Low energy activation analysis for geochemical exploratio

Ferguson, G. A.

English

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R E I' O
	 T
LOW ENERGY ACTIVATION ANALYSIS
f or
GEOCHE MICA L EXPLORATION
Submitted By
H o w a r d U n i v e r s i t y, W a s h i n g t o n, D. C. 2 0 0 0 1
to
National Aeronautics and Space Administration
Washington, D. C. 2 0 5 4 6
Dr. George A. Ferguson, Principal Investigator
NASA Grant NGR 09-011--022
1 August 1970
	
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ACKNOWLEDGEME.-IT
The principal investigator (G.A. F.) who directed this investigation
gratefully acknowledges the assistance of Mr. Sami S. Sherbini and Mr.
Balbir Singh who spent many laboratory hours in equipment construction
and data acquisition.
IL
INTRODUCTION
The National Aeronautics and Space Administration has demonstrated a
continued interest in techniques suitable for geochemical exploration of lunar
and planetary surfaces. 01 Diagnostic techniques to Assist the astronaut in
sample selection or -'insitu'' analysis have been the subject of intensive
study in recent years. (2) One such technique is the artificial excitation of
the specimen and its identification by the measurement of the radiation
spectrum produced by its subsequent characteristic de -ex citation. This
method is popularly called activation analysis. Me artificial excitation may
be achieved by a variety of means, for example, by gamma, beta, alpha,
neutron or Bremstrahlung sources. The major r.roblem in the identification of
samples by activation ana.ysL3 is the determination of the radiation spectrum
incident u pon the detector by an analysis of its experimentally measured
response.
Three basic types of detectors have been used in the measurement of
radiation. They are gas--filled counters, scintillation counters and solid-state
counters. When radiation cou:iiers are emr- loved during the explorations allud-
ed to above, their characteristics should include ruggedness, high efficiency
and good resolution. No one of the three basic detector types optimizes all
of these desirable characteristics. Gas filled counters may be made rugged
but are slow in their reslonse, thereby rendering them relatively inefficient.
The solid-state counters exhibit excellent resolution, but in its present state
of developmerit it is cumbersome and also relatively inefficient.
-2-
The scintillation counter offers a satisfactoa-y compromise of the required
characteristics of detectors for exploration. Fr - this reason, this
investigation will concentrate upon the response of scintillation counters.
Identification of unknown samples by activation analysis may be
achieved either by a direct comparison of the spectrum of the unknown with
that obtained from an identical sample of known composition, or the unknown
spectrum may be decomposed into its monoelemental components While the
former technique has some application, the later offers a more fundamental
approach to the problem of identification. The process of decomposing a
complex radiation spectrum into its monoelemental components, may be
accomplished by fitting the upper end of the observed spectrum with the
appropriate distribution function of a monoenergetic type of radiation (say,
monoenergetic gamma rays) . This specific spectrum, including its Compton
tail (produced by scattering within the scintillator) , is then unfolded. The
remaining spectrum is operated on in the same manner until the original
complex spectrum is completely analyzed.
To accomplish the unfolding, one must have available the pulse height
spectrum produced by a monoenergetic gamma ray over the range of interest,
in the same scintillation detection system as used to record the spectrum
being analyzed. Moee. n digital computers may then be employed in the
analysis. The procedure utilizes a large matrix whose elements describe the
response of the detector to monoenergetic gammas. The unfolding of the
observed spectrum, to obtain the incident spectrum, requires operations with
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-- 3 -
the inverse of this matrix. Problems are encountered in performing this
inversion due to the statistical variance in detector response which forms
the elements of the matrix. (3) More importantly, this method of analysis
is limited by the available known monoenergetic pulse height spectra.
The complete success of this approach requires a complete library of care
fully measured spectra which covers the entire energy range of interest .
Pulse height spectra of a large number of gamma ray sources have
been tabulated (4) , however, these spectra have generally involved radiation
sources of high energy. There exists a scarcity of such information at energies
below a few hundred kilovolts although many important and useful sources emit
radiation with energies in this region. One of the purposes of this study is to
investigate the response of a selected scintillation detector to incident mono-
chromatic radiation with energies below one-hundred kilovolts (100 kv) .
DESCRIPTION 01' RESEARCH.
(a) . Review of Techniques-
A review of existing techniques of the measurement of complex radiation
spectra was conducted. From this review, and the helpful preliminary discuss-
ions of the proposed research program with Dr. S. Singh (Howard University),
Dr. R. Deslattes (National Bureau of Standards), Dr. W. Johns (Lawrence
Radiation Laboratory) and Prof. D. A, Lind (University of Colorado) it was de-
cided that the technique of bent crystal spe --trometry (DuMond) would be
employed for monochromatization of the radiation source.
