(I?I!(lrilllcrl {rom Nalllrl'.
1',,/ 1fi.1.
11. ('~R. Orlnl",r 15. 1(41)1
An Examination of Praseodymium,
Neodymium and Samarium for tX-Activity
. \ 1'110'1'0(1IlA I'I[](' Jl1('lhod ('01' 111(' wet<' 'UOll 0(' ,'x'
I!'en! Iy weak?t-twtivitioR hn~ r('('cl1tJy been dcscriht'd I.
'rho method llus 110W beon l\pplicrl Lo the xnminn.Lioll
!Ir elen1<,nts G!l, 60 ami ß2. 'I'ho rcsults have Homo henring on th qllostiO!l w1l01.1Ier tU1 1I1lidentificd p!t'(w!tl'oi
lmlo of a ct'rtain typ. fOUlHI in PI'Y 01<1 mit'll. iR duc'
10 an ?(·active isotop" of "I<'mont lil '.
Jlford ('2 pIstes of i;() microll. thicknes>t wero
impr gnated for ao min. wil h floJu!'ions of pI'SSOOr1yrnimu acetato (2 -:1 pOl' cent), neodymiUTI1 nilTl\to
(O·j POl' oen!,), 01' Immaritull nill'al (0·1 and 1 per cent
nnhydrous ~alt.) M'lli 7. 'L'ho rIal s wer!' k pi for
I
22, J 9. 10 Ami!) daYt:I, rospectively, clovclo)ll~tl,
I\lld a certain 0.1' 0. 01' plo.te searched for (X·tracks in
I ho IIHllal Wfty. 1';"oI'Y t!'fl.rk ObSOl'V cl wils mOItRlIl' d.
In the nooclymillm experiment, 00 tracks runnin~
cntirely inside tha emulsion wero fOlIUd in n = 1,000
l1eids of view undor the microscope (each covflring
a. vohune v,....,.J. X 10- 7 c.c. if the upper a.nd lower
border zones aro deductod), excopt eighteen thorhun
stars and two samarium tracks. Tbe half.life ""a is
~ven byl
nvofAt ln2
N
where c is the concontration of the impl'Ognating
solution (gram.atomsfc.c.), . t is Avogadro's Dl'mbor,
and N is tbe nurnber of l'elC\rant tl·aeks. The 'penetration factor' f givos the ratio of the coneentrations in
Lllc !'lllulsion ttfLel' drying and in the impr gnating
HO!IILion. Oll IWCollnt of the eh mical tlirnilarity. Lho
ponolration fa.ctors in tllo neodymium and in tho
0·1 pf'r c nt samarium e".-perim nt could bo ll8Ilumed
LI) 11 cquo.l. I J1 the 100Lt r e},,-pel'imellt penetro.tion
could be mensur d dir ctly by counLing the samarium
tracks of 1·13 cm. rungo in airS. l!'ourteen tracks of
this kind were found in LOO neIds of view. Usill~
I·n x 10 12 aB the apparent half-life of samari1l11l
(1U1tura! isotopic mixture)3, on bM
J·O X JOu
JOO ;< (4 X 10-') X (:I X ]0-') xix (tlx
14' - 365
t0 13 ) X
10 X 1112
11 aneo J r-v 10 -1\ reaflol1lLbl valuo for 0. neutral Rolntion of sllfTiciently smo.lI cOlle ntration 1,0 mal.
orption very nfrC'()tivfI ' ,4,
ninm
>
T J{2
•
IIIlO -« 4
____ , -
Thoraforfl, for nflo ·
-
IO- 'J x (:! / JO-') x IO X(fi X I (l2aJ X l!l x l" 2
A
- :lfiJi
-
'" 2·6 X loH yeaJ'H.
an entil'ely analogons way a half·liro
T > 1·3 X lO J' years is derived for praseodymium;
no single tracfta at all wore found in 1,000 fiold s of
view of this plate.
In
48
-
-
-
I ~·--
-
r\ -o
o
~
I'\.
6
10
'fraok IOllgth (m1crons)
-j
j
16
lJl the plate impl' gnated with 'concontl'ated' solu·
tion of samarium, U 8 tracks wore COWlt d in 400
neIds. 'rbis leads La a penetration factol' f = 2·-1.
Tho spectrum of all tracks observed is givon in tho
nocnmpanying diagram.
