Y. LEVY et al.
29Pu which has been electroplated on a
stainless steel planchet is exceedingly thin
(i.e. all the alpha particles emitted are of
5.1 MeV energy). Further, when theresults of
these experiments are applied to determining
respect to
lanchet to
75) source
ae
matrix it is assumed that (a) the difference in
sensitivity of alphas in the range of 5.0—-
Fit WOee, ae ee
ficiency of
rix to be
ibrated by
- efficiency
» matrices,
standards
iurce elec-
the alpha emitter concentration in a calcite
powdered
1a-emitting
il growth section
iments
alpha and mass
following radioation
imma spectrometry
Ipha spectrometry
Ipha spectrometry
neutron activation
detectors
background levels
Thomson (1974)
are homogeneously distributed within the
calcium carbonate matrix.
(1) Calibration of Kodak LR-115, Type Il
using polycarbonate film as an energy
degraderof alpha particles
Kodak film LR-115, with a thickness of
10 pm (designated as Batch A henceforth)
was irradiated with a known dose of alpha
particles of select maximum energies by
ship between alpha particle tracks detected
and the concentration of alpha emitters can
be calculated. Thus, for calcite it was cal-
culated
with
polycarbonate
films
of
different
thicknesses (4, 8, 12, 20, 24, 28, and 30 um).
This results in alpha particle energies in the
range of 0-5 MeV. The irradiated detectors
the number of tracks counted in an area of
1.2 mm? (same area each time) by using 625
magnification.
The detection efficiency of the combination
of Kodak film LR-115 and the polycarbonate
is plotted against the polycarbonate thickness
(Fig. 1). It can be seen that the Kodak LR-115
film is not capable of detecting alpha particle
energies above about 4.5 MeV and that the
sensitivity of the film depends upon the alpha
particle energy. The highest sensitivity occurs
at 4.1 MeV.
Values of the stopping power for alpha
particles in a polycarbonate matrix have been
published (Be68). A relative stopping power
of 2:1 for alpha particles in calcium car-
that
one
track/0.04mm7/30
day
exposure is equivalent to 40 pCi of alpha
emitters per g-calcite using Kodak Batch A.
Kodak Batch B (Kodakfilm LR-115, Type II
thickness of 6m, 1974), gives a value of
104 pCi/g for one track/0.04mm7/30 day
exposure. Etching conditions for Batch B are
2.5N NaOH, 25 min, 58°C.
{2) Calibration of Kodak LR-115, Type
using calcite film as an energy degrader of
alpha particles
Calcite films of various thickness between
70
using the electroplated *’Pu source covered
were etched (2.5N NaOH, 58°C, 70 min) and
fae “ara *
of which
nergy conve isotopes
re energies
gflieta aye Shas
rom a sur-
a ike:
a by use of
k film LRa particles
’ while the
onium and
inge of 5-
a nn An
im.
5.5 MeV to that of the 5.1MeV energy of
Pu is negligible, and (b) the alpha emitters
tributed in a calcite matrix, (2) the relative
stopping powerfor alpha particles in a calcite
matrix compared to a polycarbonate matrix is
2 to 1 and (3) the calibration experiment for
polycarbonate is valid (Fig. 1), the relation-
-
and
aipha particle emitters are uniformly dis-
wn
nium
Schilling’s techniques (1970) in conjunction
with Benton’s polycarbonate alpha particle
energy loss data (1968). Assumingthat (1} the
Detection efficiency, %
yots in the
was calculated by using Northcliffe and
x
29pu source to calibrate the Kodak LR-115
film for this range of alpha-particle energies.
For both the polycarbonate and calcite
energy degrader studies it is assumed that the
bonate as compared to polycarbonate films
hw
uw
in
n
Zs oO oO Qo oO
oOo
ie
ranging from zero to the maximum alphaparticle energy emitted by the isotope’ under
study. Two types of alpha-particle energy
degraders, polycarbonate and calcite, were
used in conjunction with the electroplated
a emitters
ment. The
adiochemor
are
expected
from the
d thorium
concentralred are in
rdingly, at
711
Alpha particle energy, MeV
Q
24
Oo
|
6
@3
8
10 12 14 16 18 20 22 24 26 24 30
uM of paiycarbonate
45
6
7
8
9 i0
Il
j2 13
14
15
um of calcite
Fic. 1. Detection efficiency curves for Kodak LR-
115, Type II cellulose nitrate using alpha particles
from a °Pu source with polycarbonate and calcite
energy degraders. 100% detection efficiency is one
track per one alpha particle. ——-— Batch A+
polycarbonate,
batch B+ polycarbonate
—+—-—-— batch B+ calcite.