CALORIMETRY OF
97
BEAMS
TO BRIDGE
surrounding the electrodes, was made of l-in.-thick copper.
This provided a large heat capacity and, being protected
inside the vacuum system, was affected only very slowly
by changes in room temperature. A thermistor soldered to
it measured the ambient temperature about the target.
In the application4 for which this target assembly was
designed, the secondary electron emission was also measured. Collector No. 1. with a bias of +60 V attracted these
electrons, which were measured with a micromicroammeter. Incident particles that missed the target struck the
back wall of the shield box and were monitored by their
secondary electrons attracted to collector No.2.
One side of each of the thermistors was grounded and the
two were used in adjacent arms of a conventional Wheatstone bridge. Each of the arms was about 2000 Q, and one
contained a variable resistance so the bridge could be
balanced. The bridge voltage was 1.5 V so each thermistor
dissipated about 2.8X 10-4 W.
Two bridge detectors were used which gave about the
same sensitivity. The first was a 10-mV Brown recorder
(model S153X16V - X -156) with a 3 X 104 Q input resistance and i-sec balance time. After approximate balance
using the variable resistor, the voltage change indicated on
the recorder plot was taken as a measure of target temperature rise.
The second detector was a Fluke Manufacturing Company 801 HR differential dc voltmeter which was used as
a null detector. Bridge balance was obtained by the variable resistance which included a decade resistance box
(General Radio Company, type 670F) with O.l-Q steps.
The voltmeter was also used to adjust the bridge voltage to
within about 1 mY. Both detectors could be read to within
about 0.02 mV.
Using the above dimensions, the calculated value of heat
conductance through the support leads is G1 = 2.66X 10-3
W
It is assumed that the conductance through the
seal-throughs is high. Using the emissivity for scraped
copper, € = 0.07, the conductance from radiation is G2 = 3.86
X 10-5 WrC. Since the calculated specific heat C is
0.0473 JrC, the thermal time constant is r=CjG=17.S
sec. The observed time constant was about 30 sec. The
calculated G1 may be high because of the finite conductance
of the seal-throughs. Thus by waiting 4.6r = 2.3 min, the
target temperature difference will differ by only 1% from
the final equilibrium value. A beam power of 22 p. W
changed the bridge 0.1 mV and, since readings could be
taken to 0.02 mY, the approximate sensitivity was S p.W.
If this power is maintained for 60 sec (= 2r), the temperature will rise within 14% of its final value and 0.3 mJ
of energy will have reached the target. Interestingly, this
is the energy sensitivity quoted by Passell.2
It can be shown that if l::i.T < 2. 7°C, the thermistor
resistance is linear with l::i.T to within 1%. In measuring
l::i.T to within 1%, the bridge equations show l::i.T,....,l::i.Rvar
provided the change in the variable leg of the bridge,
l::i.Rv • r < 1%Rvar. Also, the shield box temperature should be
held constant within O.soC unless a correction is made.
THE REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 35, NUMBER 1
FIG. 3. Calorimetic
target assembly.
rc.
JANUARV 1964
System for Continuous Generation and Fast Transfer of Radioactive Gases
C. H.
FRANCES PLEASONTON AND
JOHNSON
Oak Ridge National Lahoratory, Oak Ridge, Tennessee
(Received 3 September 1963)
The system was developed to provide a stable and strong source of Re6 for the Oak Ridge National Laboratory
recoil spectrometer. A continuous flow of water-vapor swept the He 6 through about 80 ft of piping from the generator to the laboratory in 3 to 4 sec; the gas was then purified and delivered to the spectrometer in an additional 1 to
1.5 sec. Modifications in design for other experiments and for use in leak detecting are discussed.
INTRODUCTION
tudies of the energyl and charge2 spectra of Li 6 ions
recoiling from the
decay of He 6 required a continuous and stable supply of source gas. The system to be
S
1 C. H.
132, 1149
2T. A.
129, 2220
rr
Johnson, F. Pleasonton, and T. A. Carlson, Phys. Rev.
(1963).
Carlson, F. Pleasonton, and C. H. Johnson, Phys. Rev.
(1963).
described was developed for this purpose; later it was
also used in measurements of the half-life3 and internal
bremsstrahlung4 (IR) of He 6, as well as for spectrum
measurements in the
decays of5 Ne23 and 6 Ar41. The
rr
J. K. Bienlein and F. Pleasonton, Nuc!. Phys. 37, 529 (1962).
J. K. Bienlein and F. Pleasonton (to be published).
6 T. A. Carlson, Phys. Rev. 130, 2361 (1963).
6 T. A. Carlson, Phys. Rev. 131, 676 (1963).
8
4
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98
F.