It mllch res mblcs the
spcctl'llm given by 'uer and Latt 8 13 • 'rhe po~tk
would I e oven sharpcr if st ep tracks. whieh are
harcl to measure, were exclnded; it ia significltJÜ
(,hat no track of apparcnt lengLh > 9 iJ. is inelined
less than :32° . Cloarly, no seeond group is PI'OI:! nl,.
; 1'hel'Ofol'O, the half·lij; of samarium for an altemntivo
mode of 'X.deeay, 01' for d ay hy proton nm iRAinn 5 ,8.
lIlust b
T , {.
->
Ion X (.J. X
-
JO~')
x (:I x w-a) x 2·-/ x (n x LOD') X 9 x In 2
:16Ji
----
'" '·2 X 10 14 yeal's.
." !'!imilal'
Bult was rcpol'l,ed by Kollm' fl.ne!
~rn.th 1'1', buh no flP tl'um was given.
'I'he l1uident.ified pleochroic halo hr:ts n. rn.,lillR "I'
I ·H 2·(l cm. air equivalent7,8,2, cOl'rcflponcling I.n
n,bmti, 11 ·2 12·1)1J. of emulsion. 11. i~ saidl,1) 0(:('1\1'
'Inly in ufl::;ociaÜon with tho halo prodl\ood hy
samarium.
ow if, in fact, the halo were rille t, fI.H
xisting isotope of element 61, the Cl·ray gl'OllP shoulc1
appeal' in the pre ent plate experiments. Element 61.
along with the other rare earths, would be contained
in the monazite, from which the neodymium and
samariu= salts have been prepared. In fractional
crystallization it would go partly with neody=iu=
and partIy with samariu=. In view of the three·
fold natural abundanco of noodymiu=8, compared
with samariu=, probably tluee.quarters would be
found in the neodymiu= fraction. omparison of tho
tmck counts in the 'dilute' neodymium and samarium
l..'"j)eriments « 1 and 140, respectively, in 1,000
Jelds) shows, then, that the present aotivity (defined
as tho ratio of abundance to half.life) of 6 I in rock
140
cannot be more than -4 X -1- - X - = -]
I'
3
11
1000 3
]000
36
(,im fl that of samarium. Presumably this is inflllfli·
cient, to explain the halo.
J ncident.ally, eonsideration of the 'amow quatiou
flhows tbat the assignment of tbe l·g·em. Cl·radiation
to 61 would require ß·decay of neodymiwn, 01' J(.
capture in samariu=, of the same order of intensity.
Thi fact has been pointed out by Feather (private
eommunication). Arlmitt dly the Gamowequation do fl
not hold weil for the 1· 13·0111. radiation of samarium.
Yet one should obtain at least a v ry crude idea
of th half.life of an Cl-my mittel' of Z = 61. and
rnng I·
m. by \lsing for the nucleal' radius tlto
valu del'iv cl from the appli ation of the Gamow
qlmtion to samarium.
half·life of
I Y a1' iH
Nl,l<;ulat. cl in 1;hi9 manner. Consequently, the aetiv<,
~p cies conld p l'Sist only aS long as it is replenishcd.
'('h only conceivable ways for thi to hapr n !Wl'
ß-decay f neodymhlffi12 01' K·eapture in l:lanlarilUll.
Ilccaul'; of its Rffiall intonsiLy, the l' quircd radiatio n
\Vill be hal'cl t.o Ami. Th radiat.ion l'eport.ed by
LibbylO as boing emitted by n odymium is lIluch too
intcnR .
In view of the absence of an «-radiation in the
neodymium fraction, the only remaining possibility
of assigning the halo to 61 is through the assumption
I.bat, the sp cies, 01' its prccursol', has pl'actically
dien out now. J]owever, tllel'e is a general obj CtiOll
(0
!I eribing
gcooh mienl offeets to r1ecayed
spceies l l • Quali~tiv. ly, th obj etion is that if a
r--.J
\
I
I
:ipeeies was still aetiv!' 1\ (·onsiderablo tim after L1lP
ol'igin of nuclei, B,nd ImH diH!lpP arad now, its half-li(!'
