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Neutron activation analysis of high-purity selenium
Analytica Climica Artn Elscvicr Publiding Company, Printed in The Ncthcrk-mds
NEUTRON
Amsterdam
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
SELENIUM
In previous papers’ .2 the determination of bromine and tellurium were described. Other important impurities are phosphorus, chlorine and especially sulfur. These elements have already been determined in various matcrials3~4 and in seleniums-8. Chlorine is usually determined by y-spectrometry of the short-lived a&Cl after separation by volatilisation and precipitation as silver chloride. For large concentrations (> IO p.p.m.) non-destructive activation analysis of chlorine in selenium can be applied 7. When a double irradiation technique is used, sulfur and phosphorus can be determined through the isotope 33P or sulfur and chlorine through the isotope 3%. A separation of the matrix activities was obviously required: precipitation of selenium in the elementary state and sulfide precipitation of arsenic and germanium were found to be satisfactory. SUCLEAR
DATr\,
IRRADIATIOS
COSDITIONS
AKD INTERFERENCES
Reactor neutron irradiation of the elements phosphorus, sulfur and chlorine gives rise to the isotopes listed in Table I 0- 1”. From this Table the various methods of dete:mination for these elements and the nuclear interferences are apparent. In the present work, chlorine was determined by counting 3*Cl while sulfur and phosphorus were determined through the ““P-activities from cadmium-covered and uncovered samples. These activities were corrected for the contribution of the exo-ergic 3sCl(n,a)3eP reaction and for the siP(n,y)3*P and a*S(n,p)3eP reactions. This method seemed preferable to the counting of the low-energy 36S, especially since this reaction is interfered with by the exo-ergic reaction aKl(n,p)a%. Short irradiations favour the s%(n,p)a?P over the 3JS(n,y)3% reaction, as the half-lives are respectively 14.3 and 87 d. The counting of 3% was not considered, as the natural abundance and the activation cross-section of 3% ate too small to be applied in activation analysis. When the double irradiation technique is applied with s*P counting, one can expect that the chlorine concentration should not exceed certain limits compared to the sulfur and phosphorus contents. The same consideration is true for the ratios of sulfur to phosphorus. OP DE BEECK AND HOSTESS calculated the amounts of sulfur and phosphorus
which can be determined
simultaneously,
as a function AfraZ. Claim. Ada,
of irradiation 43 (x968)
I-II
C. RALLAUX,
2 ‘rmI.I!
R. DAMS,
J. HOSTE
I
NIICLI!AIt
I’HW’liHTlliS
01’ I’IIfJSl’lIOHlJS,
time and thermal-to-fast
SIJLI’IJH AND
CIILOHINI!
neutron fluxes:
(1) (2) where g~,g~ =amounts
of sulfur and phosphorus k,iY =cadmium ratios of phosphorus and sulfur E =counting efficiency N =Avogadro’s number - -’ &;-fi~thermal and fast neutron fluxes M = atomic mass time of the samples t1,&? =counting Equations (I) and (2) include the condition that the count rate of 32P through the 3lP(n,y)ssP reaction is at least two times higher than the standard deviation on the reaction for the irradiation without cadcount rate of aSP through the sG(n,p)s”P mium and vice versa for the irradiation under cadmium. When the amount of sulfur is much higher than that of phosphorus, an irradiation in the reflector of the reactor can be performed; in the inverse case an irradiation in the core of the reactor becomes necessary, The activities Al and AZ (counts/min/g sample) in the cadmium-covered and in the uncovered sample are as follows: Al
=m[P] +h[S]
A2=a2[1’]
+br[S]
+c1[Cl]
(3)
+cn[Cl]
(4)
where the square brackets represent Awl.
Chitn. Ada,
43 (1968)
I-II
concentrations
in ,ug/g sample.
NEUTRON
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
Se
3
cc= counts/min W? per ~6 P due to the 31P(n,y)3”P reaction b = counts/min 3zP per ~6’s clue to the a%(n,p)3”P reaction c = countslmin 3zP per pg Cl due to the 35Cl(n,a)““P reaction. Values for a, h and c can be determined from pl~ospl~orus, sulfur and chlorine standards. After the determination of the chlorine concentration by an irradiation for 37 min at a flux of S * 1010 n/cms,‘sec (Thetis reactor) and counting 3X1, the sulfur and phosphorus concentrations can be calculated from cqns. (3) and (4). The specific activities of 321’, due to pl~ospl~orus, sulfur and chlorine, irradiated for 4 days at a site of the MC-2 reactor, are listed in Table II. The ratios /z=&z/n*, k’ =Ira/bl and #I k =c~/ct are tlie well-known cadmium ratios, with values of 141, 1.11 and 1.14, respectively. As can be seen from this Table, if the 3 elements are present in qua1 amounts, the interference of sulfur and chlorine on the phosphorus content will be 2.1 and o.Gy/,. If these elements are irradiated under cadmium, a positive error in tile sulfur content of 37.0”/~ will be causccl by phosphorus and a. positive error of 26.574 by chlorine. TAl3L13
II
SP’BCII;IC
ACTIVITllZS
OF
----.--_-_---_---_..
D21’
DUE
.__-.___-
--_-._.
--_-.
Irradiation 1, = 0.1013 Irmdiation
..__-__.-_.--_
TO
-
PIIOSI’IIORUS.
.___
(L
without Cd n/cm”/scc
--_--.--
22.784
SULFUR
..- .----..-.L
AND
CIII.ORINIS
.--C -.._ __ _ __.-_
132
487
with Cd
j-r = *+’ IO’ ’ 11/c111*/scc -_--._-----..-.-.
- -. ..__.-_
Under the stated irradiation conditions, the cadmium ratio of phosphorus is large while those of sulfur and chlorine are rather small. The litnits of the sulfur to phosphorus ratios, which can be calculated from eqns. (I) and (2), arc therefore relatively large : 61’ > 3.5 ’ 10-S
The experimental
j&
gY >,1.1~10-1
@
values :
gp 2 I .g ’ 10-3
pug
6s >0.5g’Io-1
pg
for I pg of sulfur or phosphorus respectively are in reasonable agreement with these data, if one considers that gil andgs are proportional to the square of the neutron flux. Furthermore, it appears from Table II that the chlorine content should not exceed 20 times that of sulfur and about 952 times that of phosphorus, if the same statistical conditions are taken into account. The interferences 33S(n,p)saP and 30Si(n,y)3iSi % 3rP(n,y)a”P on the determination of sulfur and phosphorus and the interferences 38Ar(n,p)38Cl (a=0.7 mbl”), 41K(n,a)a*Cl (a=2.4 mbi”) and a%(n,y)37S --+ e- 37Cl(n,y)sXl on the cletermination of chlorine can be neglected as the concentrations of these elements, if present at all, are very low. And.