PLEASONTON AND
necessity for a strong source, coupled with an 0.8-sec
half-life, imposed a stringent condition on the permissible
transit time of the He 6 gas from its creation in the reactor
to the laboratory, a distance of about 80 ft. It will be shown
that this system satisfied the criteria of purity, strength,
and stability required of the source gas.
DESCRIPTION OF THE EQmPMENT
Radioactive Rare Gas Generators
The He was produced by the Be9 (n,a)He 6 reaction in
the poolside facility of the Oak Ridge research reactor. An
aluminum cylinder (2.760-in. i.d.) containing 23 closefitting, shallow trays served as the generator box; it is
sketched, in limited detail, in Fig. 1. The trays (2.750-in.
o.d. and! in. deep) were arranged in a vertical stack that
was centered on the horizontal midplane of the reactor
core. They held a total of about 150 g of BeO powder,
packed with an estimated average density of 0.3 g/cm3•
The BeO was high purity, Brush Beryllium, grade UOX
powder 7 with a measured surface area of 15 m 2/g (B.E.T.
method). Small, off-center "chimneys" in the trays permitted a sweep gas to flow through the box; their slanted
tops, plus the alternation of the chimneys' positions by
180 0 in successive trays, forced wide circulation of the gas
over the powder. In order to prevent migration of the
6
C.
H.
JOHNSON
powder, loosely packed layers of quartz wool were included at the top and bottom of the box. Convection cooling sufficed to overcome the heating of the box by reactor
'Y rays « 4 W/ gat full power) and maintained the exterior
of the can slightly above the ambient temperature of the
pool (about 100°F).
The Ne23 and Ar41 were produced by (n,p) reactions on
about 100 g of Na- and K-alurninum silicate, respectively,
in the lattice of the reactor. This change in location of the
generator added about 40 ft to each of the flow lines, but
this was easily tolerated because of the longer half-lives:
about 40 sec for Ne23 and 110 min for Ar41. It did, however,
necessitate a change in the design of the box because only
vertical access was possible. Details of the trays and the
assembly of the generator are shown in Fig. 2. The trays
were ring-shaped, t in. high, and formed a IS-in. stack in a
2.S00-in.-i.d. cylinder. The inlet pipe (0.7S0-in. o.d.)
traversed the axis of the cylinder and admitted the sweep
gas at the bottom of the can. Intimate contact between
the trays and the coaxial cylinders was achieved by allowing radial clearances only in the lower half of the trays'
edges; slots at 10 0 intervals in the upper half, where
diameters matched or overlapped slightly, allowed one to
force the trays into position. Such contact was desired
for better heat transfer, but was not feasible in the other
generator because of the health hazard in handling BeO
powder. The box was cooled by the reactor coolant water
GAS OUTLET
COOLANT
WATER
rli:~~~~~-2.760
GAS
OUTLET
in. ID
~~=:~a-~o~'O" '~~l"-)
,
\12
'-u'----I+tt- GAS
INLET
in.OD CHIMNEY
L
\12 i n . .
P----1iiIi.!li""""~~__ SEE DETAIL OF TRAYS
r
'
f
(0.032-in. WALLI
2.500 in. 10
AT RIGHT
t'&~2:lll~lliL-.Li"" PERFORATED
02foinllC:::ULi._.J,_ '--,_~_~~(g
r
1.
~=11IJ.-0.750 in. OD
u
2.490in.OIA--~.1
SECTION A-A
I
DISK
SEE DETAIL OF TRAYS
AT RIGHT
SWEEP GAS INLET
IL--LATTICE PIECE
,1
'h
PERFORATIONS ON
OF
CIRCUMFERENCE ALONG BOTTOM
FIG. 1. Gas generator for use outside the reactor tank. The trays
contained loosely packed BeO powder from which He" was produced
by the (n,a) reaction. A continuous stream of water vapor passed
upward through the "chimneys" and carried the He" away from .the
generator can. The pool water cooled the generator by convectIOn.
7
R. L. Hammer (private communication).
/I
,I
i'-.------.~ ~COOLANT
~ll
WATER
FIG. 2. Gas generator for installation in the :~actor lattice. The
trays held shallow layers of Na- or K-aluminum sllicate powders from
which Ne'" or Ar4' were produced in (n,p) reactions. The a&sembly
was cooled by a stream of water flowing downward through the
annulus between the generator can and the lattice piece.
