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New challenges in clinical comting
With the introduction of the so-called "nedical spectrometer" into
nuclear medicine, the reliability of the measurements has improved con-
siderably, for the couter's attention can now be focussed on the informative
components in the radiation while background and other undesirable juns
can be greatly reduced. The scattered radiation that is always present
when the source lies inside a patient can now be made less troublesome,
-
-
-
-
-
-
;
-
-
thus making it easier for us to design a realistic "phantom" in which the ;
standard of comparison will be counted. The spectrometers may also be
able to untangle a mixture of two or more radionuclides, they can check
the purity of a questionable radioactive drug, and so on. This selective
counting of gamma rays was not possible in the Geiger-tube days, and it
was difficult even in the early days of scintillation counting, when most
of the instruments operated on the integral or "threshold" basis. Ir
the spectrometer's full potential is to be realized, however, the instru-
ment must be operated with intelligence and insight. Our doctors and
technicians are learning to do this.
Stability - Returning to the threshold counters for a moment, it is
not always realized how easily disturbed they are by factors that change
the electronic amplification (including that in the photomultiplier tube)
..
.
-
-
-
...
-
.
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.
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or cause a drift in the location of the electrical threshold, which has
-
.
..
.
..
.
somewhat the same effect. Figure 1 shows superimposed integral and differ-
ential spectra for a sample of barium-133, the integral spectrum being
scaled down by a factor of about 16 to keep it from running off the chart.
-
.
.
.
.
In search of stability in a threshold counter, we look for a place where
-
.
..
-
.
.
.
.
i
-
.
the integral curve is flat, since in such a location the threshold could
f
-
-
-
-
-
:
.
* Text of a paper presented at a meeting of the Japanese Society of
Nuclear Medicine, Tokyo, November 18, 1964. Work sponsored by the U.S.
Atomic Energy Commission, under contract with Union Carbide Corporation.
-2-
move a little without changing the count rate. But the integral curve
climbs continuously and is not flat anywhere, so the best one can do 18
to find a place where the slope 18 minimum. Or, we should really say,
a place where the percent slope 18 minimum.
With the spectrometer, on the other hand, there are several places
where the curve goes through a maximum, and here the window could shift
a little without doing any harm. For several reasons (1), the most useful
counting windows are fairly wide, and if the spectrum shows were run
again with a wide window the peaks would become quite broad, thus offer-
ing regions of good stability.
Reduced irradiation of the patient -
.
.
This is one of the prominent concerns in nuclear medicine today.
Nearly everyone recognizes it as desirable in principle, but it does
.
. .
-
.
create problems. The smaller the dose of radioactivity one gives to the
.
.
patient, the less radiation there will be for the counter to measure.
-
-
--...
Thus we are being pushed iuto the field of low-ectivity counting, and con-
.
.
siderations of efficiency acquire new importance.
.
Geometrical efficiency. - Rays come out of a source in all directions;
only a fraction of them will reach the detector, and we call this fraction
the "geometry".
Too often it is distressingly low, and Figure 2 shows
why. We can imagine a spherical sheil with the radioactive source at
its center and the detector covering part of the spherical surface. The
fraction of the surface covered gives a measure of the geometrical.
efficiency. Taking a practical example, if the diameter of the crystal 18
6 cm, anå the distance to the source is 15 cm, the occupied fraction of
the surface will be very nearly IX3 o, or about 1 percent. Thus the
40 x 252 of about 1 percent
:
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!
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very first thing that happens is that 99 percent of the radiation is lost.
We can improve the geometry nearly 100 times by switching to the "well
crystal" type of detector, but an ordinary well won't accept samples
larger than a few cubic centimeters.
Cassen has designed a well counter that can accept a
120-cc
sample
(4). Sodium iodide makes an excellent scintillating material,
but the cost becomes prohibitive when one tries to surround a large
sample with solid sodium iodide. Accordingly Cassen uses broken pieces
of the crystal material, imersing them in a silicone fluid of high re-
fractive index. His counter's geometrical efficiency 18 good, but its
energy resolution 18 pnor. We will come back to the question of resolution
in a moment.