Chim. Acta, 43 (IgGS) x-11
C. BALLAUX,
4 DBTERM INATION
R. DAMS,
J, HOSTE
OF CHLORINE
Chemical sefiaralion Non-destructive activation analysis of chlorine in the selenium samples under investigation appeared to bc impossible. y-Spcctrometry shows only the photopeaks of “3Se (r.3og and 1.880 MeV) in the energy rc@on of the 3X1 photopeaks (1.642 and 2.1668 MeV). Discrimination against lower-energy radiation and the use of a lead absorber do not substantially improve the situation. Thus, a fast and efficient separation technique for chlorine had to be developed. The volatilisation of chlorine from hot concentrated nitric acid was studied with tracers; 98.6% of the chlorine’ could be absorbed in ice-cooled silver nitrate solution. The decontamination from the isotopes 7sSc, 7eA.s and ““Na was very satisfactory. About 45 min after the end of irradiation, the silver chloride precipitate was ready to be counted.
50C
6 .l.O= 1
400
e%lr I.01 + 1.04 + 1.08 MeV
e3Br
0.529 MeV Tq/2-2.33h
300
1.32MeV
wanneinumber
Fig.
I.
y-Spectrum
of the JW-fraction
scparatccl from irradiated
sclcnium.
Co1cnti9g
Fipre I shows the y-spectrum of a silver chloride precipitate, separated from 2 g of selenium which had been irradiated during: 37 min at a neutron flux of 8 - 1010 n/cme/sec. The sample was measured for 30 min with a 3 x 3” NaI(T1) detector, coupled to a 4oo-channel pulse-height analyser. Decay curve analysis of the activities measured at 1.64 and 2.17 MeV resulted in half-lives of 37.5 and 36.0 min, values which agree well with the half-life of e*Cl (37.3 minlc). Anal. Cihr.
Ada,
43 (1gG8)
I-II
NEUTRON
ACTIVATION
ANALYSIS
The o.52g-MeV
OF
HIGH
PURITY
SC
5
peak is due to “3Br formed by the nuclear reaction :
3 A3mSe\
The half-life of 2.33 11corresponds with the value given in the literature (2.39 1110). This isotope does not show any photopeaks above 1.4 MeV. The other peaks are due to W3r: one day after irradiation, wllen the sample was measured again, a pure *%r-spectrum was obtained. The following counting technique was eventually applied: 3 h after the first measurement of 30 min, counting was repeated during 3x.8 min i.e. 30 min corrected for the decay of a”Br. This spectrum was stripped from the first and a I.%min background correction added. In Fig. 2 the spectrum obtained after these operations is compared with a pm-c SAC1spectrum. It is clear that the e?Br interference is no longer present. Moreover, results calculated from measurelnents of the x.Q-MeV and 2.17-MeV peaks give rise to the same values. Interferences in these energy regions thus seem unlikely.
A selenium
sample
of 2 Q was irradiated
together
with
a chlorine
standard
1.64 MeV !I .-
(i ;?
: I
;
I 2.17 MeV
)-
Fig.
2.
y-Spectra
of 38Cl. (---)
59.4 (ug Cl; (-)
chlorine separated
from selenium.
Allal. Claim. Acta, 43 (1968)
I--II
6
C. BALLAUX,
R. DAMS,
J. HOSTE
(2 mg of ammonium chloride) during 37 min at a flux of 8.1010 n/cmz/sec. After irradiation, the sample was etched twice with 4 N nitric acid and transferred to an all-glass distillation apparatus. The selenium was dissolved by adding 40 ml of 14 N nitric acid containing IO mg of chloride carrier. During the dissolution, chlorine was volatilized by means of a modcrate stream of carbon dioxide and collected in 20 ml of ice-cooled 0.02 N silver nitrate solution. The distillate was filtered and the precipitate washed 4 times with 0.x N nitric acid and once with alcohol and ether. The silver chloride precipitate was then counted for 30 min. ‘The standard was dissolved in water and the solution made 2 N in nitric acid, and IO mg of chloride and I ml of 0.4 N silver nitrate were added. After filtration, the precipitate was washed as described above and counted for 3 and 3.2 min at a 3-h interval. Time was not available to dry and weigh the selenium after the etch. The amount of selenium analyzed was determined afterwards from the ratio of the activities of the nitric acid solution and the etching solution.
A
2.17MeV
I x=2.04 f 0*3*pgcP2g se (16.6%12 x81.01f 0.31 pg cl / 2g se (17.1v.d
Fig.
3.
I p:CI
added
I 6
5
Addition method of annlysis for chlorine.
To investigate possible systematic errors, an addition method of analysis was performed. Powdered selenium was etched with 4 N ultra-pure nitric acid and washed with demineralized water. In clean quartz ampoules, 2-g samples were weighed and spiked with sodium chloride solutions. The quartz ampoules were dried, sealed and their content thoroughly mixed by shaking. The samples were irradiated two by two, together with a 1% cobalt-aluminium wire as flux monitor. Chlorine was separated and counted as described above. From the results (Fig. 3)) no systematic error was found in the considered concentration range. Because of the double counting technique and the low count rates, relatively large standard deviations occur. Typical results for three selenium samples are given in Table III. Anal.
China. Ada,
43 (1968) I-II
.:
NEUTRON TABLE
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
SC
7
III
DETERMINATION
OF
PIfOSPHORUS,
SULFUR
AND
CHLORINE
(p.l>.lll.)