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HANDLING
RADIOACTIVE
99
GASES
PUMP
t
REACTOR TANK
TO
SOURCE
VOLUME
REACTOR CORE
TRAYS OF
AND TRAYS
SeO
POWDER
i--
N2 TRAPS
AND
PUMPS
WATER
DRAIN
PUMP
WATER _~~~==~~~
SOILER-
ICE WATER
RESERVOIR
POOLSIDE
FACILITY
o
MANUAL VALVES
I:8J
SOLENOID VALVES
o
PRESSURE GAUGE
.=r-. PRESSURE CONTROLLED
LABORATORY
U
MICRO SWITCH
FIG. 3. The He 6 gas flow and purification system. Automatic protection for the flow lines to the gas generator was provided by solenoid valves,
which are controlled by the pressure-sensitive MicroSwitches. After the water vapor carried the He 6 to the laboratory it was condensed and returned to the water boiler. The remaining gas was purified by passing over hot Cu and CuO and through the final cold traps.
flowing at high speed through an 0.046-in. annulus between
the can and the Allattice piece that contained it; in this
position the 'Y heating is about 10 W/g at full power, i.e.,
30 MW. No temperature measurements were made, but
auxiliary experiments 8 indicated that the temperature of
the powder remained at least below 400°C, which is very
much lower than the melting points of these silicates. If
fusion had occurred, emanation of the gases would have
ceased.
Gas Flow and Purification System
The source gas was supplied to the spectrometer by a
three-stage vacuum system in which the pressure was reduced, in successive steps, from 40 to about 10-6 Torr.
The basic elements of the apparatus are diagrammed in
Fig. 3. The first paat of the flow system was concerned
only with providing and controlling a sweep gas with which
to carry the radioactive gas, at high speed, from the generator box to the laboratory. The choice of a suitable gas was
dictated by considerations of its corrosive properties, the
"logistics" involved in maintaining a constant supply, and
radiation effects on the gas. Although alcohol is perhaps
superior in the first two respects, water vapor was chosen
because no serious complications result from its irradiatiOli,9 such as the excessive evolution of gas and the polymerization associated with hydrocarbons. The trivial
nature of the radiation damage allowed us to incorporate
in the gas-flow system the attractive 'feature of continuous
recircu~tion of the water.
8
9
T. A.'Carlson (private communication).
R. F. Firestone, J. Am. Chern. Soc. 79, 5593 (1957).
About 2 liters of water were adequate for operation. A
high evaporation rate was attained by heating the water
vessel above room temperature and by using a magnetic
stirrer to create a vortex in the water and thus increase
its effective surface area. The stirring was also useful in
eliminating spasmodic bursts of vapor. The boiler outlet
pipe was also heated in order to avoid condensation of the
vapor on its way to the reactor pool. The usual operating
pressure in the boiler was about 40 Torr.
In order to minimize the pressure drop between the
water supply vessel and the gas generator box, relatively
large piping (1-in. Ld.) was used for the ",80-ft connection.
Smaller piping (!-in. i.d.) was used for the equally long
return trip in order to transport the radioactive gas to
the laboratory under conditions of fast, viscous flow.
After the gas arrived at the laboratory most of the water
vapor was immediately removed by a condenser at icewater temperature. The remaining gas then went on to
the next part of the system, while the liquid returned to
the supply vessel through a vacuum-jacketed manometer
which was situated immediately below the condenser. The
normal pressure above the manometer was about 8 Torr.
The second part of the flow system contained a Cu-CuO
oven for removing the dissociation products of the irradiated water vapor, followed by a dry ice trap for condensing the remaining water vapor. The oven consisted of
about 100 parallel strands of No. 14 Cu wire, packed in a
tube which was 3 ft long, 1i in. i.d., and surrounded by
well-insulated tube furnace elements. It was preconditioned
for use by heating for a few hours, in the presence of an
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100
F.
PLEASONTON AND
J
DETAil OF BASE BAFFLE
4 112 in. 00
'I.
"4 in. WALL
SECTION A-A
FIG. 4. Vertical section of the titanium pump. A Ta filament carried
a double winding of Ti wire and a single winding of Nb wire. Water
cooling was provided for the filament through coaxial Cu tubes and
for the body of the pump through channels in the baffle and Cu
tubing soldered to the exterior of the case. About 2 kW of power were
required to maintain a suitable evaporation rate of the Ti.