IlUIHt ba fail'IY!'lhol't. In IhiReasc, howcver, it.-;f\blIlHI
anee at the fllftrl l1Wst hfWC bOfm Im'ge \I hieh iN
IIrdikely for 0.11 IIIlHLnblo Hpc(;ies. Fol' IR,rk of "f~ln
(In tho age of tho micl'l., el,c., it.is not. yel, po~~ihlo I (I
tLrgue this point quftntitatively,
.Lt ia thol'efore concluded: (.1) J[ pl'll.seodymium
is (X-active, the half-lire must be groater than 1·3 X
10 16 years. (8) lf neodymium js (X-active, tbe halflife must be greater thnn 2·6 X lOH ycars. (C) Jf
samarium emit.':! a soeond gr01,;lp of ce-rays, the intcnsity must be at least ] 00 timf'S less than that of the
1·13-em. group. (D) Any prcsent ce-activity of element 61 in rock cannot oxcoed 3 per cent of the
activity of Blll'Tlarium. (E) Heasons against ossigning
the unidentified pleochroie halo of radius 1· cm.
air-equivalent to lemont 61 aro given. (P) H, however, the assignm nt ware true, it would have to ho
aBSllIDed that the species is short-lived and ia 01' was.
replenished from 0. 10ng-livod source.
OUI t.hanks are due to lrof. K . }'rzibrnm for l liH
ilrt r'OHt in thi" york. und 10 UI'. O. ,mol'l\,nll, <Ir
'f'r ibachor Chamisehe \\' l'ke for the vfLlnnhlo gift.
01' pure pr paratiOl1R of tbe raro carLhs.
lor·oovor·.
ono of IIS (E. 13.) is indebtecl to Prof. . FoaLh r
fol' giving the stimulus 10 1his jnvesLigl~Lion by
diHC IIAA iollH or Lho prohlllm of element (j I .
1<'. BESTEN RE! N I~ It
E. HRODA
Tl. Physikalisohes Institut
der niversität, Wien.
Mareh 31.
'Jcokncrnnd 13rO<!n, Nature, 164, 4J2 (1011).
• SchinllmeL~ter, Wien" Ber., 144, 475 (1930); 145, IIU ( I U:ln).
, Iloscmalln, Z. 1'11118" 99, lOG (lO:!tl).
• Broo", Nature, 160, 231 (1947).
• Milder, Z, PI/li'" 88,601 (1931).
• 'l'ayloT, Prof. ROll. S~., A, 150 , 3 3 (l(l35).
'.JOly, Proc. ROll. 'oc., A, 102, 0 2 (1023).
' lfcnderson and l'urnbuU, 1'roc. ROll . • '0(., Al 145, ;,H:! (In:ll),
lJenderson, Proc. 1101/. 8oc., A, 173, 2GU (10311).
• Nod~ackl NatUrun..I, 18). 757 (l9:1c). Uolw.chmillt, Skrift,'r NflrRk"
Vta . .ilkaa., Q. 4 (1 v37),
10 Llbby, Ph1l', R.v., 46, 19G (1934).
11 Iray, Glückauf '\ud Paneth, Proc. ROll. Sor., A, 166, 2:\1\ (I !l:UI) ,
.. 131\11011, l'li1J~ , Rt •. , 73, 6;JU (JOI8).
" ( '\1er :\nd 1~'\tl~5, 'nture, 158, t 117 (19411).
" J(.'lIer :\lul MaU",r, Pli",. RN'., 74.1121 (I!ll>l).
I'rinred in
Gr~nt
Uritain hy Fi.ber, JCnlRht &
(;0.,
Ud., St. "lhano.
PB 52658
NATURE
LETTERS TO TH E EDITORS
the partia l pre3sura of atoms and r adicals p~o
relatively high, a mixture of chlorine at 1 em. p
and carbon tetrachloride at 2 cm. pressure s
oan increase of press ure of about 0·3 cmo, and
eonsidera ble amount of solid product was fortned,.
Chlorine ancl carbon monoxi de. This we ll . ~
photochemiea l system exhib its one of the
striking examples of the effeot of intensity on ,.
course of 0. r eaction. Illumination in the o~
way of n mixture of chlOl·ine ulld ca rbon mon~
produces phosgene quite rapidly, and the ~
can be t a ken almost to completion. \-Ve find ...
irradiation by a 4,1J00·Joule fl ash, however, prod
no detecta ble pl'essure cha nge in this mixttlrl!,
tha t if phosgene is addecl it is rapidly decomJ)Olletto chlorine and carbon monoxide. The reactiOll
quite rev ersible amI ca n be ml1.de to proceed eithr
way at will by ehang illg the intens ity of the illumin..
tion. Th f~t the imporwnt prima ry ae t is the ph.
oh emical d ccompo it ion of chlorine in both cß.,<':< ....
shown by th e v ory small pr ::;sure ch al\ge~ prc..h~
when p1.U°O phosgene was l1sed.