--
Phosfdorus
Sample
Mellroli
710.
aualysis
of
foloui
I
Addition method
0.5 I “fo.o.$3
Classical method
0.079’L
Sulfur
(8.3%)
0.0612
0.064~&0.003~
CltenticaZ
(4.8%)
0.0683
-
SIMULTANEOUS
DETERMINATION
(3.0%)
o.g7 zto.17
(17.5%)
0.66 O.G8
I .Gz
0.079’
0.083”&0.004O mcthotl
4.6olo.14 I.36 x.51
o.ogrz
Classicnl
Chlorine fortrtd
fOll?ld
(5.0%)
1.50~0.08
(5.3%)
o.G7 &0.0X
2.02
0.40
I x$3
0.32
2.00fo.02
(r.oO/,)
0.36&0.04
(1.5%)
(IT.I”/~)
-
OF
SULFUR
ASI>
PHOSPHORUS
scpavation
Phosphorus is usually separated as magnesium ammonimn phosphate or as ammonium molybdophosphate. Arsenic forms similar compounds whereas considerA prior separation of these elements is able amounts of selenium coprecipitate. therefore necessary. Distillation permits the simultaneous separation of the two elements. From a sulfuric acid-hydrobromic acid solution, more than gg.ggg% of while 98.8% of the phosphorus is selenium and arsenic can be distilled at 200-220°, recovered in the residue, as was shown by tracer experiments. Phosphate can be precipitated from the residue as ammonium phosphomolybdate; owing to the presence of sulfuric acid, which delays the precipitation, an increased amount of ammonium molybdate and a longer digestion time are necessary. It appeared that only about 98% of the carrier-free 3*P, from the WS(n,p)ezP reaction, is present as orthophosand other phate; the other 2% is probably a mixture of pyre-, tri-, tetraphosphate long-chained polyphosphates, chemical forms which do not react with ammonium molybdater4. Under the experimental conditions used, more than 99% of 32P was The yield of the total operation recovered in the phosphomolybdate precipitate. was at least 95%. Cotrnting The radiochemical purity of the phosphomolybdate precipitate, separated from 50 mg of selenium, irradiated without a cadmium filter during 4 days at a flux of 6.101" n/cm?/sec, was checked by three different methods. (I) The absorption curve was compared with a pure eel?-standard. As shown in Fig. 4, after subtraction of the contribution of the y-component, a value of Eg-(Max) =x.75 MeV was obtained by Feather analysis. This value agrees well with the literature value (r.708 MeVrO). (2) From decay-curve analysis (Fig. 5), a small long-lived contamination was found. After correction for this interference, a value of T, =x4.3 d was obtained, which agrees well with the value given in the literature (14.3 dro). Aaal.
China. Act&
43 (1968)
I-II
C.
8
RALIAUX,
R.
DAMS.
J,
HOSTE
(3) y-Spectromctry of the lxccipitate was applied. Only small photopeaks of could Ix! cletcctc!d. For the determination of sulfur and phosphorus, the precipitates were counted for IO min with an end-window G.M. tube (thickness 3 mg/cm”, dead time : 300 ,USCC). The samples were also measured for 30 niin by j+qxxtromctry. In the samples not covered with cadmium, sonic 7fiSc appexrcd to lx present, wliei-cas this interfercncc was negligible in the caclniiuln-coverecl samples. When 7GSe was present, a correction which was counted with the (s-xoO/;‘,) was applied by means of a 75% prccipitatc two radiation cletcctors. (75% dots not emit P---particles.) 75sc
1'~
__ -,--
100
Fig. .I. Absorption 5. rol:! ll/Clll+icc. Fig:. 5. lhcay
L.
_-
.L...
300
d (mg/cm2)
curves.
._I_._
.__._,
500
___-
0 3W tracer.
of tllc :‘“L’-frxtion
1oL
I
700
SCpilYXtcd
..-
“_.,b___.’
0 :]=I’ scparatccl from
20
__
from
1
30
_-
-8
-__
40 Time
L
__..
50 (days)
L
60
._.._‘.
70
_.
50 mg SC irrncliatctl
t___.,_...‘-.___-_)
80
so
loo
for .t cl;tys at
sclcnium.
Procedure In a cadmium box (thickness I mm, length 2 cm, width x.9 cm) 6 quartz ampoules were placed containing three so-mg selenium samples, IO mg of diammonium hydrogen phospllate, Lo nig of @assium sulfate and xo mg of potassium chloride. This cadmium box was placed at the bottom of an aluminium capsule. Above this an empty aluminium cylinder was placed (length 1.3 cm, widtll 1.9 cm) and above this anotller aluminiunl cylinder (length 2.0 cm, width ~9 cm) containing 6 identical samples. The empty aluminium cylinder serves to eliminate most of the influence of the cadmium box at the other end of the irradiation capsule. The capsule was irradiated for 4 days at a tllermal flus of 0 * 1012 n/cm”/sec ancl a fast flus of _c.IOl~ n/cm+ec. One clay after the end of irradiation, the selenium was washed with dilute nitric acid ancl water to remove surface contamination. After drying and weighing, the samples were dissolved in IO ml of 14 N nitric acid containing 2 mg of phosphorus and I mg of arsenic (AscOa). This solution was transferred to a Scherrer (( NH&Hl’O.r) distillation apparatus, 20 ml of 35 N sulfuric acid were added and a moderate stream of carbon dioxide was bubbled through the solution. For the collection of the matris activities, three vessels, each containing Go ml of 2 N sodium hydroxide, were used. After heating to 2oo”, 35 ml of 40% hydrobromic acid were added dropwise from the separatory funnel at zoo-220° (time cn. I: Ii). After cooling, xoo mg of selenium carrier were added, the solution was heated Aural. Claim. rlctn, 43 (rgGi3) 1--1x
NEUTRON
ACTIVATIOS
ASALYSIS
OF
HIGH-PURITY
SC
9
to 200” and 20 ml of 40% hydrobromic acid were added in the same way. The addition of carrier and the distillation was repeated twice. The residue of the distillation was transferred to a beaker and ncutralised with G N ammonium hydroxide. After addition of 15 ml of 14 N nitric acid and 30 ml of 34% ammonium nitrate solution, the solution was heated to 8o” and 35 ml of 3oj ammonium molybclatc was added dropwise. After standing overnight, the solution was filtered and the prccipitatc washed with a solution containing 50 g of ammonium nitrate nnd 37.5 ml of 14 N nitric acid per liter. The precipitate was dried with alcohol ancl ether, heated for I 11at zoo”, weighed and counted. The standards were dissolved in dilute (cu. I N) nitric acid to eliminate aclsorption of 3oP on tlic glass wall 14.After dilution, alirluots containing I0 rug of chlorine, 2 pug of sulfur, 0.1 ~6 of phosphorus or 10 ,~g of phosphorus for the ‘cadmium-covered sample were taken and 2 mg of phosphorus carrier and 15 ml of 35 iV sulfuric acicl were added. After heating for r 11 at zoo”, ammonium I~llospllomolyl,datc was precipitated as described above.
