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CitmplimtHti of tht AutMtr

ON THE DECAY OF EXCITED RADIOACTIVITY FROM
NATURAL GASES

BV MiM L. 0. JOHNiON

[Reprinted from the Physical Review, Vol. XX., No. 3, March, 1905]

iid^Ws^'riWiti^.'^W!^-:mrr;^*m

[Kcprinud fioiii ili<- I'liv^n \\ Ki\1k\v, \,.|. \\ , N... j, M.inh, |i>i5.J

On tiik DfccAV ok Iaiitek RAnicACTurn ikum Naui;ai ("ia>i>.

I!V Ml- I,. I!. I..MN-..\.

IN a rerent jiajjer by I'roftssor J. C. M< Itnnan '• On the Radioai tivily
of Mineral Oils anil Natural (iases" ' it was shown that the natural
gases of Ontario contain an emanation whiih is siniilir to tli il fuinul
by other investigators in the gases lontaineil in certain s|)ring waters and
in the air drawn from the surface layers of the earth, and whi< h, from
its rate of decay as well as that of the radioai tivity excited in bodies
exjiosed to it, bears a strong resemblamc to the; emanation from railium.

In a i>a]ier by Miss lirooks^ the rtsiihs are jiresented of a scries of
measurements on the radiation from rods rendere<i railioa< live by e\|)Osurc
to the emanation from thorium, radium and actinium. In this |.a|ier it
s shown that, in the case of rods exposed for short perioils to the ema-

ion from thorium the radiation at first increases, reaching a maximum
.1 -■'bout two hours after removal, and then deiays ai< ording to an ex-
ntial law. A similar result was obtained w ilh rods exposed to the
eniunation from actinium, the only difference being that the niaximiim
intensity with this substance is reached in about ten minutes alter
removal. Hut when the rods were exposed to the emanation from
radium, it was found for all exposures that there was a rapid initial de-
crease in the intensity of the radiation for the first ten minutes ; after
which it remained constant for a length of time which decreased with
the increase in the i)eriod of the exposure ; then it droi)pcd gradually
and approached a zero value.

From these results it is evident that ai tivities excited in rods by ex-
posure to radium, actinium or thorium have certain characteristic s by
which they may be definitely distinguished.

The experiments described in this paper were c arried out in order to
examine more closely the emanation found in the natural gases mentioned
above, and to throw further light on the ipiestion whether the radio-
activity was due to the presence of the emanation from radium or due to
the materials themselves possessing the intrinsic jiroperty of radioactivity.
The behavior of the emanation was studied and a series of curves obtained
showing the decay of the radiation due to the excited activity on rods
exposed for different periods in the gas. It was found for all exposures
that the activity dropped at first and reached a steady state which length-
ened as the peiiod of exposure was shortened. It then gradually died
down to and approached a zero value. These curves are ver similar to

' -Mistract of a paper presented at the Philadelphia meeting of the Physical .'Society on
Deceitilier !o, 1904-

' Paper esented at International Electrii.d Congress at Si. I ouis, Scpltniber, KJO^.
3 Philosttphical Magazine, .September, lycH-

^"Wi . \.wmm':^irsrr.s'^irf^-ysr'mr"''^KP- *:sfiis,^'«ipw«5*6»':iff «•■

17«

Tin: AMiiRiCAX riivsic.ii. society.

[\,.i., xx.

those yivcn by Mis-* lirooks for roils exposed to the emanation from
radium and, therefore, support the theory that the emanation found in
the natural gases dealt with is due to the presimce of radium at their
source. Moreover, I'rofessor Rutherford in his work o • Radioactivity '"
gives results showing that the amount of acti\ ity excilcd on a rod expose<l
to the emanation from radium is always directly jjroportional to the
amount of emanation present. To see if this was true of the active
emanation in natural gases, exposures of the s.nne period were made in
the same gas on different days and it was found that, as the activity of
the emanat on decreased, there was always a proiiortionate decre.\^e in the
amount of activity uhich it exciteil.

The gas under investigation was confined in a cylinder about 60 cm.
long and 25 cm. in diameter. I'.rass rods were suspended in the gas by
insulating supi)orts, and conncled to ih- negative terminal of a I'oepler-
Holtz machine, which maintained a potential of 11,000 volts. Alter
exposure the rods were inserted in a seiond receiver of the same size
through which a gentle current of air was drawn in order to remove any
emanation adhering to them, Their activity was then determined by
means of measurements upon the comluctivity they imparted to the air
in the receiver. These measurements were made with a sensitive l)ol-
ezeleck electrometer, and as the rods were directly exposed to the air,
the conductivity imparted by them was due to all the types o'i rays which
they emitted.