Two oITcc ts seem to pla ya p a rt in ehanginjOt Lbo
m echa llism of photochemical r ea eti ons a t high ,.
tensitics. First, since the corcentra ti on of tlle in e
m ediate is compa rable with t h at of tho relletam
inter.atomic and intel··l'adical collis iol\s aro 03 fr..
qu ent IlS collisions of th cse intermed iates wi th m.
stable m o lec ul cs. Thus, fOI· cxample, th o revel'l!ibJin,.
of t ho pho:>g ne r action m ay b o expltt illcd by 111' f"I'o
v orsibilit v of the r eaetion OCI
CI , = CO 12 - 1
followcd hy COCl
Cl = CI .
CO, in ruH agreemtn1
with tho schemes of B od l\S toin, a nd Lehn r :uJ
Rollefson. Secondly , the peoc SB is n a rly ndiI\11I\1w-.
a nd V Ol'Y high ins ta ntaneo 18 t empClotl.turcs may c..
roach ec1, du tl in p a rt t o the oXllth ermic I·clletion oft""
labile int rffi di a t es with each oth r and in p nl't to th.
therma l d eg ra dation of tho nb;;orbec1 light en erg~·o
Tho m echa nis m of somo r encti ons at high 1..:::
intens itv will be disoliSl ed in m or o d eta il el ewherr :
the a bo·vo x ampl s S l've to show tha t thi tyP<1
soure , whieh extenc1s th o availa ble l.Ippt:!l' limit J
intcns ity some 10' times, produ cos pliotochemir..l
oft·ects of ulll1sl1a I m agni turle, allel in some cn l~ 111111
altor prof, l'ndl r t.ho nonll.d l ll \1l~O uf tlle reactp,n.
'I Ite Editor8 do not hold themaelve8 reaponsible
JOT opiniOT/8 expre8sed by their correapondents.
No notice is taken oJ anonymoua comrrmnicati0rt8
Chemical Reactions Produced by Very High
Light Intensities
IT has beon a matter of general experience that
photochemical reaetions are not muoh a ltered in theü·
courses by change of light intensity. The range of
light intensity hitherto available, however, has b een
limited by timt obtaülable from the Sl.ill and from
such sources as high·pressure m ercury vapolu· lamps,
the t otal usable output of which in the r egion between
2,000 a nd 5,000 A. does not exceed IO ~ o quanta/second.
Thor e are many eases, p arti cularly those photo.
chemical rea etions whera fl-ee radicals are illvolved,
where it wOlild b e desirable to oxtend investiga tions
to rouch higher intensities into the r egion whero ~h o
conc ntra tion of int.ermediutes is compa mb le ,.... Ith
tha t of the l'eact ttnts .
The gas.discharge Aash.lamp which h l\s b een
d e e loped recently fOl· photographio pllrpose seemotl
t o ofiotogr eat p ossibilities for eJo..l erimonts of this kind,
and we h a ve fowld that this type of lamp is a very
elllcient source of light in the photochomicully llse ful
r egion. The fl ash is produced by the dischnrgo of a
la rg condenser through a tube mied wi t h rar gns,
a m I Hp to energies of nbollt IOD Joules almost any
of the 1.ls Lmi types of diseh a rge Illll1p m ny be m:ed o
By u sing qunrtz tube. {md Im-ge tungs ton electl'od es ,
we h ave workeu with energ ie up to 10,000 Joules/fl ash
in a tube 1 m flt r long . Tho duration of this flash
was 4 rn.sec., o.nc\ tho use fLiI light output in tll o
r egion of absorption of uranyl oxalate was lO U
quantnjseconel. The output is contilluous down &0 ~ L
leAS t 2,000 A. and appears to b o fairly e venly diS·
tributed, anel the output at wave.lengths I 5S tha n
3,500 A. may be furth er incroased by the addition of
a. Httle liquid m ercury to the gas filling.
The following figures givo the p Ol' entago d e?om .
p osition ohta inerl in a fow typioal s ubstances wl th a
single \slt of 4,000 Joules Io.sting 10::ls thun 2 rn.scc.,
tlle s ub tan ce b e ing contained in 0. qUM tz tube, on o
m etre long, Iying p a raJl I to the lump:
+
n
G IlS
NO.
H,CO('H.
CJl,COlJr
CII.COCOCIl.
