ?)1/2,
• N number of photoelectrons,
• ? - the variance of PMT gain.

7
Intrinsic resolution

• Non-proportional response of the crystal.
• - ?-rays absorption secondary electrons due to
  different processes, as photoeffect and
  secondary X-rays or Auger electrons, Compton
  scattering, etc.
• - scattering of secondary electrons (?-rays).
• Non-uniformity in the crystal.

8
Study of energy resolution

• 1956 Kelly et al. first observation of
  the intrinsic resolution in NaI(Tl),
• 1956 Engelkemeir non-proportional
  response of NaI(Tl),
• 1961 Iredale excellent discussion of
  intrinsic energy resolution in NaI(Tl),
• 1968-1969 Prescott and Narayan verified
  influence of non-proportionality on
  intrinsic resolution.

9
Modern studies

• 1994- Dorenbos et al study of
  non- proportionality for Ce-doped crystals,
  discussion of intrinsic resolution in relation
  to the non-proportionality, etc.
• 1997 - M. Moszynski et al study of
  intrinsic resolution with APD and
  non-proportionality of different crystals,
  intrinsic resolution of NaI(Tl) and pure NaI.

10
Modern studies

• 1994- Valentine et al. development of Compton
  Coincidence Technique to measure electron
  response for crystals, study of electron
  response, simulation of intrinsic resolution due
  to ?-rays.

11
Modern studies

• Valentine et al
• non-proportionality of electron response for
  some different crystals alkali halides and
  non-alkali halides crystals.
• Note a very good proportionality for YAP.

12
Modern studies

• 1999 Kapusta et al, study of energy resolution
  and intrinsic resolution for YAPCe
• Energy resolution 4.380.11
• Intrinsic resolution 1.30.5 !!!

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Intrinsic resolution of NaI(Tl)

• Intrinsic resolution in comparison to Monte Carlo
  simulation of ?-rays component due to
  non-proportionality of electron response.

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Intrinsic resolution of NaI(Tl)

• The geometrical difference between measured and
  calculated curves
• ?e (?sc)2 - (?cal)21/2,
• ?e 2.20.15, corresponding to 5.20.35, at
  FWHM.
• Intrinsic res. of NaI(Tl) is 5.8 0.24,
• ? -rays component!!!

15
Analysis of escape peaks in BGO

• Energy spectra of 320 keV and 412 keV ?-rays from
  51Cr and 198Au sources.
• Escape peaks were analysed in respect to an
  escape of Ka1, Ka2, Kb1, Kb2, and Kb3 components
  of the bismuth X-rays.

16
Intrinsic resolution

• All the studies carried out in 1990s finally
  confirmed that the intrinsic resolution of
  scintillators originating from the
  non-proportional response of the crystals is a
  fundamental limitation of obtainable energy
  resolution.
• It is due to the stopping process of ?-rays and
  due to the scattering of electrons (?-rays).

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Non-proportionaliy of light yield

• Murray and Meyer non-proportional response
  attributed to the specific energy loss dE/dX,
• Balcerzyk et al. LSO, GSO and YSO showed the
  same character of the response.
• Mengesha et al. NaI(Tl), CsI(Tl), CsI(Na) again
  the same shape of the curves.
• Non-proportionality depends on the crystals
  structure.

18
Non-proportionaliy of light yield

• Piotr A. Rodnyi in his recent book Physical
  processes in inorganic scintillators (1997) has
  concluded that
• - The non-proportionality of scintillator
  response is an intrinsic property of the (host)
  crystal and therefore cannot be improved
  substantially.

19
BGO - Non-proportionality of the light yield

• Non-proportionality measured at LN2 and room
  temperatures represnts a common curve.
• Note good positions of points corresponding to
  escape peaks.
• A precise analysis showed about 1 excess of the
  light yield.

20
BGO - Intrinsic resolution

• Note a common curve independent of the crystal
  temperature. It agrees with a common
  non-proportionality curve.
• Points corresponding to the escape peaks fit very
  well to the curve.

21
Crystals at liquid nitrogen temperatures

• Studied crystals pure NaI, pure CsI, BGO, CaWO,
  LSO
• Preliminary tested YAP and BaF2
• Non-proportionality characteristics and intrinsic
  resolution are compared to those measured at room
  temperature.

22
Crystals at LN2 temperature

• Crystals mechanically coupled to LAAPD and cooled
  down to about 100 K in a typical cryostat used
  for Si-detectors.
• LAAPD 16 mm diameter, crystals 10 mm diameter.
• Be window allows to detect x-rays as the
  reference to measure number of e-h pairs.

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Pure NaI

• NaI (A), ?10 mm x 5 mm,
• Measured with a peaking time of 50 ?s!!!