Intrinsic crystal efficiency - Recognizing that many of the rays
emitted from a source will miss the crystal, we must also recognize that
some of the rays that actually enter the crystal will go right through it
without making any ions, in which case there will be no scintillation
and no count in the scalor. Moreover, some rays do interact but gonerate only
weak scintillations, because the first interaction may be a Campton ovent (it
usually 18) and the scattered ray sometimes escapes from the crystal, carrying
part of the energy with it.
As we shall 360 in a moment, we can't permit the
spectromotor to count all suintillations; whether weak or strong, because to do ...
.
1.0
.
2
T:
FM
.
80 would bring in too many background counts. The "primary peak" in a spectrum
(see Fig. 3) reprosents strong scintillations that result from the absorption of
all the energy of the incoming photon (hence we call it the "total-absorption poak"),
and it is in thio part of the spectrum that we can count the greatest number of
informative rays cambd ned with a minimum of background. Accordingly the most
important numbor describing crystal efficiency is the "poak intrinsic efficiency",
by which won the ratio photons absorbed totally
all photons ontoring crystal
T. This ratio will
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be energy-dependent, for more of the scattered radiation will be lost if
the incoming epergy is high. The ratio will also depend on crystal size,
since escape is easier in small crystals. Curves are available for crystals
of verlous sizes, showing intrinsic efficiency plotted against energy (2,3).
1
T
A
.
1
3
2
"Total intrinsic efficiency" Includes all the photons that make any kind
of scintillation; the "peak-to-total ratio" 18 total-ab
all scintillations
Thus, peak intrinsic efficiency is the product of the other two.
Window efficiency - In very low-activity counting one should take care
to use the spectrometer's window properly, for the window 18, essentially,
a mechanism that throws counts away. Ironically, it turns out best to
throw away about 15 percent of the counts in the total-absorption peak
of the spectrum. We normally place the window so that it covers the pri-
mary peak, and we muet decide how .wide its opening should be. If we make
it too narrow it will throw away too many of the informative ("signal")
counts, whereas if it is too wide it will let in too many undesirable ("noise")
counts -- background and what-not. The compromiss giving best statistics is
shown in Figure 3, where the window catches 85 percent of the pulses in
the primary peak; if you try to get more than this it will cost too much
in terms of "noise". The top and bottom of the window are adjusted to
ir
i..
1
-
131
2
cut the curve at the points where the count rate is 1/3 of the maximum
1.!
or peak rate. Fortunately this choice of opening is not very critical;,
0
if stability is also a serious consideration, the window can be made somewhat
wider (1, p. 28) without much loss in statistical reliability.
The foregoing discussion shows why it is important to have good reso-
lution in a spectrometer. If the resolution 13 poor, so that the primary
peak is spread out, the "85% window will have to be made wide in order
il
to fit the peak properly, and this widened window will bring in more back-
.
7
.
.
.
"
-50
In short, statistics are made worse when the resolution is poor.
ground.
Quite apart from this, good resolution is important in distinguishing
the primary radiation from scatter, in sorting out the several components
of a mixture, in plotting a spectrum, and so on.
Figure 4 shows a large-sample counter that retains good resolution (5).
It uses a single sodium iodide crystal, 125 x 10 cm, and a 10-liter sample
surrounds it on all sides except that needed for the phototube. With
this arrangement, all parts of the sample lie within about 8 cm of the
crystal.
The average geometry is considerably less than 50 percent, but
the large pass of the sample helps to compensate for this.