CI/,CO
rre. ure (emo)
%
3
S
2·5
10
1&
~g
Tho ga.'l volume was 500 c.c. In o~ch cß.'le, ancl
m er eul'Y had b n adued to !~e lamp III the ?ascs of
a cet one and k e tene. In additIOn, wlth chlorIne at 0.
prN<Slll·e of 1 cm., the enormOliS Dudde effect of 2 cm.
wa observed.
Thr e oxampl es of the different b ehavio w·oof \~Oe ll .
known photochemical sys tem,> under the. hIgh lIg ht
intens ities will be givon.
Acetone a rid keten e. Ins tcad of the h y drocarbo ns
and carbon monoxide which are nonnally producecl
with these eompounds , 0. complex mixtur of s ub·
stane s including hydrog n and a high proportioon of
carbon was obta ined . The carbon appeal' cl In nn
unusua l fin e cobweb·like A>rmf\ tion whi ch hun g aeross
the tube fr om w nll to wall.
Ch[orine arid carbo1l t tTochloride. Curbon tetra.
chlorid i us uull~ cons ider cl to b the final prodl1et
of tha phot och mical reGcti on of chlorine with m '
thane. A t these high light inte ns iti . , h owever, wh n
+
+
H. G. W. }iORIl I~H
G. PonTE n
0
J; pfwtmcnt of Phys i c ~I Chemis try,
Ca mbridge.
wo
15
.
,
]) eeom po~ ilic u
"eatl)"
4
Odober 15, 1949
/
A ug. G.
-_._-
.
An Examination of Praseod ymlu":I' .
Neodymium an d Samarium for cx-Actlvlty
.
cx.
A P]{ OTOc:ni\PllIC m eth d for the d e tcctlOn ?~I
trem ely w a k cx.activities has rccently b een desC;f' i~
The m eth od has now b een appli d to the Xtl man at
?f elements 50, ~o and 62. The r csl;llts h!l've som ~
ang on th ques.tlOn whether A.~ Ul1I cl en t died pi ~ d~
halo of a c~ r ta~n type, found In v ery.old mleß, t3
t o I1.n /.X·nc tl vo ll:\o to lX' of elemo.nt 61'. . . • were
I1ronI (' _ pla t s of 50 rnlCrons othlckn e_s ~
impr gna t cl for :10 min. with Soilltions oof p~ ,_
dymillJ1l uc to.t e (• . ;l p er c~nt), n cod y mmm J~.t~
(0·1 p er COIl t), 01' sama rium mtrat e (0 · 1 and I ~ t rar
ß1Ihyclt"Ou>l salt ) at p H 7. 'rhe plu~e wore ~:fopeJ.
t = 22, 10, 10 und 9 days , r pe tJ\' Iy, de ks in
and a. c rt a in arca of plM senreh cl for cx·trnc ..L
th e 1I S U J\1 W I\ Y . E v ry t rack ob rvcd wru m 1\.>1Il"t'ing
In th e n dymium IJxperim nt, n o tr~cl(s ~ut(j()O
onti l"Oly ins ide th o emulSion wpr°C' fOllod In n - ,
r,
0
*
->-...
'Ii.
O ctober 15, 1949
No. 4172
fjelds of view und e r the mieroscope (each cQvering
volume v '" 4 X 10-' c_c. if tbe upper and lower
bOrd er zones 0.1"0 d eaueted), except eighteen thorium
stars and 1,"-0 Sf.I,mllrium tracks. The half-life Tl/2 is
given byl
nvcfAt 1n2
TI/: =
N
'
I>
"hero c i8 tbe conool1tl'ation of the impl'egnating
solution (gram-llt,oJns/c.c. ), A is Avogo.dro's number ,
and N is tho numb "r of relevant tracks _ The 'penetrati on factor' j givoi:! tlle ratio of tlle concontmtion'l in
lila emlll!:!ioll aner <lrying and in the impregna,ting
solution. ()Il II ceollnt of the chemical sirnilnrity, the
penoh 'nt,ion fll cwrs in the neodymium amI in the
0.1 (ler ( '0111, smn41l'ium experiment eould be asswned
to iJo c'lllu L In Lbo Ja.tter oxperiment penetration
could bo m ' fl SIlJ'cd direoLly by counLing 1,110 sam arium
trlv:ks (,r J. 1:1 ·m . range in nir'. 'FOlll"tcen t rocks of
this killl i ". I'U foulld in 100 fjelds of viow, Using
'1,1) X HlI Z [\,!1 1,110 o.pparent half-life of samarium
(na tural iaCJ Lopic mixtw'e)", ono h as
1·0 X 10"
WO X (-I X 10- 7) X (3 X ]0-6) x jx (13 X 10 23 ) X 10 X In 2
14 X 365
Heneo f "-' 10- :\ rCltSonublo vo.lue for a n eutral solutiun of s lltncion tly smaJl eoncontrat.ion 1,0 make
IIdpOI'plion vory offoOLivo"'_
ThcrC'fore for neodymiutn
T I /!