In the paper referred to above by Professor Mi I.ennan it was shown
that when measurements were nnde upon the condm tivity of the gas as
it came from the well, it exhibited a steaily decrease, and fell to one half

T.Mii.i: I.

Ci'ii.lu li: ''v ,>f X.iliiril (/.; .

Time.
Hours. Minutes.

Current.

Arbitrary
Scale.

Time.
Hours. Minutes.

Arbitrary
Scale.

288

',41

292

340

296

327

301

310

307

283

313

215

320

172

331

135

335

119

104.5

340

145

' Ruth<.rforJ. RiiliaacUviiy, page 274.

iiRSEH'IW

^aki2^ r>.:^ja^'is^:^^- jgf^^?" .:^^'/

nil- AMi-.Kh.w rinsiiM. >i.{iin.

value in al)Oiit i.-S days. In the iirt-fnt in\c>iit:.ition the j;as was
• tiulieil unilrr slifjlitly dilTerent < iniiiustaiK l>. As sonn as it was rec eived
from the «ill a rod was exposKl in it at dinVreiit tunes and nnder nega-
tive electrification for ilitTerent periods. The rod in eac h e\|iosiire was
rendered active by the deposit of the subslar.ie caiisui); evi ited radioac-
tivity, and it was always found that the removal of the active rod was
followed by a rise in the conductivity of the gas to a n)a\iinum value,
after which it gradually fell ai cording to an experimental law in the same
manner as before. .\ typical set of observations on this \arialioi; in the
conductivity is given in Table I , and a curve representing them is shown
in Fig I.

In this case a negatively charged rod was exposed at i i,ooo volts in
the gas for ten hours, and it will
be seen from the values given al ive
that the conductivity steadily rose
and reached a maximum value m
about four hours after the removal
of the negatively charged rod. It
will also be seen that alter the
maximum value was reached the
conductivity fell to one half value
in about seventy hours.

In order to investigate how the
activity excited in a negatively
charged rod varied with the time
of ex|)osure, a series of exposures
for different p 'ods was made on
one day and tlic results of the^e
observations are given in Table II.
and curves illustrating them are
shown in Pig. 2.

Before commencing this set of exposures a rod connected to the nega-
tive terminal of the electrical machine was susjiended in the gas for about
two hours. It was then withdrawn and the t'lrst exposure was made.
During the intervals which elapsed l)ctween the different exposures, the
electric field was always ajiplied in the same manner to the gas, with the
object of making the exjwsures in the presenc e of a freshly produced
disintegration ])roduct.

The values given in Table II. and represented graphically in Tig. 2
show that for all periods of exposure the radiation decreased initially,
then reached a steady state which lengthened as the period of the
exposure decreased, after which it dropjied steadily and approached a
zero value.

Fig. 1.

^^^:m^mmm^^mms^m;^s^\'', ::■■-. m'i' \ 'n,%rpj:istf

r--% "

I So

nil AMI.KIiAX rin:

II .socii.iy.

I\..i \x.

T\r,i,i. II.

T wo-i

11 1 n M t c

Klvc-ii

ilr.ute

Tfn-m

inut«

Twenty

-minute

Kxp.

^%UTt.

Enpo

»jrr.

Expoi

kurc.

E<)w

sure.

Cii'fcnt

mrrtrt

Current

Time in
Minute*.

Current

'line tn
linute^.

Arbitrary

Tim« in

Mirutf.

Arburary

ScaU-.

Tinre in
Miuutes.

Arbitrary

Stals.

Arbitrary
Scale.

25.0

4.H.9

61.0

79.5

18,0

28.0

55.0

71.5

1').9

39.5

56.5

U.5

7.0

19.3

32.5

51.5

16.5

4 5

17.2

29.0

49 5

21.5

4.0

16.2

31.0

51.0

26.5

-■■.il

14.9

30.0

50.5

4."

12.0

31.5

49.0

5,0

11.7

30.0

511.0

4.5

11.6

29.0

44.0

.? 5

9.7

24.0

44.5

J. 5

23.0

44.5

3.5

21.0

51
56
61

39.5
36.5

35 5

Korty

-miniitf

Oni--

lOijr

Two-

■ hour

I- our

• hour

Hxp

osur*'

tvpu

S-; re.

Expo

sure.

E.xposure.

