A RADON PUMP*

BY L. F. CuRnss

This paper describes a pumping system for removing radon from asolution of radium salts. Several improvements have been madewhich make it more efficient and easier to manipulate than the usualforms. The chief advantages of this design are that it eliminates allrubber tubing and all stopcocks through which mercury must pass.This avoids the annoyances from dirty mercury which are otherwiseencountered. The arrangement here described makes the use of liquidair very convenient and therefore permits somewhat more rapid manipu-lation than is possible with a pump depending solely on purificationby chemical means. The only stopcock in the whole apparatus isbetween the drying bulb and the radium bulb. Although this is exposedto the radon it has given no trouble during the year which this outfithas been in use in the radium laboratory at the Bureau of Standards.All gaseous products which develop from the grease used on thisstopcock are apparently removed by the subsequent purification.

Fig. 1 is a scale drawing of the pump. The radium bulb, stopcockand sparking bulb are not shown but would be attached to the opentube leading from the P205 bulb in the upper part of the figure. Theapparatus consists of two Toepler pumps in series with a few acces-sories to provide for purification of the radon and for manipulatingthe mercury in the pumps. In operation the impure radon is drawnfrom the bulb containing the radium solution through a small spark-ing bulb provided with electrodes between which the spark from asmall induction coil continually passes. This ignites the hydrogen andoxygen. The moist mixture then passes over through the P205 bulbwhere all water vapor is removed if sufficient care is taken. The gasesthen pass over molten KOH to remove CO and CO2 and finally overheated copper oxide to remove any excess of hydrogen. The radon asfinally drawn into the bulb of pump No. 1 is comparatively pure butmay contain small amounts of permanent gases. These are removed bypumping the radon over into the vertical tube supporting the Dewar, D,which has been previously evacuated by pump No. 2. As soon as all

* Publication approved by the Director of the Bureau of Standards of the U. S. Depart-ment of Commerce.

77

[J.O.S.A. & R.S.I., 17

the gas containing the radon has been pumped over, the mercurylevel being held above the side arm going to pump No. 2, liquid airis poured into the Dewar and after ten or fifteen minutes the residualuncondensed gases are pumped off by pump No. 2. Then the liquid

Fic, 1. A ssembled radon pump.

78 L. F. CURTISS

RADON PUMP

air is removed and after a few minutes the radon in a fairly purestate may be pumped off and collected over mercury from the deliverytube of primp No. 2. If a higher degree of concentration is required itmay be allowed to stand for several hours in contact with KOH.

The above method is suited for preparing radon for an exposingapparatus where it is desired to make sources of the active depositRaB+C. However the design may be modified slightly so that itmay be used either for this purpose or for filling small glass tubes withradon itself. The only modification required is in the Dewar. The

X G>~~~~~~~~~~FELT PAD

4-D STEEL CLAMP

' FT.

(A)

FIG. 2. Dewar and clamp for use with radon pump.

Dewar in Fig. 1 must be replaced by one as shown in Fig. 2A. Thegeneral operation remains the same but instead of pumping the radonoff after removing the liquid air it is forced up through the centraltube of the Dewar into the capillary where it may be sealed into theglass tube. When forcing the mercury up for this purpose it is necesaryto prevent an excessive loss of mercury through the delivery tube ofpump No. 2. The small clamp shown in Fig. 2B fits over the end ofthe delivery tube and effectually prevents the escape of mercury dur-ing this operation.

July, 1928] 79

[J.O.S.A. & R.S.I., 17

A few other details of the pumping system should be described.

C1 and C2 are glass check valves ground into their upper seat to pre-

vent mercury from rising above them. When the arrangement shown

in Fig. 2A is used it is desirable still to include the check valve. Un-

less very carefully ground it will leak mercury fast enough to provide

for forcing the radon into the capillary and it prevents a rapid rush of

mercury into the stem of the Dewar when filled with liquid air which

of course will break the Dewar. The mercury reservoirs for the two

pumps are connected each to their respective pump by a barometric

column and the top of each reservior has a small glass tube sealed into it.

By means of the special two-way metal valves V1 and V2 these reser-

voirs may be connected to an air reservoir of about 25 pounds pressure

or to the atmosphere. Thus by proper manipulation of these valves

the mercury may be run up and down in the pumps without lifting

any reserviors and without the use of rubber tubing. In the course

of operation these reservoirs gradually lose mercury which passes

over through the pumps. To make it easy to replenish the mercury

two steel unions U1 and U2 are provided. Since four or five "strokes"

suffice to remove practically all available radon this does not require

very frequent attention. The part attached to the glass tubing run-

ning down to the reservoir is rigidly supported on the angle-iron frame-

work while the upper part of the union attached to copper tubing can

be readily taken off and allow mercury to be poured in.If the bulbs containing the KOH and CuO are about the sizes

indicated they will not require refilling over long periods of time.

However the P205 bulb under ordinary conditions must be renewed

about every six months.

BUREAU OF STANDARDS,

WASHINGTON, D. C.APRIL 5, 1928.

L. F. CURTISS80