Nonproportionality of organic scintillators and BGO

1
Non-proportionality of organic scintillators and
BGO
Winston-Salem June 4, 2007

Antoni Nassalski
2
Motivations

• The non-proportionality of the light yield of
  scintillators appears to be the fundamental
  limitation of energy resolution 1.
• A more profound studies of the non-proportional
  response of organic scintillators were performed
  in comparison to that of a BGO crystal 2.
• The studies covered tests of
• BC408 plastic,
• BC501A liquid scintillator,
• Anthracene organic crystal.
• We put the question whether we will observe the
  influence of much lower density of the organic
  scintillators on their non-proportionality?

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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Experimental Details
Table I Main properties of the studied
scintillators
PMT - Photonis XP5200 photomultiplier, Blue
sensitivity of 13.3 µA/lmF, High quantum
efficiency of 35 allows to observe

• E g, X lt 122keV full energy peaks,
• E g, gt 122keV position of the Compton
  edges.

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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Results
The energy spectra recorded with the anthracene.

• Energy peak for 57Co, well separated from Compton
  spectrum, photofraction 1.

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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The energy spectra recorded for the 238Pu-9Be
source, measured with the BC408 plastic.
A typical broadened Compton spectrum Lmax
position of peak, Lc true position of Compton
edge, L1/2 position of half-height of observed
edge.
The positions of Compton-edges were analyzed
following the method described by Hawkes and
Adams 3. In the experiment LC was set as 0.66
of LMAX.
Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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Table II Number of photoelectrons for 0.447 MeV
g-rays.

• A factor two lower number for BC501A liquid
  scintillator than that of BC408 plastic. It is in
  contrary to the listed nominal light output given
  in Table I.

It could be associated with

• aging effect in the BC501A, delivered about 10
  years ago,
• scintillator diameter of f50 mm, covers whole the
  photocathode of the 2 inches XP5200 PMT ? edge
  effects.

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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The non-proportionality curves, light yield
relative to energy.

• The measurements pointed out a much larger range
  of energies presenting non-proportional response
  compared to that known for inorganic
  scintillators 1.
• BGO light yield saturates above 100 keV, while
  that of anthracene above 500keV.
• BC408 and BC501A scintillators present
  non-proportional character up to 4 MeV.

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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Absolut light output as a function of the photon
energy of the studied scintillators. Error bars
are within point size.
Anthracene crystal, scintillation response to
electrons, protons and a-particles 4.
experimental data. theoretical curves
following Birks 4.

• All the tested scintillators, appear to have a
  linear character of the light output dependence
  on g-ray (electron) energy.
• It confirms non-proportionality and linearity
  discus by J. Valentine and P. Dorembos in More
  on the scintillation response of NaI(Tl).
• The measured characteristics of the
  non-proportionality do not differ of the old data
  collected in the past for organic scintillators.

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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Conclusions

• The study showed a much larger non-proportionality
  of the light yield of organic scintillators than
  that known for the inorganic crystals.
• The light yield of inorganic scintillators
  saturates at energies above 100 keV. In case of
  anthracene the non-proportional response is
  observed up to about 500 keV, while for the BC408
  plastic and the BC501A liquid scintillators, the
  non-proportional character is measured up to 4
  MeV energy lost by gamma quanta.
• The observed effect can be related to a strong
  quenching of the light for charged particles in
  organic scintillators 5, which is much larger
  than that observed in inorganic scintillators
  6.
• The larger non-proportionality found for BC408
  plastic and BC501A liquid scintillators in
  comparison to anthracene suggests that the
  non-proportionality is also affected by the
  energy transfer process in organic scintillators.
  

Antoni Nassalski - Non-proportionality of organic
scintillators and BGO
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References
1 M. Moszynski, Energy resolution of
scintillation detectors, SPIE Proceedings, Vol.
5922, 592205-1. 2 M. Moszynski, M. Balcerzyk,
W. Czarnacki, M. Kapusta, W. Klamra, A. Syntfeld,
and M. Szawlowski, Intrinsic energy resolution
and light yield nonproportionality of BGO, IEEE
Trans. Nucl. Sci., vol. 51, no. 3, pp. 1074-1079,
June 2004. 3 N. P. Hawkes, J. M. Adams, D.S.
Bond, S Croft, O.N. Jarvis, N. Watkins,
Measurements of the proton light output function
of the organic liquid scintillator NE213 in
several detectors, Nul. Instrum. Methods, vol. A
476, pp.190-194, 2002. 4 J. B. Birks, The
theory and practice of scintillation counting,
1964. 5 J.B. Czirr, The ?/ß ratio of several
organic scintillators, Nucl. Instrum. Meth.,
vol. 25, pp. 106-108, Jan. 1964. 6 E.V.
Sysoeva, V.A. Tarasov, O.V. Zelenskaya, V.A.
Sulyga, The study of a/? ratio for inorganic
scintillation detectors, Nucl. Instrum. Meth.,
vol. A414, no. 2-3, pp. 274-276, Sept. 1998.
Thank you