. air stream, in order to oxidize some of the Cu. Maximum
efficiency of operation was obtained at a temperature of
about 500°C. An auxiliary liquid N 2 trap and mechanical
pump, following the CO 2 trap, were available for evacuating the system during start-up procedures or for maintaining steady flow conditions on a "stand-by" basis. Without
the disturbing influence of the oven, the gas flow in this
part of the system through about 6 ft. of i-in. tubing
would also have been viscous.
A single cold trap comprised the final stage of the flow
system for the He 6 and Ne23 spectral measurements. It
contained an N2 "slush" which was obtained by pumping
on liquid N2 and stabilizing the pressure over the mixture
at about 25 Torr above the triple-point vapor pressure.
Connecting piping in this section was large in diameter
and as short as possible, in order to obtain high conductance for molecular flow of the purified source gas to the
spectrometer.
Because the IB measurements on He 6 were made in
another laboratory, a 4O-ft extension of the piping was
required in the second stage of the flow system. This,
unfortunately, enhanced the strength of the contaminant
activities relative to the Heo content of the gas, a serious
situation because IB accompanies only about 2% of the
He 6 tr decays. The N 2 slush trap was, therefore, abandoned
in favor of an ordinary liquid N 2 trap followed by a
high conductance, continuously evaporating Ti pump,
in order to obtain more positive control of the final
C.
H.
JOHNSON
purification of the gas. Figure 4 shows a vertical cross
section of the pump. The filament lO was made of 0.170-in.
Ta rod with a single winding of 0.030-in. Nb wire and a
double winding of 0.035-in. Ti wire; the purpose of the
Nb is to alloy with the Ti and raise its melting point, thus
pennitting more easily controlled evaporation. About 2.0
kW (410 A at ",5 V) were required to maintain minimum
effective evaporation of the Ti. Cooling was provided by
water flowing through coaxial Cu tubes to the filament
clamps, through a tight winding of Cu tubing soldered to
the exterior of the housing, and through three channels in
the base baffle. Continuous evaporation of the Ti was
required because of the high partial pressure of hydrocarbons in the residual gas, which arose from an extensive
use of Lucite in the IB vacuum system. A filament normally lasted for about 60 h of operation, although a few
required replacement sooner. After termination of the IB
experiment the Ti pump was installed in the original
system and used during the measurements 6 of the charge
spectrum of Ar41. The longer half-life of the Ar41 permitted the source gas to be held in a reservoir, in communication with the pump, thus maximizing the purging
action of the Ti. At equilibrium, the pressure in this chamber built up to about 7X 10'-5 Torr and the gas was
admitted to the spectrometer through an adjustable leak.
Manually operated valves were strategically located to
pennit almost any section of the system to be isolated for
individual attention. In addition, three solenoid-operated
valves were installed in the flow lines to the genemtor to
provide automatic protection and equalization of pressure,
in the event of an unexpected loss of vacuum or electric
power while the gas was flowing. Bellows-type, pressureoperated MicroSwitches controlled these valves; they were
set to operate at about 65 Torr. There was, unfortunately,
no protection against the human errors that could result
in a sudden mass transfer of the water supply from the
boiler to the CO 2 trap, as the authors can testify!
The water vapor that had accumulated on the dry ice
trap was returned to the supply vessel between periods of
operation. This was usually accomplished overnight by
removing the dry ice and isolating the trap except for
vacuum communication with the ice-water condenser; the
direction of the vapor flow was thereby reversed and the
water slowly returned to the supply vessel. (A continuously
operating system· should be possible with the use of a
parallel, or alternate, system of dry ice traps and an
auxiliary condenser.) After the water vapor was returned to
the boiler, other gases that had been released were then
pumped out of the trap.
A high radiation background accompanied the flow of
the source gas. During measurements of He 6 with the
10 R. E. Clausing, in Transactions of the Eighth National Vacuum
Symposium (1961), editorial supervision of L. E. Preuss (Pergamon
Press, Inc., New York, 1962), Vol. 1, p. 345.
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HANDLING RADIOACTIVE GASES
recoil spectrometerl the most intense, localized sources
(in mR/h) were the glass ion-gauge tube on the source
volume (2500), the entrance to the water condenser (1750),
the mechanical fore-pumps on the source volume and the
magnetic-deflection chamber (800 and 200), and the N2
slush trap (400). One-inch-thick plywood shielded against
{3 rays that were not stopped in the walls of the vacuum
system; it reduced the above readings by a factor of about
10. Additional Pb shielding protected personnel and minimized backgrounds for the particle detectors of the spectrometer; except for brief excursions into fields as high
as 200 mR/h, the experimenters worked in a field of only
2 mR/h. Protection from inhalation of radioactive gas was
provided by exhausting all pumps through the reactor
"off-gas" system, at slightly less than atmospheric pressure.
