A LIQUID XENON RADIOISOTOPE CAMERA Page: 3 of 12
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In the positron mode, the blurring of the image due
to scattering in the tissue is reduced by accepting only'
unscattered gammas, and a high counting rate is
achieved by the elimination of the collimator. Lavoie
showed some advantages of the liquid xenon scintillator
for the detection of 500 keV annihilation y rays. 7 The
importance of having a large photofraction is greater
in this mode of operation, since a coincidence of two
photopeak electrons is required. The number of co-
incidences is proportional to the square of the photo-
fraction. In thin liquid xenon chambers the photo-
fraction at 500 keV is about five times larger than in
3. Electronegative Impurities in Liquid Xenon
As the free electrons drift in the liquid under the
influence of an electric field, some are lost by attach-
ment to electronegative impurities. After drifting
through a distance, x, the fraction surviving is given
N = Number of electrons surviving capture and
reaching the anode
N0= Initial number of electrons
x = Distance of travel (mm)
k = Probability of capture per mm of drift per
C = Concentration of electronegative impurities
The fraction N of the number of initial free elec
trons N0 that survive attachment and multiply near the
anode produce a pulse of magnitude:
Q = Q G e-kCx,
where G is the gain and Q0 the initial charge. In
order to insure the detection of all of the converted y
rays in the liquid, the pulse height due to the electrons
must be higher than the detection level of the readout
electronics. Electronegative impurities degrade the
energy resolution by relating the pulse height to the
position of the initial ionization. High purity is there-
fore essential for good energy resolution, especially
in thick chambers. Thick chambers are desired for
good conversion efficiency with high energy gamma
rays. Impurities could thus limit both the energy
resolution and the chamber thickness.
We have found that for oxygen in liquid argon at
fields of a few kilovolts per cm, k = 0.26+0.1/ppm/mm.
This means, for example, that only 80% of the initial
electrons arrive at the anode after traveling in a cen-
tral anode chamber 1.5 cm thick containing 0.1 ppm
02. Using techniques described in ref. 8, we have pre-
pared xenon gas that reliably contains less than 50 ppb
of oxygen and nitrogen (measured with a Varian helium
ionization gas chromatograph). Yet, we have observed
that the impurity level in a large size chamber con-
taining liquid xenon sometimes exceeds kC =1 per mm.
We find no difficulties in consistantly obtaining
clean liquid (kC < 0.02) by means of the following 2
a. Gas purification:8 The xenon gas continuously
circulates, by means of a positive displacement pump,
for several hours over a hot copper catalyst (200"C)
and cold (-77*C) molecular sieve 4A material (see
Fig. 4). The copper reacts with oxygen while the mo-
lecular sieve traps other unknown electronegative im-
purities. Every few weeks of operation, excess nitro-
gen is removed by passing the gas over hot calcium
b. Liquid purification by electronegative ion
pumping (ENIP): We found ENIP to be an excellent
means of further purifying the liquid in the chamber.
This is often necessary due to impurities not removed
by the purifier and probably introduced by the walls of
the chamber or by the purifier. In order to perform
this ion pumping, the voltage of the fine wire anode is
reversed and becomes negative. As the voltage is
raised, the wires emit electrons that are captured by
the impurities. The negatively charged impurities
that drift to the now positive electrode remain there
for several hours, even when the voltage is reversed
during the normal mode of operation. If kCx is high,
a current of 1 A can remove 10-6/1.6X10-19 or
6X 1012 molecules/sec. In a typical chamber
(5 cm long, 2 mm wire spacing. 1 cm thick) this
amounts to - 0.2 ppm impurities for 10 minutes of ENIF
Typically, 10 to 30 minutes of ENIP are sufficient to
provide full expected pulse height. The harmful im-
purities must be those present in a very small con-
centration, below the ppm level and with very high k
value. There are materials known to have an electron
attachment cross-section several thousand times
larger than that of oxygen (for example, SF6). 9 We
believe that outgassing is the source of most of such
4. Calculated Spatial Resolution and Detection Efficiency
We have measured the spatial resolution of liquid
xenon chambers to be better than 15 for alpha parti-
cles. 1 For y rays the resolution is limited in principle
by the radial distance traveled by the photoelectrons.
For 511 keV annihilation y rays we have measured the
spatial resolution to be better than 1mm rms (see
Section 5). In the following section we estimate the
ultimate spatial resolution for a gamma ray chamber.
The following scattering processes contribute:
(a) A primary photoelectric interaction in which all
of the gamma ray energy is absorbed by a photoelec-
tron. For the energies considered, the error due to
the path length of the photoelectron is small (see be-
low). The probability of generating a photoelectron
diminishes rapidly with increasing energy.
(b) A single Compton interaction followed by the escape
of the secondary gamma ray from the detector.
(c) A Compton interaction, followed by a secondary
scattering, may occur at a distance away from the
original gamma ray track, and may generate either a
photoelectron or another Compton electron.
The gross detection efficiency (Fig. 5) is the sum
of the three processes described. This was computed
for 0.75, 1.5, and 3 cm thickness of liquid xenon,
using the data given by McMaster4 and Hubbell. 5 The
only source of spatial error in processes (a) and (b) is
the movement of the conversion electron away from the
point of they-ray interaction. In xenon, multiple scat-
tering of the electron is so severe that the net dis-
tance that the electron travels is only a fraction of its
" range". It has been estimated by a Monte Carlo
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Zaklad, Haim.; Derenzo, Stephen E.; Muller, Richard A.; Smadja,Gerard.; Smits, Robert G. & Alvarez, Luis W. A LIQUID XENON RADIOISOTOPE CAMERA, report, February 1, 1972; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc898301/m1/3/: accessed September 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.