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This source, for low energies (below 100 kv), was a modified Flaxitron
X-Ray System (Model 8050-311) purchased for this purpose. (For energies
above 100 kv in future studies, the use of the University Cockcroft-Walton
accelerator is anticipated.)
(b) . Production of Radiation:
The Flaxitron X-Ray System (Model 8050-311) was used as a source
of variable energy radiation. This self rectified system operates on a
continuous duty cycle with a variable output voltage within the range 10-:110 kv.
The 0.5 mm emission source emits radiation through a 0.025 " beryllium window
which is directed through a two meter long lead tunnel onto a single crystal of
germanium serving as a monochrorriator.
(c) . Monochromatization-
Descriptions of the interaction of gamma rays with matter, as well as
the dynamical theory of diffraction of gamma rays by a crystal, have been
extensively discussed (5) . It is known (6) that only for, the incident angle, AB,
and spacing, d, of a given set of crystal planes, will coherent diffraction of
wavelength T be observed acc rding to	 n %_= 2d sirr: GB* Now when
a crystal slab is elastically bent to a cylindrical surface such :hat the
diffracting planet are parallel to !,nd intersect the cylinder axis an increase
diffracted intensity is observed (7) . DuMond (8) described and constructed
such a bent crystal spectrometer. The essential features of the arrangement
-of source, crystal and detector used by DuMond are given in Figure 1. The
DuMond geometry has been employed to construct the bent crystal spectrometer
F.
- 5 -
used for monochromatizing -the x--ray source in this study. A single crystal
of germanium was used as is monochromator. .
(d) . Detection:
The monochromatic radiation diffracted by the germanium crystal was
directed onto the scintillation detector under study after passage through a
sixteen inch lony Soller slit collimator (see Figure 1) . Collimation of the
diffracted intensity was required to accurately define the scattering angle
(and hence the energy of the diffracted beam) as well as to reduce background
radiation resulting from incoherent scattering. The scintillation detector used
for this study was a rucigedized "INTEGRAL LINE" Assembly, Type 12SPF12-x,
serial # DQ-376 supplied by the Goddard Space Flight Center. The assembly
consisted of a 3-inch NaI (T1) scintillation crystal optically coupled to a 10-
stage head-on type pliotomultiplier (RCA # C31009E) 	
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(e) . Recording:
Pulse height spectra were collected using a 512-channel analyzer
(Kicksort Model 701) and stored on paper tape by memory read-out throz: `gh a
Model 33TZ Teletype. These data are available for future analysis.
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EXPERIMENTAL RESULTS-
PreDminary pulse height spectra have been obtained using radio-
isotopic calibration sources and monochromatic radiation produced by the
bent crystal spectrometer.
(a) . Cs 137 pulse height spectra.
Figure 2 is the pulse height spectrum obtained from a 0.5 microcurie
Cs 13 7 source. The identifying features of this spectrum have beer, labeled.
The measured resolution of the 662 kev photopeak is 12 % (which compares
favorably with the value 8.8% measured when the scintillator was originally
tested at Harshaw Chomical Company prior to shipment) . The measured
resolution of the 32% kev barium x-ray produced by this source is 60%.
(b) - Ba133 pulse height spectra.
Figur,	 is the pulse height spectrum obtained from a 0.5 microcurie
Ba l33 source. This source produces radiation with two discrete energies,
namely 32 kev and 85 kev. The response of the scintillator, as shown in
this figure, clearly distinquishes these two energies. Analysis o! these
data (and subsequent measurements) indicated that the response of the detect-
ion system was essentially linear with approximately 0.3 kev per channel.
(c) . Monochromatic radiation.
In Figure 4 is shown a detailed study of the 32 kev Ba x-ray line
(from Cs i37) together (for comparison) with th y, pulse height spectrum from
the detector when exposed to 33 kev monochromatic x-rays obtained with the
spectrometer by diffraction from the (220) crystal planes of germanium.
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NEED FOR FURTHER INVESTIGATION.
(a) . Additional measurements are necessary for the completion
of this project. The satisfactory preliminary data, given in this report, and
f
the ease with which a complete tabulation of these important pulse height
spectra may now be obtained (with the equipment now assembled) makes the
continued support of this project essential. It is clear that this program will
yield results which are required for the research program of NASA.
(b). The measurement of the response of the detectcr system for
monochromatic incident radiation in the energy range 20 kev 4:E< 100 kev
should be completed in order that shape functions may be generated. The
analytical techniques which have been developed by these (and other)
investigators should be employed to unfold complex spectra into its mono-
elemental components.
(c) . This research program was funded under Grant NGR 09-011- 022
($6000) and ended 1 August 1970. An interim request for continued support
during tha period 1 August 1970 - 30 September 1970 has been filed under NASA
Control # 09--011---022 (032) . A research proposal to support further investigation
is being made.
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-9 -
REFERENCE
(1)	 Space Research Direction for the Future.
Space Sciences Board, National Academy of Science, Part 1 and II,
January 1966.
(2)
	