PERFORMANCE OF THE SYSTEM
Flow Rates
Calculations of conductances, flow rates, and transit
times were made for the flow system based on Poiseuille's
equations for viscous flow in a straight, cylindrical tube.H
Our system could hardly be described as straight, but it
should fulfill the major criteria for viscous flow, with the
possible exception of behavior in the hot Cu-CuO oven.
Using the dimensions and operating pressures of the He 6
poolside system the calculations predicted a volume flow
rate of about 2X 105 JLI secI, for pure water vapor, and a
transit time of about 0.6 sec for one flushing of the gas in
the generator box over to the laboratory. A preliminary
experiment using a helium leak detector with an approximate model of the system suggested, however, that the
time would be more nearly in the region of 3 or 4 sec;
later experience in leak checking of the actual system
confirmed the suggestion. This is not an unreasonable discrepancy, considering the many bends in the piping and
changes in temperature to which successive portions were
exposed. A transport time of 3 sec would imply a flow rate
of about 4X 104 JLI secl of water vapor, or the equivalent
of 150 g/h of liquid water.
Without the disturbing influence of the oven the gas
flow in the purifying stage of the system would also be
viscous, with a transit time of less than 0.2 sec. Comparative radiation measurements between the high and low
pressure ends of the oven were, however, indicative of a
"bottling-up" of the gas at the entrance to the oven. For
the final oven geometry, which was developed experimentally to give a reasonable compromise between purification
and flow rate, it was concluded from such measurements
that a reasonable estimate of the time would be about 1 to
1.5 sec.
11 Saul Dushman in Scientific Foundations of Vacuum Technique,
edited by J. H. Lafferty (John Wiley & Sons, Inc., New York, 1962),
2nd ed., Chap. 2, p. 82.
101
Source Strength
An estimate of the rate of production of He 6 gives
7.2X 1011 atoms created per second in the generator box.
This is based on 150 g of BeO powder, a fast flux of about
2X 1013 n/cm2-sec, and a total "effective" cross section12
of u.= 10-26 cm2 • To determine the efficiency of the flow
system, this value is compared with our best estimate of
the equilibrium amount of gas delivered to the source
volume. This quantity is obtained from the observed
counting rates of singly charged Li 6 ions integrated over
the energy spectrum and corrected for the relative abundance of charge-two ions, for detection efficiency, for energy
resolution and transmission of the spectrometer, and for
the geometry of the conical source volume; this yields a
value of 8.1X 109 decays per second for the source strength.
After adjusting this for the decay rate of He 6 and for losses
caused by pumping on the source volume, one obtains a
value of 9.9X 109 He B atoms arriving per second at the
spectrometer, or about 1.4% of those produced in the
generator. If one assumes 100% emanation and collection
of the He B, then this efficiency implies a total time interval
of 6.15 T t , or 4.9 sec, between creation and use of the
source gas. This is in good agreement with the empirically
determined estimates given above.
For Ne23 and Ar4I, the longer piping from the reactorcore position could be expected to about double the transport time to the laboratory. As many as 10 sec could,
therefore, elaspse before the gas reached the spectrometer.
In relation to their half-lives this represents about 85 and
95% efficiency in use of the gases, if the assumption is
again made of 100% emanation and collection of the
radioactive gases. That this is a good assumption for
emanation of these gases was shown by an auxiliary experiment S ; regarding collection, it is probably better for
this generator, because of shallower layers and looser
packing of the powders.
Radioactive Impurities
The use of triply distilled water as the vapor source
minimized radioactive contaminants in the gas; however,
enough extraneous elements were present in the vacuum
system itself to contribute several species. Analyses of
several samples of condensates from the liquid N 2 trap of
the "stand-by" pumping system showed the presence of
U235 fission products (Ba l40 , Lal40 " Cel41 and CS137) , rather
large amounts of S36, and traces of mercury isotopes
(H g197m, H g197, and H g203); in addition, the shorter lived
Hg205 was observed with the IB scintillation counter. The
presence of these isotopes indicates that we were not completely successful in preventing migration of Hg from the
spectrometer diffusion pumps to the flow system. The
source of the other contaminants is presumed to be the
12
R. S. Rochlin, Nucleonics, Data Sheet No. 28, January 1959.
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102
F.