Background problems are likely to be bad around a hospital, because
soice of the sources of background will be variable. They get turned on
2
.
and off (e.g. X-ray and therapy machines), or they may be moved from
place to place (e.g. doses, patients, excreta, etc.). If very low-level
counting is to be done, therefore, a hospital wing should be designed
to have a "hot end" and a "cold end", to keep sensitive counters as far
as possible from the sources of variable background. This has been
partially achieved at the Medical Division of the Oak Ridge Institute of Nuclear
Studies (@INS). There the whole-body counter, and the 10-liter sample count or
referred to above, are isolated by themselves at one end of the hospital wing
and, for good measure, they are buried in a bank of earth. Special materials,
selected for rear-zero radioactivity, are used in the construction of the
walls of the "cave" where the patients are counted (5). Figure 5 shows
the floor plan of the underground annex; note the 4 meters of earth between
the cave and the hospital proper. There is considerable additional earth
betweon the cave and the teletherapy machines, which are off to the left in the
7 o'clock" direction. The subjects to be counted take a shower and put on
21
.
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a clean X-ray gown before going into the "clean laboratory", where only
the subjects and the operator of the equipment are allowed to go. Visitors,
carrying who-knows-what on their shoes, are expressly kept out, but they
may watch the operations from tive office through a lerge window in the
wall. While being counted the patient lies on a canvas stretcher and is
.
surrounded by a thin, plastic box so that only 8 percent of the air in
the cave need be ventilated. In addition, the air is carefully filtered
..-..
first. The
provisiona lould considerably reduce the contamination
hazard from radon, fallout, etc.
Low-energy emitters - Not long ago the medical people suddenly
realized that the electron-capture nuclides give off neither alpha nor
beta particles, and thus their destructiveness to surrounding living
tissues is greatly reduced. They do deliver a definite "on-site dose",
but usually it is rather mild, being due mainly to conversion and Auger
electrons, plus a contribution from photons having energies of 10 kev
or less. These considerations brought the electron emitters quickly into
HC
L
.
"
the diagnostic field, where they are enjoying a surge of popularity. It
so happens that most of the medically useful members of this group are
.
low-energy emitters, so now the instrument designers are presented with
I
the problems of counting photons in the 20-100 kev energy range, and of
locating their points of origin with a direction-sensitive detector. .
We fully sympathize with doctors who want to keep down the radiation
-
E
.
dose received by the patient, but it should be pointed out that low-energy
$
rays have a few unfortunate habits. The patient will absorb many of
.
..
them, for one thing, which will make quantitative counting difficult, and
more than usually dependent on the "realism" of the phantom in which the
..
standard of comparison will be counted. No phantom, of course, can hope
IYI
..
.
2
.
TEX
.
to match the absorption and scattering in all patients: people are just
too variable. But the quantitative, in-vivo procedures are not the only ones.
in trouble, for the low energies also bother the directional devices ---
notably the honeycomb collimators. Figure 6 shows the two kinds of rays
that can deceive a directional detector: (1) rays that penetrate the septa
between the holes in the honeycomb, and (2) rays that come into the field
of vision from off-target locations and then are scattered up into the
detector. At low energies the penetration problem is lessened, but the
scattering problem becomes worse (6). There are three reasons for this.
First, the Klein-Nishina function (8) shows what there is more scattering
at the lower energies -- mors deflected rays to worry about. Second, the
Compton equation (7) shows that the scattered energies lie close to the
primary, making it harder for a spectrometer to identify and reject the
deflected rays. Third, the spectrometer approaches this task partially
ti
crippled, for its resolution gets worse as the energy goes down. These
la finestra
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three hazards are shown pictorially in Figure 7.
We can offer the spectrometer a helping hand by moving its analyzing
window somewhat to the right of the normal position. This utilizes the
steeply sloping, left side of the Gaussian response characteristic to
sharpen up the differentiation between the primary rays and scattered rays
whose energies have been only mildly degraded. Figure 8 shows how such .
an asymmetrical window is applied in the case of cobalt-57. This trick
se
t
in the entire
involves a loss in count rate, but it can clean up a scan considerably,
suppressing off-target rays that would make the outlines less sharp.
in the internet
Securities
This
problem 18 discussed more fully in reference (6). For reasons of stability,
:
the asymmetrical window should not be used unless it is needed.