	
A. E. Metzger, R.' E. Packer, and J. I. Trombka, IEE Trans.
Nuclear Science Vol. NS -13, No. 1, Feb. 1966; J. I. Trombka and
I. Adler, "Analytic Method for a Non-Dispersive Anal y sis, "
Presented at the First National Conference on Electron Probe
Microanalysis, University of Maryland, College Park, Md. , May
19 66 .
(3)	 J. I. Trombka, I. Adler, R. Schmadebeck and R. Lamothe,
Report X-641-66--344,  Goddard Space Flight Center, Gi eenbelt, Md .
(4)
	
	 R. L. Heath, "Scintillation Spectrometry Gamma-Ray Spectrum
Catalog, " Phillips Petroleum Co. , Atomic Energy Division, Idaho
Falls, Idaho.
(5)	 W. H. Zachariasen, Theory of X-Ray Diffraction in Crystals,
(John Wiley and Sons, Inc.) 1945. A. H. Compton and S. K. Allison,
X-Rays in Theory and Experiment (D. Van Nostrand Co. Inc.) 1935
(6)	 W. L. Bragg, Proc . Cambridge Phil.
Soc . , 1.7, 43 (1912)
(7)	 D. A. Lind, W. West, J. W. DuMond, Phys. Rev. 77, 475 (1950);
O. I, Sumbayev, Soviet Union Jour. Exp. Theo. Phys. 32, 1275 (1957)
(8)	 J . W . M. DuMond and P. A. Kirkpatrick, Rev. Sci . Inst . 1, '88 *(19 30) ;
J. W. M. DuMond, Rev. Sci. Inst. 18, 626 (1947)
--	 _.  
- 10 -
Salaries*
Research Assistant
Secretarial Assistance
BUDGET REPORT
Requested
1500
600
ExFe ►lded
1722.16
Overhead
(32.2% of salary related costs) 	 676	 557.06
Equipment and Expendible Supplies* 	 3224	 4090.78
6000
University Cost Sharing	 360
6365
	
6360
*Approval for deviations in these items was received (see attached correspondence)


Low energy activation analysis for geochemical exploration

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