PLEASONTON
fluxes used in soldering joints in the Cu and brass control
system. This is a reasonable assumption since only trace
amounts of U and CI could account for the observed
quantities of the fission products and of the S35, which was
probably created in the Ci3 5(n,p)S35 reaction. These
activities contributed to the radiation background in the
laboratory, but did not interfere with the experimental
measurements.
Two contaminants were, however, of importance to the
measurements, NI6 and Ne23 . The presence of NI6 was not
surprising because of the abundance of 0 16 that was
available for its production in the 016(n,p)NI6 reaction; it
was identified by half-life measurements (",7.5 sec) of gas
samples that were isolated in the N 2 slush trap. The fact
that its relative abundance, in the source volume of the
recoil spectrometer, was very sensitive to the temperature
of this trap suggests that it was present in large measure as
NO. Searches for the 'Y rays in the IB spectra showed that
the Ti pump was extremely efficient in eliminating the NI6
from the gas; they set an upper limit f~r its relative abundance of about 8Xl()-3% of the Re 6 activity. Ne23 was
found in the IB source gas to the extent of about 0.1%
of the Re 6 {j activity, or about 5% of the IE emission.
The origin of the Ne23 is not known, but it could
have been a known Na impurity of 100 ppm in the BeO
powder. Two other equally abundant impurities in the
BeO, K and Mg, could also have produced contaminant
activities; they would have been far less evident, however,
because the short gas-transport time and the continuous
pumping on the source volume of the recoil spectrometer
discriminated in favor of the Re 6.
Stability
The counting rate of a proportional counter that monitored the activity in the source volume of the recoil
spectrometer indicated that the supply of source gas was
extremely stable. During one typical run of 12 h the monitor showed a slow rise of about 4% during the first 8 h
and then fluctuations of less than 1% for the remainder of
the run, including data taken immediately after the reactor
returned to full power following a short, unscheduled
shutdown; variations in intensity of the source were usually
less than 5% from one day to the next and were attributable to variations in setting the Variac that controlled the
temperature of the water boiler. To achieve this gratifying
stability required closely controlled reactor power, a constant sweep rate from generator to spectrometer, and
steady maintenance of refrigerants in the cold traps and
AND
C.
H.
JOHNSO~
water condenser. There were no problems with the first
two condtitions; the one that required our careful attention was the N2 slush trap because of the NI6 that condensed on it. If the N 2 in the trap was allowed to solidify,
by pumping on it too fast, it made poorer contact with the
metal container and led to an immediate rise in the partial pressure of NI6 in the source volume. Since the proportional counter could serve as a proper monitor of the
source strength only if the NI6 were present as a stable
fraction of the Re 6 activity, extremely close control of the
temperature of the trap was required. Use of the Ti pump
eliminated this hazard in the later experiments.
REMARKS
As stated previously, water vapor was chosen as the
sweep gas because of its favorable radiochemical properties.
However, in a transfer system that is not subject to radiation exposure, organic solvents such as ethanol, methanol,
and acetone would be superior to water. At room temperature their vapor pressures are from two to ten times as
great as that of water, and their viscosities are about the
same or somewhat lower; thus, higher sweep velocities can
be easily obtained. Furthermore, their triple points are at
such low temperatures that the vapors condense to liquids,
with pressures of considerably less than 1 Torr, at dry ice
temperature. This permits the dry ice trap to replace the
water-ice trap in performing the function of condensing
the vapor and returning it to the boiler. Continuous operation would still be possible with the use of parallel liquid
N 2 traps following the dry ice trap.
One practical application would be as an adjunct to a
helium leak detector in testing long pipes. A unit would
include the vapor source (boiler), dry ice trap, liquid N2
trap, and appropriate valves so that a vapor stream could
be established in the pipes to be tested. Helium gas entering a remotely located leak would then be carried swiftly
to the leak detector. Such a system, using alcohol vapor,
was tested and its operation was quite impressive.
ACKNOWLEDGMENTS
We are indebted to Dr. P. S. Rudolph for his advice on
the radiochemical properties of gases. We would also like
to thank R. E. Clausing for helpful discussions on techniques of Ti pumping and R. L. Hamner for supplying
the BeO powder and allowing us to use his facilities for
handling it. Acknowledgement is also given to Dr. T. A.
Carlson for communicating information relative to his
N e23 and Ar41 experiments.
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