,
>
1000 X ( A 10- 7) '{ (3 X 10- 6) X 10 X (6 X J() '3 ) X 1!J X In 2
3135
"-' 2·6 X lOu y ears.
In an entit'oly un lo\.logolls way 0. half-lifa
~ > 1·3 X 10" yen!"!:! i8 d orived for pl'a.scodymium ;
no single t,1'I1 ks n,L all wore found in 1,000 fj elds of
"iew of t,h iR pllttA.~ 48
j
'ö
I
Z
1(1
8
o
o
LJ
5
\
10
659
NATU RE
15
Track l eugth (IDlcrolll5)
. In the pInte imprt'gnated with 'con cent.rated' solutIon of samarium, 11 8 tracks ,vere counted in 400
fields. 'l'his lead s to a pepetration fnetar j = 2 ..1..
The spectrwn of a ll tracks observed is given in the
.• &ccompnnying diagram_
It muoh r sembles th
I
S)lectrum given by Cuer and Lattes lS •
The p eak
i. lI"ould bo oven sharper if ste p, tra I\s, whieh are
hard to m easure , were excluded; it is significant
tbat no track of appa.rent I ngth > 0 (1. i9 inelined
less than 32°. Clearly, no second group is present.
Th refore, the half-life of samarium for an alt€rnative
mode of IX-deeay, or for decay by pro(,on emLsion 5 , .,
must be
TI/'
> .
400 X (4 x 10-') x (3 x 10- 5 ) x 2·4 X (6 X
365
'" 1,2,
10 2~)
X9 X
in !!
x 10" years.
A siroilal' result was r ·pOl·ted by K eller and
Jfather H , but no speetrum was given.
The unidentified pleoehroie halo ha.s a radius of
1·8- 2·0 em. air equivalent',"" eorre. ponding to
about 11,2- 12,5(1. of emulsion. It is said to oceur
only in nssociation with the h alo pronuced by
samarium. Now if, in fact, tho halo were du" to an
xis(,in~ isotope of element 61, the ex·ray grollp sho uld
appear in (,he present plate experiment.s. EI('mcnt 61.
along with the o('her rare earths, would be olltained
in tho monazite, from whieh the neod"mimu snd
sa ma,r illm salts have been prepared. 111 fra (;tional
cl·yst.nllizntion it would go partly wi1,h n odymium
and partly with samarium. In vie", of th three fold natural abul1dnnco of n eodymiUln G, eompared
WiLh samarium , probllbly 1,hr u-qwuter" W GlI!(~ bc
found in 1,110 n 'odyrnium hadion. 'ompn r i!'o n of the
tru k counts in the 'diluto' ncodymium und :,amarium
experiments « 1 nnd 140, respecLivc l~-, in I , 00
fields) shows, thon, timt the present activity (defined
as the ratio of abundnnee to half-lifo) of 61 in r ock
cannot be mol'O Lhan -4 X -1- / -1 X -140
_ = _1
3
1000 3
1000
35
tim s that of samarium . Preswnably this is insufficiont to oxplain the haloIncidenw JJy, consid ration of tho Gomow equation
shows thut tho a signment of tho 1 ·8-em. ex-radiation
to GI would l' qUil'O ß-d cay of n eodymium, 01' J\.capturo in samnrium, of the same order of int.cn8ity_
This fact b us b een point d out by Fenth I' (priyatc
eommunicution). Admittedly tho Gamow eq ua tion doe:;
nOL h old w 11 for t,ho 1 ·13-c111. radiation of samarium_
Yot one should obtain nt least 0. vory crudo idca
of tho half-lifo of an ex-rny emitter of Z = GI and
ran go 1·8 cm. by u sing for tJ-'e nuolenr rarlim th ~
voluo derived from tho application of the Gamow
quo.t,ion to samnrium. A half-life of ,-..., I year I!<
euleulated in this mann er. Conseq\.lently, the aetiy
flpeeies onld persist only as long as it is r eplenished.
