Kernfisiese metings om 'n kilocurie 60Co-bron
Date
1972-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT:
(Formulas did not scan well. See theses abstract in the PDF document p. 90)
The major part of the thesis is devoted to a study of the energy spectrum
of a 5000 Ci 60co source of finite dimensions. Measurements of the exposure
dose distribution at various distances from the source have also-been performed. The determination of mean lifetimes of nuclear states utilizing
nuclear resonance fluorescence techniques and the 60co source is also included.
This parameter is an important one in the calculation of the shape and intensity
of the emitted 60co spectrum.
In chapter 1 a general discussion and motivation for the project is
followed by a brief introduction to ionization chamber dosimetry, nuclear
resonance fluorescence and elastic scattering of gamma radiation from atoms.
The dose distribution, as measured with a Baldwin Farmer dosimeter in
roentgen units, is discussed in chapter 2. The irradiation enclosure, the
source (which is encapsulated in stainless steel) and the measurements are
described. Measurements of the dose distribution were performed (i)on and
above a flat circular table with the source on the axis of thetable at
different heights above the table surface and (ii) in the space surrounding
the source, hoisted to its maximum height, with an empty irradiation chamber.
Regression coefficients for the linear relationships (log· as a function
of log r) obtained from the measurements are given for various source dosimeter
configurations.
Standard measurements of the exposure dose at a horizontal distance,
60 cITT froITT the source in its highest position, gives value of approximately
121 less than the value specified by the manufacturers This is mainly due-to
the attenuation caused by the stainless steel capsule surrounding the source.
Finally an illustration of a practical application of the dose distribution
measurements is given.
An unique method for the determination of the continuum of energy degraded
photons emitted by the source, is discussed in chapter 3. This continuum is
mainly due to Compton scattering in the source itself and the capsule surrounding
it. The nuclear resonance fluorescence effect is used to measure the intensity
and shape of the spectrum for the energy region 0,478 to 1,265 MeV. Resonant
scatterers containing the nuclides 7Li, 63cu, 65cu, 27Al, 45sc, 59co, 35c1 and
31P were used and the resonantly scattered radiation was detected by a 20" cm3 Ge(Li) detector. The experimental set up, procedure followed and calculations
are discussed in detail. The results are summarized in Table 3.2 and
Figure 3:4 An average value of 108 photons per electronvolt per second is
obtained for the energy region 0,478 to 1,265 MeV.
(Approximately 40% of the total curie-content.)
In chapter 4 the determination of the intensity of theprimary 1,17 and
1,33 MeV photons emitted by the source, is discussed. Elastic scattering
(mainly Rayleigh and Thomson) from copper, arsenic, cobalt, zinc,
phosphorous, aluminium and hexachloro-ethane were observed to provide average
values of
(a) (b,94 ± 0,13) x 1014 photons of energy 1,17 MeV per second and
(b) (1,01 ± 0,12) x 10 14 photons of energy 1,33 MeV per second.
This amounts to 62% of the total curie-content.
The relevant formulae as well as the calculation of differential cross
sections for elastic scattering for low Z (Z ~ 33) atoms are also discussed.
The results obtained are compared with those of chapters 2 and 3.
Three methods for the determination of mean lifetimes T of nuclear energy
levels are discussed in chapter 5. The methods, which involve the observation
of nuclear resonance fluorescene are the following:
I • (1) The selfabsorption technique, where the incident spectrum N(iEr), detection
efficiency and angular distribution of the scattered radiation need not to
be known.
(2) Determination of the incident spectrum making use of elastic scattering
from lead (the angular distribution must be known, but not the detection
efficiency).
( 3) Observation of nuclear resonance fluorescence in the direct beam, from
the source, making use of the determined shape of the energy spectrum of the
radiation from the source. (This method relies on accurate knowledge of the
detection efficiency of the spectrometer and angular distributions as well as
lifetimes and spins of the levels used to determine the continuum.)
The second and third methods are new in the sense that N{E~) is obtained
in a unique manner. The results are summarized in the following table: (Due
to two possible (N(El')~h) and N(Er)~ 1
)) for the continuum of the third method,
2·values are given for T for.each energy level.)
Energy T
Method Nuclide (MeV·) (xl0-12 s)
(1) 64zn 0,99 1,9 ± 1,2
(2) 64zn 0,99 2,0 ± 0,3
66zn 1,'04 2,3 ± 1,0
68zn 1,08 l,0 ± 0,3
(3) 64zn 0' ,99 1,7 ± 0,7 (a)
2,9 ± 1,2 (b)
66zn .1,04 1,8 ± 0,8 (a)
3,0 :!" 1,3 (b)
68zn 1,08 1,8 :!" 1,6 (a)
2,9 ± 2,5 (b)
75As 0,865 1,2 ± 0,3 (a)
1,8 :!" 0,4 (b)
1,075 o' 24 +- O, 03 (a) ·
0,39 ± 0,06(b)
(a): N(E j(l)
r V
An assessment of the three methods is also given.
Description
Thesis(Ph.D.)--Stellenbosch University, 1972.
Keywords
Nuclear reactions, Dissertations -- Physics, UCTD