In short, the low-energy emitters have vices as well as virtues, and
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both should be considered in deciding whether or not to use one cť them.
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Currently we think that for scanning purposes an energy of around 150 kev
is about ideal. Below this energy the difficulties begin to mount, and
one should avoid them if possible.
Because these newer nuclides are currently receiving so much atten-
tion, we might do well to tabulate a few of their advantages and draw-
backs..
3
-
1
Drawbacks
1)
Advantages
less dose to the patient per uc
(but the real question is:
? more actual information
per unit of patient damage ?)
1) poor geometry
(if crystal is small);
.
2) special crystal and can;
3) special colliinator, for
full efficiency;
a) less strain on the detector:
a) good intrinsic efficiency
with small crysta.l.;
b) negligible collimator leakage;
4) "escepe peak" in spectrum; .
5) poor energy resolutiou;
mow nae mouston.
c) reduced background;
6) electronic noise problems;
a) smaller size, weight, cost,
etc.
7) phantom troubles;
8) scatter troubles.
The foregoing review, without pretending to be complete, presents
some of the developments in counting and shielding techniques that are
needed to meet nuclear medicine's increasing instrumental demands. They
re-emphasize the need for good spectrometry in the clinical counting of
!
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gamma rays.
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AXX
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References
-
materinsti
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-
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(1) D. A. Ross (59): Medical gamma-ray spectrometry, U.S. A. E. C.
report ORINS-30, pp. 24 & ff (off. Tech. Serv., Dept. of Commerce,
Washicgton 25, D.C.)
(2) C. C. Harris, D. B. Hamblen, and J. E. Francis, Jr., ('59): Basic
7
n'hidant
a
principles of scintillation comting for medical investigators, V.S.
A.E.C. report ORNL-2808, pp. 9 and ff. (Office of Technical Services;
Donnais
-
-
this report is bound together with (1), above).
*
,
'
'
(3) R. L. Heath ('64): Scintillation spectrometry, Second edition, vol. 2,
-
.
taurimit
appendices 2 and 3, U.S. A.E.C. report IDO-16880-1 (or TDD-4500)
(off. Tech. Serv., Dept. of Commerce, Washington 25, D. C.)
(4) B. Cassen ('62): J. nucl. Med. 3, 128.
(5) D. A. Ross and A. C. Morris, Jr., ('62): Medical Division Research
Report ORINS-42, p. 48 (off. Tech. Serv., Dept. of Commerce, Washington,
25, D. C.),
(6) D. A. Ross, C. C. Harris, M. M. Satterfield, T. R. Bell, and J. C.
Jordan ('64): Fadioaktive Isotope in Klinik rand Forschung, Band 6,
108 (in press; Urban und Schwarzenberg, München).
(7) A. H. Compton ('23): Phys. Rev. 21, 483. See equation (3), p. 486.
(8) O. Klein and Y. Nishina ('29): Z. Physik 52, 853. For graphical
development of the equation see A. T. Nelms ('53): U: S. National
Bureau of Standards Circular 542, Washington, D. C.
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Legends
Fig. 1.
Differentia). and integral spectra for a small source of barium-
133, in air. The integral curve was run at about 1/16 sensi-
tivity, to keep the graph from running off the chart. This
curve rises continually, offering no flat regions where sta-
Fig. 2 -
differential spectrum shows & peak. (Spectra courtesy of Medical
Division, Oak Ridge Institute of Nuclear Studies.)
Detector "geometry" can be estimated by imagining a small source at
the center of a sphere (radius R), with the detecting crystal
(radius r) occupying part of the spherical surface. The ratio of
the occupied area (approximately ar2) to the whole surface (45 R2)
gives the fraction of the radiation that reaches the crystal. With
the source only three crystal diameters away, more than 99 percent
of the rays are lost.