To investigate possible systematic errors, an acldition methocl of analysis was used. Powdered selenium was etched with ultra-pure 4 N nitric acid and washed with tridistilled water. After drying, 5o-mg samples wcrc weighed in quartz ampoules and spiked with diammonium hydrogen phosphate and potassium sulfate solutions. After drying, sealing ant1 mixing, the ampoules were irracliated and the W’ separated. By mcans of eclns. (3) ancl (d), the activities due to sulfur ancl phosphorus were plotted as a function of the amounts of sulfur and phosphorus acldcd. From Figs. 6 and 7 it appears that no systematic errors occur up to 46.1 ng of pl~ospl~orus and 1.02 pg of sulfur added. Finally, in two other selenium samples, the sulfur and phosphorus contents were determined. The results are given in Table III. Table III also shows that the
[/b,.26.3-’
A
b,,.735
1.7cm/ng
P(6.OV.1
2 41 cpm (5.6%)
5
Xm25.9’2lng
&2%1
25
50 ng P added
Fig.
G. Addition
method
of analysis
for phosphorus.
Fig.
7. Addition
method
of analysis
for sulfur. Awd.
Cirin,.
Ada,
43 (rgG8)
I-II
C. BALLAUX,
IO
elements are present in ratios such that the simultaneous and phosphorus is satisfactory.
R. DAMS,
determination
J. HOSTE
of sulfur
The authors thank Mr. ‘~0~113~ of the “Metallurgy Hoboken” for providing the selenium samples. Tire technical assistance of Mrs. J. Gor
Chlorine was determined in selenium by irradiation of 2-g samples for 37 min at a flux of 8. xor0 n/cin”/sec. Chlorine was volatilised from hot concentrated nitric acid ancl precipitated as silver chloride. The isotope a%1 (T 1 ‘37.3 min) was counted by y-spcctromctry, Sulfur and phosphorus were determined by irradiating 5o-mg samples with and without cadmium shielcling for 4 days at a thermal flux of 6*10ra n/cm3/scc and a fast flux of 4.rorr n/cm~/sec. The matrix activities were separated by distillation from sulfuric acid-lrydrobromic acid at 200--220~. The isotope 321’ =14.3 d) was then precipitated, togetlrer with phosphate carrier, as ammonium v, phosphomolybdate, and counted with a G.M. tube. Amounts of 0.4-1 p.p.m. chlorine, 65-520 p.p.b. phosphorus and x.5-4.6 p,p.m. sulfur were found in high-purity selenium samples. RBSUMl!:
On dose le chlorc dans le selenium par irradiation d’dchantillons de 2 g pendant 37 min h un flux de S~xol%/cm~/sec. Le chlore est volatilisC de solutions acide nitrique concentr&, chaud et precipitc comme chlorurc d’argent. L’isotope “*Cl (T,=37.3 min) est compte par spectrometrie gamma. Soufre et phosphore sont doses par irradiation d’dchantillons de 50 mg avec ct sans couche de cadmium pendant 4 jours a un flus thermique de 6. Iol%r/cmz/sec et un flux rapide de 4’ Iolrn/cm”/sec. Les activites de matrice sont sc!parees par distillation dc milieus acide sulfurique-acide bromhydrique h 200-220°. L’isotope azl? (Tb = x4.3 cl) est ensuite prircipitd avcc entraineur phosphate comme phosphomolybdate d’ammonium et comptd avec tube G.M. On a trouve ainsi 0.4-1 p.p.m. de chlore, 65-520 p.p.b. de phosphore et 1.5-4.6 p.p.m. de soufre dans des &hantillons de s&nium tres pur. ZUSAMMENPASSUNG
Es wird die Bestimmung von Chlor, Schwefel und Phosphor mit der Neutronenaktivierungsanalyse in hochreinem Selen beschrieben. Zur Ermittlung des Chlorgehaltes werden 2-g Proben 37 min bei einem Fluss van S x Ior%/cm4 x set bestrahlt und das Chlor aus heisser konzcntrierter Salpetersaure verdampft und als Silberchlorid gefillt. 3RCi (7‘1=37.3 min) wird y-spektrometrisch gez!ihlt. Zur Bestimmung des Schwefels und Phosphors werden 5o-mg Proben mit und ohne Cadmium-Folie 4 Tage lang bei einem thermischen Fluss von 6 x xol%r/cm”- x set und einem schnellen Fluss von 4 x 101ln/cm~ x set bestrahlt. Die Matrisaktivitaten werden aus einer Mischung von Schwefelstiurc und BromwasserstoffsZure durch Destillation bei 200-220’ abgetrennt. Das Isotop azP (T1 = 14.3 d) wird zusammen mit einem Phosphattrager als AmmoniumAwl.
Chinr. Actu.
43 (rgG8)
I-II
NEUTRON
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
Se
II
phosphomolybdat geftillt und mit einem Geiger-MiillerZihlrohr Selenproben wurden 0.4-1 p.p.rn. Chlor, 65-520 p.p.b. Phosphor Schwefel gefunden.
16
(1965)
655
geztihlt. und I.++
In den p,p.m.
Stde Corms., .+ (I cjh(i) 307, 3 I I, Mikvockiru. Ickuouud. Ado. I (1905) IO. I<. c. IiOCll. .4ctivdiorr R wlysis, l-I~~rltll~ool~,~A&Am~ic Press, New York ant1 London, IgGo. 7’crDrIlc~r~tlrr A tomkrrnc, ‘I‘cil 1, 13;t11d I, I’cr~amot~ Press, W. IiUNZ AND J. SCIIINTL~II:ISTIIR, J9fi_3. I<. I)Anfs AND F. AI)A~~s, i~adiochiru. Acta, in press. J_ C. ROY ANID J. J, I~AWTON. Ckulk River Cmurirc. 1ao3. (1g60). J_ 1’. 01’ DIS UEECK ANIB J. HOSTI;. J. Rdionml. Chtwr., in press. J. 13. DAHL AND 0. I<. HIRKELUND, l.A.E.A. Confevcme. I~udioisotoprs in tire PkysicdScicnces and I rdttstvy. Copeuhageja. 1960, hoc. Vol. 11, p. 471.