1:5.0

154.3

196.7

252.3

122.0

141.3

174.7

218.3

102.5

130.8

)

173.7

184.3

96.0

• '.i.a

163.2

152.3

91.5

126.3

154.2

137.3

93.0

120.3

148.2

1243

90.0

116.3

141.7

108.0

82.5

105. S

134.2

95.3

76.0

100.8

; 'hi

89.5

73.5

90.8

113.2

66.5

83.3

108.7

61.0

7S.3

91.7

56.5

92.7

'I lu'st' curxf'- on a< i nuiit u\ iluii >imilaiity to those of Miss ISrook^ lor
CNposiire in the eniaiintifiii ol raihmn I'oriii another link in the i ham of
f\i(!cn(e whi' h leads to the conilusioi'. that the emanation Ibmul in
natural ;_'ases lias radiuni tor its sonrce.

.\s stated above the results )i\\t:\\ in Table I!, were obtained from
observations iii.ide in one day upon the same i,'as. Uurini; this jieriod as
I'ig. T shows, there was a gradual deeay of the emanation. To iletermine
the relation existing; between e>ccited rad oactivity and the strength of
the emanation iirodn( ing it a -eries of e\i)Osiires of five minute^ duration
was made on three sui eesive da\s in the same gas.

ifii^l'^JMMSPfS^^m^IJK^^i^'

N". ,!

nil. .iMi i\'i j.y rins/it! mhii.j):

I Si

In m.iNint; tho- iiiei->ui ■iciil^ ihf < oiului tivitv i>\ tlio ":i-< ».!■> I'lr^t

made in ihc gas thii-^ tit-aUMl, 'I'lic -aim- |mi)( icl;iiv «a^ lollownl on fai li
(lay so as to maintain as far as |M>^>ililo tlu' viuio i ondition^. llii' rLsiilts
of tlu'sc obsorvations arc .L;ivt'n in ral)lc III. wliii l; i ontains a nc ord ot
the dctay of tlie activity L-M-itcd on each of the days.

T.vi;:.i. HI.

l-'.\-.il.;l l\,!./h<.i.ti: ilv (l-'r. -miniitr l^yf' ll'( i.

Column I.

1 First Day ■.

Ctjlumn H.

Time
1 Minutes.

Current
1 Arbitrary-
Scale .

Time
in Minutes.

1.52

no. 2

1.5

7l).2

o7 2

40.2

J7.2

2'.)

3H.7

39.2

.3S.X

3S.8

4')

37.3

33.8

31.3

30.3

31.8

Current
Arbitiary
Seal .

Time
in Minutes

1.5

4iJ

2.5

35.6

31.1

30.1

26.1

23.1

Column III, Third Day..

Current

I Arbitrary

Scale .

64.5

55.5

24.5

27.5

2'i

28.5

28 5

27.5

25.5

l82

THE AMEKICAX Pf/yS/CAL .S0C/E1Y.

[Vol. XX.

The strength of the emanation jiresent in the gas on each day is given
in Table iV.

TAIiI.E IV.

Exposure.

First ilay

Second d.iy.
Third d.iy....

Strength of EniKnKtion.

476
394
320

From an inspection of these values together with the results in Table
III. it will he seen, that a direct proportionality e.xists between the
strength of the emanation and the amount of activity excited.

Taiii.e V.

E.xtiltii A'lhfii'ihfi: ify {Ei:'e-iiifiititt' Jijc/^osiirt').

Time in Minutes.

Column I,
First D«y)
Current.

Column II.

(Second Day)

Current.

Column III.

(Third Day)

Current.

110

108

94.8

40.2

38.4

38.7

41.1

38.8

37.3

40.4

37.3

36.1

37.4

33.8

36.1

36.7

Table V. contains a few of the results reduced on this basis. The
values of the activity obtsined in the first day are recorded in Column I.
and the intensity of the emanation is assumed to be unity. Columns 11.
and III. contain the readings obtained on the second and third days cor-
rected to an emanation of unit intensity. The general agreement of the
values recorded in the three columns justify the assumption of the law of
proportionality.

In the experiments tiescrilied in this jiaper the effects found were jiro-
duced by all the rays given off from the excited body. Experiments
are now in progress to investigate these effects still further and especially
to determine the decay curves on the basis of ,s and ;- radiations.

THt PHYSICAL REVIEW

A JOVRNAL OF hXPERIMENTAL AND THEORETICAL PHYSICS

CONDUCTED
WITH THE CO-OPERATION OF THE

AMERICAN PHYSICAL SOCIETY

EDWARD L. NICHOLS
CRKS8T UKRBITT, AlfD FRIDKBICK BSDKU

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