A gpectrum of chromium-52, with the background spectrum below.
When the sample is very weak, the spectrometer's window should
see only 85 percent of the pulses in the total-absorption peak,
for beyond this point the rapidly increasing "noise" overpowers
the slowly increasing "signal". (Spectra courtesy of ORINS
Medical Division.)
Low-activity counter for 10-liter samples. The large mass of
Fig. 3 -
Fig. 4 -
sample, which surrounds the crystal except on the bottom,
makes up for a detector geometry considerably less than 50
percent. Courtesy of ORINS Medical Division; redrawn from
(5), page 51.
Fic. 5 -
Plan of the whole-body-counting facility at ORINS Medical
Division (unpublished, by permission). This annex is buried
-ll-
in a bank of earth, which shields the patient-counting "cave"
from the variable radiation in the hospital. The dotted line
shows the path of a subject from the office, through the shower,
to the "cave", and back to the office. The "clean laboratory"
18 a restricted area, ventilated with carefully filtered air
at slight poeitive pressure. Visitors can watch the procedures
through the window in the office wall, but are not allowed to
go in.
Fig. 6.
Showing two linds of rays that a honeycomb detector 18 not
supposed to see, since they originate outside the legitimate
field of vision. The penetrating rays go through the septa
between the collimator's holes. The scattered rays come
into the field of vision and are deflected upward into the
crystal. At low energies there is less trouble from pene-
tration, but more from scattering. From (6), courtesy of
* See page // A Urban
as Urban' und Schwarzenberg, München.
118_1 - Showing the three ways ta which scattered radiation becomes
troublesome at low ecergies -- 100 kev compared with 500. Upper
curves: Compton's equation (7) shows that the scattered energies
(single collision) will be crowded together when the primary energy
18 low; Klein-Nishida calculations (8) show that the scatter 18 al.
Bo more abundante Lover curves: These show how a spectrometer's
resolution gets worse, at low energies, just when good resolution
18 needled. From (6), courtesy of Urben und Schwarzenberg, München.
Fig. 8 -
Cobalt-% scans, showing the cleanup value of the asym-
metrical window (lover loft) when the primary energy is low.
From (6), courtesy of Urban und Schwarzenberg, München.
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Fig. 1 -
Differential and integral spectra for a small source of barium-
133, in air. The integral curve was run at about 1/16 sensi-
tivity, to keep the graph from running off the chart. This
curve rises continually, offering no flat regions where sta-
bility would be good. It rises mout steeply wherever the
differential spectrum shows a peak. (Spectra courtesy of Medical
Division, Oak Ridge Institute of Nuclear Studies.)
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Fig. 2.
Detector "geometry" can be estimated by imagining a small source at
the center of a sphere (radius R), with the detecting crystal
(radius r) occupying part of the spherical surface. The ratio of
the occupied area (approximately ar2) to the whole surface (48R2)
gives the fraction of the radiation that reaches the crystal. With
the source only three crystal diameters away, more than 99 percent
of the lays are lost.
B
WINDOW

COUNT
RATE
Cr 51
-FWHM
TAX FWHM
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KEV
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Fig. 3-
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A spectrum of chromium-51, with the background spectrum below.
When the sample is very weak, the spectrome ter's window should
806 only 85 percent of the pulses in the total-absorption peak,
for beyond this point the rapidly increasing "noise" overpowers
the slowly increasing "signal". (Spectra courtesy of ORINS
Medical Division.)
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Fig. 4.
Low-activity counter for 10-liter samples. The large mass of
sample, which surrounds the crystal except on the bottom,
makes up for a detector geometry considerably less than 50
percent. Courtesy of ORINS Medical Division; redrawn from
(5), page 51.

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Plan of the whole-body-counting facility at ORINS Medical
Division (unpublished, by permission). This annex is buried'
in a bank of earth, which shields the patient-counting "cave"
from the variable radiation in the hospital. The dotted line

..---..