W.
; Solid
IWHNSCH,
Arrrtl.
C/rim.
Actn,
43 (1968) T-II
NEUTRON
Amsterdam
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
SELENIUM
In previous papers’ .2 the determination of bromine and tellurium were described. Other important impurities are phosphorus, chlorine and especially sulfur. These elements have already been determined in various matcrials3~4 and in seleniums-8. Chlorine is usually determined by y-spectrometry of the short-lived a&Cl after separation by volatilisation and precipitation as silver chloride. For large concentrations (> IO p.p.m.) non-destructive activation analysis of chlorine in selenium can be applied 7. When a double irradiation technique is used, sulfur and phosphorus can be determined through the isotope 33P or sulfur and chlorine through the isotope 3%. A separation of the matrix activities was obviously required: precipitation of selenium in the elementary state and sulfide precipitation of arsenic and germanium were found to be satisfactory. SUCLEAR
DATr\,
IRRADIATIOS
COSDITIONS
AKD INTERFERENCES
Reactor neutron irradiation of the elements phosphorus, sulfur and chlorine gives rise to the isotopes listed in Table I 0- 1”. From this Table the various methods of dete:mination for these elements and the nuclear interferences are apparent. In the present work, chlorine was determined by counting 3*Cl while sulfur and phosphorus were determined through the ““P-activities from cadmium-covered and uncovered samples. These activities were corrected for the contribution of the exo-ergic 3sCl(n,a)3eP reaction and for the siP(n,y)3*P and a*S(n,p)3eP reactions. This method seemed preferable to the counting of the low-energy 36S, especially since this reaction is interfered with by the exo-ergic reaction aKl(n,p)a%. Short irradiations favour the s%(n,p)a?P over the 3JS(n,y)3% reaction, as the half-lives are respectively 14.3 and 87 d. The counting of 3% was not considered, as the natural abundance and the activation cross-section of 3% ate too small to be applied in activation analysis. When the double irradiation technique is applied with s*P counting, one can expect that the chlorine concentration should not exceed certain limits compared to the sulfur and phosphorus contents. The same consideration is true for the ratios of sulfur to phosphorus. OP DE BEECK AND HOSTESS calculated the amounts of sulfur and phosphorus
which can be determined
simultaneously,
as a function AfraZ. Claim. Ada,
of irradiation 43 (x968)
I-II
C. RALLAUX,
2 ‘rmI.I!
R. DAMS,
J. HOSTE
I
NIICLI!AIt
I’HW’liHTlliS
01’ I’IIfJSl’lIOHlJS,
time and thermal-to-fast
SIJLI’IJH AND
CIILOHINI!
neutron fluxes:
(1) (2) where g~,g~ =amounts
of sulfur and phosphorus k,iY =cadmium ratios of phosphorus and sulfur E =counting efficiency N =Avogadro’s number - -’ &;-fi~thermal and fast neutron fluxes M = atomic mass time of the samples t1,&? =counting Equations (I) and (2) include the condition that the count rate of 32P through the 3lP(n,y)ssP reaction is at least two times higher than the standard deviation on the reaction for the irradiation without cadcount rate of aSP through the sG(n,p)s”P mium and vice versa for the irradiation under cadmium. When the amount of sulfur is much higher than that of phosphorus, an irradiation in the reflector of the reactor can be performed; in the inverse case an irradiation in the core of the reactor becomes necessary, The activities Al and AZ (counts/min/g sample) in the cadmium-covered and in the uncovered sample are as follows: Al
=m[P] +h[S]
A2=a2[1’]
+br[S]
+c1[Cl]
(3)
+cn[Cl]
(4)
where the square brackets represent Awl.
Chitn. Ada,
43 (1968)
I-II
concentrations
in ,ug/g sample.
NEUTRON
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
Se
3
cc= counts/min W? per ~6 P due to the 31P(n,y)3”P reaction b = counts/min 3zP per ~6’s clue to the a%(n,p)3”P reaction c = countslmin 3zP per pg Cl due to the 35Cl(n,a)““P reaction. Values for a, h and c can be determined from pl~ospl~orus, sulfur and chlorine standards. After the determination of the chlorine concentration by an irradiation for 37 min at a flux of S * 1010 n/cms,‘sec (Thetis reactor) and counting 3X1, the sulfur and phosphorus concentrations can be calculated from cqns. (3) and (4). The specific activities of 321’, due to pl~ospl~orus, sulfur and chlorine, irradiated for 4 days at a site of the MC-2 reactor, are listed in Table II. The ratios /z=&z/n*, k’ =Ira/bl and #I k =c~/ct are tlie well-known cadmium ratios, with values of 141, 1.11 and 1.14, respectively. As can be seen from this Table, if the 3 elements are present in qua1 amounts, the interference of sulfur and chlorine on the phosphorus content will be 2.1 and o.Gy/,. If these elements are irradiated under cadmium, a positive error in tile sulfur content of 37.0”/~ will be causccl by phosphorus and a. positive error of 26.574 by chlorine. TAl3L13
II
SP’BCII;IC
ACTIVITllZS
OF
----.--_-_---_---_..
D21’
DUE
.__-.___-
--_-._.
--_-.
Irradiation 1, = 0.1013 Irmdiation
..__-__.-_.--_
TO
-
PIIOSI’IIORUS.
.___
(L
without Cd n/cm”/scc
--_--.--
22.784
SULFUR
..- .----..-.L
AND
CIII.ORINIS
.--C -.._ __ _ __.-_
132
487
with Cd
j-r = *+’ IO’ ’ 11/c111*/scc -_--._-----..-.-.
- -. ..__.-_
Under the stated irradiation conditions, the cadmium ratio of phosphorus is large while those of sulfur and chlorine are rather small. The litnits of the sulfur to phosphorus ratios, which can be calculated from eqns. (I) and (2), arc therefore relatively large : 61’ > 3.5 ’ 10-S
The experimental
j&
gY >,1.1~10-1
@
values :
gp 2 I .g ’ 10-3
pug
6s >0.5g’Io-1
pg
for I pg of sulfur or phosphorus respectively are in reasonable agreement with these data, if one considers that gil andgs are proportional to the square of the neutron flux. Furthermore, it appears from Table II that the chlorine content should not exceed 20 times that of sulfur and about 952 times that of phosphorus, if the same statistical conditions are taken into account. The interferences 33S(n,p)saP and 30Si(n,y)3iSi % 3rP(n,y)a”P on the determination of sulfur and phosphorus and the interferences 38Ar(n,p)38Cl (a=0.7 mbl”), 41K(n,a)a*Cl (a=2.4 mbi”) and a%(n,y)37S --+ e- 37Cl(n,y)sXl on the cletermination of chlorine can be neglected as the concentrations of these elements, if present at all, are very low. And.