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Non-proportionality of NaI and NaI(Tl)

• Note different non-proportionality of studied
  samples
• Non-proportionality of NaI (C) comparable to that
  of NaI(Tl) at RT.
• Is it influence of traces of Thallium observed in
  the emission spectrum?

25
Pure halide crystals at LN2 temperature

• A very high light output of 124000 ph/MeV, for
  the best CsI crystal and very a high energy
  resolution of 3.8 for the best NaI.
• A good proportionality of the light yield for the
  best samples.
• Non-proportionality curves depend on purity of
  the crystals.
• The non-proportionality characteristics in the
  halide crystals are not (only) their intrinsic
  property.

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LaBr3 in comparison to NaI(Tl) crystals
Non-proportionality of LaBr3 and NaI(Tl)
Intrinsic resolution of LaBr3 and NaI(Tl)
27
LaBr3Ce crystal
662 keV ?-ray spectrum, as measured with 3x3
LaBr3Ce and NaI(Tl) crystals, data compiled by
Saint-Gobain Crystals.
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Discussion and conclusions

• The non-proportionality is the fundamental
  limitation of energy resolution.
• - secondary ? and X-rays in the stopping process
  of ?-rays,
• - scattering of secondary electrons (?-rays),
• - a high energy resolution of YAP, LaCl3 and
  LaBr3 is corelated with a good
  non-proportionality
• The non-proportionality of the undoped oxide
  crystals, as BGO, CWO and CaWO seems to be a
  fundamental characteristic of scintillation
  material.

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Discussion and conclusions

• In contrast, studies of pure undoped halide
  crystals and Ce doped crystals showed that the
  non-proportionality can be affected by both the
  host crystal and the doping agent.
• It seems to show that the non-proportionality and
  intrinsic resolution characteristics are altered
  by accidental doping by impurities.
• The last observation suggests that a selective
  co-doping of crystals may improve the
  non-proportionality and the intrinsic resolution.

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Discussion and conclusions

• Dorenbos proposed recently to analyze
  non-proportionality in terms of two processes
• - The first one is related to the
  non- proportionality of the host material,
• - the second process is associated with the
  transport of the energy to the activator.
• Both processes are in fact correlated with the
  ionization density, as it was postulated by
  Murray and Meyer.

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Discussion and conclusions

• This approach seems to be confirmed by the recent
  experimental studies.
• The non-proportionality of the undoped oxide
  crystals, as BGO and CWO, represents the
  fundamental properties of the scintillator
  materials.
• In the case of doped crystals, one observes
  variation of the non-proportionality
  characteristics depending on doping agents,
  particularly for halide crystals.

32
Discussion and conclusions

• Moreover, the pure, undoped halide crystals, as
  NaI and CsI, are very sensitive to the
  impurities, which affect the non-proportionality
  curves.
• It suggests that a selective co-doping of
  scintillators may improve the non-proportionality
  and their energy resolution. No doubt that
  further studies are necessary.

33
BGO, CWO and LSO

• Non-proportionality of BGO, CWO and LSO
• Intrinsic resolution at 662 keV
• BGO 5.3?0.4
• CWO 5.4?0.4
• LSO 7?0.5

34
New observations NaI(Tl)

• Light pulse decay of NaI(Tl) versus temperature

35
New observations NaI(Tl)

• Energy resolution of NaI(Tl) and its components
  versus peaking time, as measured at -20? C.
• Acc. to L. Swiderski et al.

36
New observations CsI(Tl)

• Non-proportionality curves for 3 ?s and 12 ?s
  shaping time constants
• Acc. to A. Syntfeld
• et al

37
New observations CsI(Tl)

• Intrinsic resolution of CsI(Tl) for short and
  long shaping
• Acc. to A. Syntfeld et al.
• Energy resolution is improved accepting all light
  of scintillation pulse.

38
New observations LGSOCe Non-proportionality
characteristics
LGSO, GSO and LSO
LGSO crystals different Ce doping
39
New observations LGSOCe Afterglow of LGSO
The crystal under the tests was illuminated for
120 seconds by a beam of ?-rays from a strong
241Am source (13.9 GBq).
The voltage drop, in the period of the
illumination, was normalized to 1 V for all the
crystals.
40
New observations LGSOCe

• Intensity of afterglow, light output and
  intrinsic resolution versus Ce doping of LGSOCe
  crystals

41
Energy resolution

• Non-proportionality is the fundamental limitation
  of the energy resolution status of 2005,
• However, new results, reported by our group in
  2006, suggest that other effects, as a
  contribution of slow components and in the
  limiting case afterglow, affects energy
  resolution,
• The origin of the effect is not clear. Is it a
  statistical spread of the population of different
  deexcitation ways in the crystal?