T- UNITED STATES AT O M I C E N E R G Y CO M M I SS I ON
AECU - 1638
ENERGY RESPONSE OF NaI(Tl) CRYSTALS TO
ALPHA PARTICLES OF LESS THAN 10 MEW
By - -
Ralph H. Lovberg ENGR 188Ary
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UNIV. OF WAs.
tº #:
September 24, 1951
|Date Declassified]
Los Alamos Scientific Laboratory
|- Technic a l l n form a tion Service, O a k Ridge, Te n n e s see
UNIVERSITY OF MICH
III.iii.
390.1508647
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INSTRUMENTATION
Reproduced direct from copy
as submitted to this office.
This report is based on LADC-loló.
AEC, Oak Ridge, Tenn., 10-24-51--555-W7552
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ENERGY RESPONSE OF NaI (Tl) CRYSTALS TO ALPHA PARTICLES
OF LESS THAN 10 MEW
Ralph H. Lovberg
Recent investigation") of the response of NaI crystals as scintil-
lation counters of charged particles has shown that while the light output
is essentially linear with energy of incident electrons, protons, and
deuterons, there is nonlinearity in the response to alpha particles of
less than 10 Mev. The investigation described here was undertaken to
determaine in detail the shape of this nonline arity.
In this experiment, energy control was accomplished by air attenuation
and the particles were allowed to strike a freshly cleaved crystal face.
To avoid the usual rapid moisture contamination of the crystal surface in
the open atmosphere, the entire operation was performed in a dry-box, with
signal and high-voltage cables brought in to the 5819 photomultiplier by
means of airtight feed-through connectors. P20 g was used as a drying
agent, and no detectable deterioration of crystals cleaved in the box took
place in a three-week periode
Two sources of alpha particles were used, both thin to their own radi-
ations. The first was a "thorium active" deposit yielding alpha groups at
8.78 Mev and 6.0l. Mew from Thº' and The disintegrations, respectively. The
239
second was Pu‘’’, giving 5.16 Mev alphas. Both sources were deposited on
the ends of probe rods which passed through a light-tight, sliding seal in
the end of the phototube housing. The l x 1 x 0.1 cm crystal was held to
the tube face by a wire clip, with a drop of mineral oil providing addi-
tional optical coupling.
The thorium probe was used in seven positions from l.00 to 6.00 cm
from the crystal, and the Pu source in five positions from 2.00 to 3.62 cm,
* Work done under the auspices of the AEC.
– l -
the near position in each case being the limit at which the source backing
reflected a significant amount of light back into the photocathode, pro-
ducing spurious heightening of output pulses.
Appropriate range corrections for the local atmospheric pressure of
60.2 cm Hg were made, and the mean particle energies were computed from
Bethe's range-energy curves"?). “s
Output of the 5819 was fed into a low gain preamplifier for polarity
inversion, through a Los Alamos model 503 pulse amplifier, and into a con-
tinuously variable single-channel pulse height discriminator. Line arity
of the electronic system was checked by applying standard pulses to the
phototube anode resistor and observing the corresponding discriminator
dial readings. No corrections were found necessary. ! ---
In the accompanying figure, mean output pulse height for each probe
position is plotted against computed mean energy. In every position, the
peak of the pulse height distribution could be determined to within one
volt.
I wish to thank Mr. Hugh Stoddart of the Los Alamos Cyclotron group
for preparation of the sources used in this experiment.
*Taylor, Remley, Jentschke, and Kruger, Phys. Rev. 83, 169 (1951)
(*)Bethe, The Properties of Atomic Nuclei II., Brookhaven National
Laboratory (1919)