Chim. Acta, 43 (IgGS) x-11
C. BALLAUX,
4 DBTERM INATION
R. DAMS,
J, HOSTE
OF CHLORINE
Chemical sefiaralion Non-destructive activation analysis of chlorine in the selenium samples under investigation appeared to bc impossible. y-Spcctrometry shows only the photopeaks of “3Se (r.3og and 1.880 MeV) in the energy rc@on of the 3X1 photopeaks (1.642 and 2.1668 MeV). Discrimination against lower-energy radiation and the use of a lead absorber do not substantially improve the situation. Thus, a fast and efficient separation technique for chlorine had to be developed. The volatilisation of chlorine from hot concentrated nitric acid was studied with tracers; 98.6% of the chlorine’ could be absorbed in ice-cooled silver nitrate solution. The decontamination from the isotopes 7sSc, 7eA.s and ““Na was very satisfactory. About 45 min after the end of irradiation, the silver chloride precipitate was ready to be counted.
50C
6 .l.O= 1
400
e%lr I.01 + 1.04 + 1.08 MeV
e3Br
0.529 MeV Tq/2-2.33h
300
1.32MeV
wanneinumber
Fig.
I.
y-Spectrum
of the JW-fraction
scparatccl from irradiated
sclcnium.
Co1cnti9g
Fipre I shows the y-spectrum of a silver chloride precipitate, separated from 2 g of selenium which had been irradiated during: 37 min at a neutron flux of 8 - 1010 n/cme/sec. The sample was measured for 30 min with a 3 x 3” NaI(T1) detector, coupled to a 4oo-channel pulse-height analyser. Decay curve analysis of the activities measured at 1.64 and 2.17 MeV resulted in half-lives of 37.5 and 36.0 min, values which agree well with the half-life of e*Cl (37.3 minlc). Anal. Cihr.
Ada,
43 (1gG8)
I-II
NEUTRON
ACTIVATION
ANALYSIS
The o.52g-MeV
OF
HIGH
PURITY
SC
5
peak is due to “3Br formed by the nuclear reaction :
3 A3mSe\
The half-life of 2.33 11corresponds with the value given in the literature (2.39 1110). This isotope does not show any photopeaks above 1.4 MeV. The other peaks are due to W3r: one day after irradiation, wllen the sample was measured again, a pure *%r-spectrum was obtained. The following counting technique was eventually applied: 3 h after the first measurement of 30 min, counting was repeated during 3x.8 min i.e. 30 min corrected for the decay of a”Br. This spectrum was stripped from the first and a I.%min background correction added. In Fig. 2 the spectrum obtained after these operations is compared with a pm-c SAC1spectrum. It is clear that the e?Br interference is no longer present. Moreover, results calculated from measurelnents of the x.Q-MeV and 2.17-MeV peaks give rise to the same values. Interferences in these energy regions thus seem unlikely.
A selenium
sample
of 2 Q was irradiated
together
with
a chlorine
standard
1.64 MeV !I .-
(i ;?
: I
;
I 2.17 MeV
)-
Fig.
2.
y-Spectra
of 38Cl. (---)
59.4 (ug Cl; (-)
chlorine separated
from selenium.
Allal. Claim. Acta, 43 (1968)
I--II
6
C. BALLAUX,
R. DAMS,
J. HOSTE
(2 mg of ammonium chloride) during 37 min at a flux of 8.1010 n/cmz/sec. After irradiation, the sample was etched twice with 4 N nitric acid and transferred to an all-glass distillation apparatus. The selenium was dissolved by adding 40 ml of 14 N nitric acid containing IO mg of chloride carrier. During the dissolution, chlorine was volatilized by means of a modcrate stream of carbon dioxide and collected in 20 ml of ice-cooled 0.02 N silver nitrate solution. The distillate was filtered and the precipitate washed 4 times with 0.x N nitric acid and once with alcohol and ether. The silver chloride precipitate was then counted for 30 min. ‘The standard was dissolved in water and the solution made 2 N in nitric acid, and IO mg of chloride and I ml of 0.4 N silver nitrate were added. After filtration, the precipitate was washed as described above and counted for 3 and 3.2 min at a 3-h interval. Time was not available to dry and weigh the selenium after the etch. The amount of selenium analyzed was determined afterwards from the ratio of the activities of the nitric acid solution and the etching solution.
A
2.17MeV
I x=2.04 f 0*3*pgcP2g se (16.6%12 x81.01f 0.31 pg cl / 2g se (17.1v.d
Fig.
3.
I p:CI
added
I 6
5
Addition method of annlysis for chlorine.
To investigate possible systematic errors, an addition method of analysis was performed. Powdered selenium was etched with 4 N ultra-pure nitric acid and washed with demineralized water. In clean quartz ampoules, 2-g samples were weighed and spiked with sodium chloride solutions. The quartz ampoules were dried, sealed and their content thoroughly mixed by shaking. The samples were irradiated two by two, together with a 1% cobalt-aluminium wire as flux monitor. Chlorine was separated and counted as described above. From the results (Fig. 3)) no systematic error was found in the considered concentration range. Because of the double counting technique and the low count rates, relatively large standard deviations occur. Typical results for three selenium samples are given in Table III. Anal.
China. Ada,
43 (1968) I-II
.:
NEUTRON TABLE
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
SC
7
III
DETERMINATION
OF
PIfOSPHORUS,
SULFUR
AND
CHLORINE
(p.l>.lll.)