.........
.
.............
shows the path of a subject from the office, through the shower,
to the "cave", and back to the office. The "clean laboratory"
18 a restricted area, ventilated with carefully filtered air
at slight positive pressure. Visitors can watch the procedures
through the window in the office wall, but are not allowed to
Li
go in.
..
.
.
.
! Fig. 6.
Showing two kinds of rays that a honeycomb detector is not
1
supposed to see, since they originate outsiảe the legitimate
field of vision. The penetrating raye go through the septa
between the collimator's holes. The scattered rays come
into the field of vision and are deflected upward into the
crystal. At low energies there is less trouble from pene-
tration, but more from scattering. From (6), courtesy of
Urban' und Schwarzenberg, München.
-
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(A) PENETRATING
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(B) SCATTERED RAYS
SOURCE IN
OFF-TARGET
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AREA OF MAXIMUM
SENSITIVITY
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PRIMARY
230
450
PRIMARY -
230
900
450
1350
RELATIVE ABUNDANCE
900
1800
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1350
1800
50
100 kev
500 kev
COUNT
RATE
RESOLUTION-
RESOLUTION --
LOW
ENERGY
MEDIUM
ENERGY
Fig. 7-
Showing the three ways in which scattered radiation becomes
troublesome at low energies (100 kev compared with 500).
Compton's equation (7) shows thet the scattered energies will
be crowded together when the primary energy is low; the
Klein-Nishina calculations (8) show that the scatter is also
more abundant. The lower curves show how a spectrometer's
resolution gets worse, at low energies, Just when good reso-
lution is needed. From (6), courtesy of Urban und Schwarzen-
herg, München.
"
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UNCLASSIFIED
ORNL-DWG 63-5840
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ME= 80 - 145 kev
AE = 105 – 145 hev


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9
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AE = 116 - 146 kev
TOP VIEW, PHANTOM
C057
30 uc EACH VIAL
SPEED: 0.4 in./sec
DOT FACTOR : 2
DEPTH (LOWER VIAL): 3 in.
Fig. 8 -
Cobalt-57 scans, showing the cleanup value the asym-
metrical window (laver loft) when the primary energy is low.
From (6), courtesy of Urban und Schwarzenberg, München.
:
-***
so that wanna worries mange k
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A2
ABSTRACT
New challenges in clinical counting.“ D. A. Ross, C. C. Harris,
M. M. Satterfield, and P. R. Bell, Oak Ridge National Laboratory.
-- In
the field of nuclear medicine the value of the pulse-height spectrometer
is now well recognized. Given an alert and informed operator, it can re-
duce the obscuring effects of background; it can at least partially suppre88
.
unwanted scattered radiation; it can provide better stability (nther
.
.
things being equal) than is available in a "threshold" counter; it can
.
.
.
check the radiochemical purity of a questionable drug; and often it can
.
..
measure separately two radioactive components in the same sample. The
..
a
•
persistent demand for still lower radiation doses to the patients, and the develop
ment of now, low-energy emitters, have created new difficulties for the counting
equipment, and skillful operation is more important than ever. Good
counting efficiency is hard to obtain, and the factors that determine it
should be well understood if weak samples are to be counted. The
selection of the spectrometer's energy band depends on resolution, as
well as on other things. The counting of large, weak samples, and of whole
patients, brings up a number of difficult problems, and calls for an all-
out attack on background. The low-energy radionuclides are focussing
renewed attention on the problems resulting from absorption and scatter.'
ing, and special techniques may be needed. Recent arrivals in the moder-
ate- and low-energy fields are: gold-199 and lodine -723 (159 kev), cerium-
141 (145), technetium-99m (140), cobalt-57 (122), xenon-133 (81), mercury-197
(67-78), cesium-131 (30), and iodine-125 (28 kev).
"Research sponsored by U. S. Atomic Energy Commission under contract
with Union Carbide Corporation.
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