The only concoivnblo wa)'s for this 1,0 hnppen are
ß-d ca.y of n eodymium U 01' K-eapture in samarium.
B cnu'e of ita amn)) intellsity, the r equir d radiation
will be hard 1,0 find . Thc radiation l' ported by
LibbylO llS b eing el1litted by neodymium is l11uch too
in tense.
In view of the absence of an ex-radiation in the
neodymium fraetion, the only remaining possibility
of assigning the halo to 61 is through thc assumption
that thc species, or its preeursor, has praetical1y
died out now. However, there is 0. general objection
to aseribing geoclJemical effecta to decayed
sp cies l l_ Qualitatively, the objeeLion is that if a
sp eies was still acLive 0. considerable time after th e
origin of nuelei, and ha.s disappeared now, its half- li~
must be foirly short_ In this oase, however, its nbundanee o.t tlle start must 1ll1ve qeen largo-, hich i.
unlikely for an unstable sp eeies. For lack of deta
on the age of tlle mica, etc., it is not yct possible to
argue this point quantitatively.
lt is tberofore concluded: (A) If praseodymium
is lX.active, tho hnlf-life must be greater t hon 1 ·3 X
101& years. (B) If n eodymium is ex-Rcth-o, the balf-
660
life must b e greater than 2·6 X 1014 years. (C) If
samarium emits a second group of ot·J'ays, the inten·
sity must be at least ) 00 times less than that of the
1· 13·cm, group. (D ) Any present ot-activity of elem ent 61 in rock cannot exceed 3 per cent of the
activity of samarium. (E) Reasons against assigning
the unidentified pleochroic h alo of radius 1·8 cm.
air-equivalent to element 61 are given . (F ) If, how.
e er, the ass ignment were true, it would have to be
assumed that the species is short·lived and is, or was,
replenished from a long.lived source.
Our thanks are due to Prof. K . Przibram for his
inte rest in this work, and to Dr. O. Smetana of
Tre ibach er Chemische 'Werke for the valuable gift
of pure preparations of the rare earths. l\Ioreover,
one of us (E. B.) is indebteel to Prof. N . F eather
for · giving the stimulus to this investiga tion by
discussions of the problem of elem ent 61.
. F . BEsTE:r.-nEINER
H. Physikalisches Institut
E.
RRODA
dcr Universität, Wien.
March 31.
Jenkner find Brodn, Nature, 164, 41!l (1949).
• SchjotluleL~ter, TVi.ner JJer., 144, '~7 5 (1035); 145, 4 19 (1 936).
• H 03emt\Jlu, Z. Plly •. , 99, 405 (J ll:l6).
. • ßrod.'\, fInit". , 160, 23 1 (J OI;) .
• )r ~der, Z. PIlV'" 88, 601 (t 9:H).
• Tarlor, Pror. R~y. Sor . , A, 150, 383 (1 035).
, .Toll', Proc. }loV. Soc., A, 102 , 682 (1023).
• EIcndcrsoo lind TurnbuJJ, Prur.. ROll. Soc., A 145, 5tl2 (10:14) .
l:! cnllrrson, Proc. RO/J. Soc. , A, 173, 250 (1039).
• S()(~~nCk, N(ltl,,~ci", 182 757 (1 0:10). (Jo!clschmillt, Skrijlcr Norsk.
l !ll. Akad. , No. 4 (I \)37).
.. Libby, T'hy,. R.v., '16, 106 (10:11).
.. Fay, G1Uckauf and PnneLh , "roc. Rov. Soc.; A, 166, 238 (1 038) .
"ßallou, l'liU8. Rev., 73, 630 ( 1018).
" t:ue r a1\(1 I.altes, Natu re, 158, ]07 (LOHl) .
" Keller nnd )I:\ther, Phv•. R et·., 74, &24 (19IS).
1
Thickness c f the He lium 11 Film
D ETE IDITNATioNS of the thickness of the film tImt
covers a ll solid surfl1,ce in eontact wiblt liCjlli 1 J,olill.m
TI h a ' -e been made by D a LUlt and i\fend e l ~sohn', a nd
by Kikoin alld L nsar ov 2 , by m eth ods which dep end
on tbe measurcmont of the volLune of h eJil1l'l'I J1
r eq uil' cl to form tlle film on sW'faces o f kn ow11 n.1'Of.1 .