--
Phosfdorus
Sample
Mellroli
710.
aualysis
of
foloui
I
Addition method
0.5 I “fo.o.$3
Classical method
0.079’L
Sulfur
(8.3%)
0.0612
0.064~&0.003~
CltenticaZ
(4.8%)
0.0683
-
SIMULTANEOUS
DETERMINATION
(3.0%)
o.g7 zto.17
(17.5%)
0.66 O.G8
I .Gz
0.079’
0.083”&0.004O mcthotl
4.6olo.14 I.36 x.51
o.ogrz
Classicnl
Chlorine fortrtd
fOll?ld
(5.0%)
1.50~0.08
(5.3%)
o.G7 &0.0X
2.02
0.40
I x$3
0.32
2.00fo.02
(r.oO/,)
0.36&0.04
(1.5%)
(IT.I”/~)
-
OF
SULFUR
ASI>
PHOSPHORUS
scpavation
Phosphorus is usually separated as magnesium ammonimn phosphate or as ammonium molybdophosphate. Arsenic forms similar compounds whereas considerA prior separation of these elements is able amounts of selenium coprecipitate. therefore necessary. Distillation permits the simultaneous separation of the two elements. From a sulfuric acid-hydrobromic acid solution, more than gg.ggg% of while 98.8% of the phosphorus is selenium and arsenic can be distilled at 200-220°, recovered in the residue, as was shown by tracer experiments. Phosphate can be precipitated from the residue as ammonium phosphomolybdate; owing to the presence of sulfuric acid, which delays the precipitation, an increased amount of ammonium molybdate and a longer digestion time are necessary. It appeared that only about 98% of the carrier-free 3*P, from the WS(n,p)ezP reaction, is present as orthophosand other phate; the other 2% is probably a mixture of pyre-, tri-, tetraphosphate long-chained polyphosphates, chemical forms which do not react with ammonium molybdater4. Under the experimental conditions used, more than 99% of 32P was The yield of the total operation recovered in the phosphomolybdate precipitate. was at least 95%. Cotrnting The radiochemical purity of the phosphomolybdate precipitate, separated from 50 mg of selenium, irradiated without a cadmium filter during 4 days at a flux of 6.101" n/cm?/sec, was checked by three different methods. (I) The absorption curve was compared with a pure eel?-standard. As shown in Fig. 4, after subtraction of the contribution of the y-component, a value of Eg-(Max) =x.75 MeV was obtained by Feather analysis. This value agrees well with the literature value (r.708 MeVrO). (2) From decay-curve analysis (Fig. 5), a small long-lived contamination was found. After correction for this interference, a value of T, =x4.3 d was obtained, which agrees well with the value given in the literature (14.3 dro). Aaal.
China. Act&
43 (1968)
I-II
C.
8
RALIAUX,
R.
DAMS.
J,
HOSTE
(3) y-Spectromctry of the lxccipitate was applied. Only small photopeaks of could Ix! cletcctc!d. For the determination of sulfur and phosphorus, the precipitates were counted for IO min with an end-window G.M. tube (thickness 3 mg/cm”, dead time : 300 ,USCC). The samples were also measured for 30 niin by j+qxxtromctry. In the samples not covered with cadmium, sonic 7fiSc appexrcd to lx present, wliei-cas this interfercncc was negligible in the caclniiuln-coverecl samples. When 7GSe was present, a correction which was counted with the (s-xoO/;‘,) was applied by means of a 75% prccipitatc two radiation cletcctors. (75% dots not emit P---particles.) 75sc
1'~
__ -,--
100
Fig. .I. Absorption 5. rol:! ll/Clll+icc. Fig:. 5. lhcay
L.
_-
.L...
300
d (mg/cm2)
curves.
._I_._
.__._,
500
___-
0 3W tracer.
of tllc :‘“L’-frxtion
1oL
I
700
SCpilYXtcd
..-
“_.,b___.’
0 :]=I’ scparatccl from
20
__
from
1
30
_-
-8
-__
40 Time
L
__..
50 (days)
L
60
._.._‘.
70
_.
50 mg SC irrncliatctl
t___.,_...‘-.___-_)
80
so
loo
for .t cl;tys at
sclcnium.
Procedure In a cadmium box (thickness I mm, length 2 cm, width x.9 cm) 6 quartz ampoules were placed containing three so-mg selenium samples, IO mg of diammonium hydrogen phospllate, Lo nig of @assium sulfate and xo mg of potassium chloride. This cadmium box was placed at the bottom of an aluminium capsule. Above this an empty aluminium cylinder was placed (length 1.3 cm, widtll 1.9 cm) and above this anotller aluminiunl cylinder (length 2.0 cm, width ~9 cm) containing 6 identical samples. The empty aluminium cylinder serves to eliminate most of the influence of the cadmium box at the other end of the irradiation capsule. The capsule was irradiated for 4 days at a tllermal flus of 0 * 1012 n/cm”/sec ancl a fast flus of _c.IOl~ n/cm+ec. One clay after the end of irradiation, the selenium was washed with dilute nitric acid ancl water to remove surface contamination. After drying and weighing, the samples were dissolved in IO ml of 14 N nitric acid containing 2 mg of phosphorus and I mg of arsenic (AscOa). This solution was transferred to a Scherrer (( NH&Hl’O.r) distillation apparatus, 20 ml of 35 N sulfuric acid were added and a moderate stream of carbon dioxide was bubbled through the solution. For the collection of the matris activities, three vessels, each containing Go ml of 2 N sodium hydroxide, were used. After heating to 2oo”, 35 ml of 40% hydrobromic acid were added dropwise from the separatory funnel at zoo-220° (time cn. I: Ii). After cooling, xoo mg of selenium carrier were added, the solution was heated Aural. Claim. rlctn, 43 (rgGi3) 1--1x
NEUTRON
ACTIVATIOS
ASALYSIS
OF
HIGH-PURITY
SC
9
to 200” and 20 ml of 40% hydrobromic acid were added in the same way. The addition of carrier and the distillation was repeated twice. The residue of the distillation was transferred to a beaker and ncutralised with G N ammonium hydroxide. After addition of 15 ml of 14 N nitric acid and 30 ml of 34% ammonium nitrate solution, the solution was heated to 8o” and 35 ml of 3oj ammonium molybclatc was added dropwise. After standing overnight, the solution was filtered and the prccipitatc washed with a solution containing 50 g of ammonium nitrate nnd 37.5 ml of 14 N nitric acid per liter. The precipitate was dried with alcohol ancl ether, heated for I 11at zoo”, weighed and counted. The standards were dissolved in dilute (cu. I N) nitric acid to eliminate aclsorption of 3oP on tlic glass wall 14.After dilution, alirluots containing I0 rug of chlorine, 2 pug of sulfur, 0.1 ~6 of phosphorus or 10 ,~g of phosphorus for the ‘cadmium-covered sample were taken and 2 mg of phosphorus carrier and 15 ml of 35 iV sulfuric acicl were added. After heating for r 11 at zoo”, ammonium I~llospllomolyl,datc was precipitated as described above.