Tho experiments, fo/' whi<.'], 110 gr 'at aCt:l1t'acy ww;
claimeu, gave an average "slue of 3·[j X 10- 0 c;n. for
the thickness, anl! no de finite indicat.ioll of 0. eh ange
of thickn s~ with ternperaturo. A tkins 3 h as llf;e u u.
static molhod involving condensation 01' h oliwn gas
to form tho film a nd bulk liquid, and a c1yn n.mie
m eth od d ep ending on oseillatory m otion of th o film .
In the lutter case, tho experiments indieMed that the
thiekness varieel inversc ly ns the h oight abovo the
level of the b u lk liquid, the thielmess at 1 cm . being
about 10 X 10- 1 em.
Theore tical treatments of the h elium fi lm have
b een carried. out by Frenkel 6 , Sehiffs and 13ij1, d e
Boer a nel )JlChe)s '. The Fronkel-Schift' theory pro·
dicts that the film thielmess should vlt.ry inversely as
th~ cube root of the h eight. The Bij l, d a BOOl' and
:.'I [lohe I theory g ives 0. variation inversely as the
s qua re r oot of the h ight. Tha basi ~ of t he latter
theory h a , howe ver, b ecn criticized by rot t 7 • No
theoretienl treatment of t h e variat ion oE thiclmess oE
the film .with t emperatllro h as yot b oen publisl1ed.
Experunents h ave b een carrieel out in th is L ab orn·
tory allowing Q. direet d etermination of thic.;lmess and
its variation with h oight and t emperaturo. 'I'he
m etho<.! d epends on tho fao t thllt when lan e pol-
,
O d ober 15, 1949
NATUR E
Vol. IM
al'ized light is reflected from
a polished m etal surface,
it b ecom es eJliptieally pol .
arized, the eccentricity and
orientation of the ellipse
d epending on the opticnl
constants of the m etal, the
angle of incielence anel the
wave.length of the light.
These cons tl\nts are ehanged
by the superposition of a film
of different m aterial, in this
case the h elituu II film, and
the m eastlIed changes give the
thickness of the film, if its re ·
frQ.etive in lex is known. The
ax periment t\l al'J'angoment is
similar to that d escribed by
Rotheu 8 fol' the m eaSllrem ent
M
of the thiekness of luy el'S of
b aritun steMmte. A staillie. s
stoal mirrol' is support-ed in
s uch a w ay that the h elilun
film ha.· n o furth er p a th fot·
cl'eep, amI no h ent can entel'
the mitTOI' by conduetion (Fig .
1). In Intel' experimen ts a
eylinc1rical copper radiation
s hielc1 s nrrounding tho minor
and cJipping into tllc liquid
h eli um \\'n~ uLlHch ed to th
support bio\\' th benket· B .
1'110 inciclent anel l' Oeet d
light pnss d thl'ough n a rl'OW
long itu lin a l sli ts in th shield .
1'he tempcruture of the S \I1··
r OtUldings of the mirror is
}';g. 1 . CrY03ttlt tnU . G,
I!!aos SUliport; B, prOlett·
mllintnined uniform to a high
iug bcaker of It,!uid helium;
d egr 0 , amI o\'llporatio n of the
.ll, ruirror
film and ( f the bulk heli.llm 1I
i. vory smnll. Tho soclilllll ligh t u scd for the exp eriments p asses thl'Ollgh n total thickness of 12
mm. of Chance'!; ON. ]!) hent-absol'bü1g g lass and
h as b oen provlld not t o evaporate the film to ade·
t eeto.l)le amOl1Ht. Undet· these c nditions, the film
s hould b e t ho 'static' onc cOlLs irl er0d in the theorie3.
D etcl'm ill utions luwe been made over the tempera·
lure rang 1.10 K . - ).. point tlt1cl fOt' h eights varying
[rom 1· :::; to l !) mm. 1'be m '(\l'lu'ell1ents im-oh'('
. U ing:i of .'l. Kieol pri. m at (i) Kom kno\\'n eOllstant
tcmporatllr · b low tlto )..- p oint (2 · 1 'u" K . ) allel (ii) O[
B
He
,
f
I
1
t
205"
N
20,0
2630
,
'.
!l02°
o
0'0
0'8
TIclght of ntm (eID.). T -
1 ' ~ 5'
] '0
1>:.
Fig. 2. RdlltiOIl beLween Xlo.o! aottlng nnd h ch:ht. AB, limits of
6pread duu to rnO"eJ1lcnt of obser".r'. e)'e frOll1 extreUle Hft to
extreme rl ght nt the tdescoj'\e