To investigate possible systematic errors, an acldition methocl of analysis was used. Powdered selenium was etched with ultra-pure 4 N nitric acid and washed with tridistilled water. After drying, 5o-mg samples wcrc weighed in quartz ampoules and spiked with diammonium hydrogen phosphate and potassium sulfate solutions. After drying, sealing ant1 mixing, the ampoules were irracliated and the W’ separated. By mcans of eclns. (3) ancl (d), the activities due to sulfur ancl phosphorus were plotted as a function of the amounts of sulfur and phosphorus acldcd. From Figs. 6 and 7 it appears that no systematic errors occur up to 46.1 ng of pl~ospl~orus and 1.02 pg of sulfur added. Finally, in two other selenium samples, the sulfur and phosphorus contents were determined. The results are given in Table III. Table III also shows that the
[/b,.26.3-’
A
b,,.735
1.7cm/ng
P(6.OV.1
2 41 cpm (5.6%)
5
Xm25.9’2lng
&2%1
25
50 ng P added
Fig.
G. Addition
method
of analysis
for phosphorus.
Fig.
7. Addition
method
of analysis
for sulfur. Awd.
Cirin,.
Ada,
43 (rgG8)
I-II
C. BALLAUX,
IO
elements are present in ratios such that the simultaneous and phosphorus is satisfactory.
R. DAMS,
determination
J. HOSTE
of sulfur
The authors thank Mr. ‘~0~113~ of the “Metallurgy Hoboken” for providing the selenium samples. Tire technical assistance of Mrs. J. Gor
Chlorine was determined in selenium by irradiation of 2-g samples for 37 min at a flux of 8. xor0 n/cin”/sec. Chlorine was volatilised from hot concentrated nitric acid ancl precipitated as silver chloride. The isotope a%1 (T 1 ‘37.3 min) was counted by y-spcctromctry, Sulfur and phosphorus were determined by irradiating 5o-mg samples with and without cadmium shielcling for 4 days at a thermal flux of 6*10ra n/cm3/scc and a fast flux of 4.rorr n/cm~/sec. The matrix activities were separated by distillation from sulfuric acid-lrydrobromic acid at 200--220~. The isotope 321’ =14.3 d) was then precipitated, togetlrer with phosphate carrier, as ammonium v, phosphomolybdate, and counted with a G.M. tube. Amounts of 0.4-1 p.p.m. chlorine, 65-520 p.p.b. phosphorus and x.5-4.6 p,p.m. sulfur were found in high-purity selenium samples. RBSUMl!:
On dose le chlorc dans le selenium par irradiation d’dchantillons de 2 g pendant 37 min h un flux de S~xol%/cm~/sec. Le chlore est volatilisC de solutions acide nitrique concentr&, chaud et precipitc comme chlorurc d’argent. L’isotope “*Cl (T,=37.3 min) est compte par spectrometrie gamma. Soufre et phosphore sont doses par irradiation d’dchantillons de 50 mg avec ct sans couche de cadmium pendant 4 jours a un flus thermique de 6. Iol%r/cmz/sec et un flux rapide de 4’ Iolrn/cm”/sec. Les activites de matrice sont sc!parees par distillation dc milieus acide sulfurique-acide bromhydrique h 200-220°. L’isotope azl? (Tb = x4.3 cl) est ensuite prircipitd avcc entraineur phosphate comme phosphomolybdate d’ammonium et comptd avec tube G.M. On a trouve ainsi 0.4-1 p.p.m. de chlore, 65-520 p.p.b. de phosphore et 1.5-4.6 p.p.m. de soufre dans des &hantillons de s&nium tres pur. ZUSAMMENPASSUNG
Es wird die Bestimmung von Chlor, Schwefel und Phosphor mit der Neutronenaktivierungsanalyse in hochreinem Selen beschrieben. Zur Ermittlung des Chlorgehaltes werden 2-g Proben 37 min bei einem Fluss van S x Ior%/cm4 x set bestrahlt und das Chlor aus heisser konzcntrierter Salpetersaure verdampft und als Silberchlorid gefillt. 3RCi (7‘1=37.3 min) wird y-spektrometrisch gez!ihlt. Zur Bestimmung des Schwefels und Phosphors werden 5o-mg Proben mit und ohne Cadmium-Folie 4 Tage lang bei einem thermischen Fluss von 6 x xol%r/cm”- x set und einem schnellen Fluss von 4 x 101ln/cm~ x set bestrahlt. Die Matrisaktivitaten werden aus einer Mischung von Schwefelstiurc und BromwasserstoffsZure durch Destillation bei 200-220’ abgetrennt. Das Isotop azP (T1 = 14.3 d) wird zusammen mit einem Phosphattrager als AmmoniumAwl.
Chinr. Actu.
43 (rgG8)
I-II
NEUTRON
ACTIVATION
ANALYSIS
OF
HIGH-PURITY
Se
II
phosphomolybdat geftillt und mit einem Geiger-MiillerZihlrohr Selenproben wurden 0.4-1 p.p.rn. Chlor, 65-520 p.p.b. Phosphor Schwefel gefunden.
16
(1965)
655
geztihlt. und I.++
In den p,p.m.
Stde Corms., .+ (I cjh(i) 307, 3 I I, Mikvockiru. Ickuouud. Ado. I (1905) IO. I<. c. IiOCll. .4ctivdiorr R wlysis, l-I~~rltll~ool~,~A&Am~ic Press, New York ant1 London, IgGo. 7’crDrIlc~r~tlrr A tomkrrnc, ‘I‘cil 1, 13;t11d I, I’cr~amot~ Press, W. IiUNZ AND J. SCIIINTL~II:ISTIIR, J9fi_3. I<. I)Anfs AND F. AI)A~~s, i~adiochiru. Acta, in press. J_ C. ROY ANID J. J, I~AWTON. Ckulk River Cmurirc. 1ao3. (1g60). J_ 1’. 01’ DIS UEECK ANIB J. HOSTI;. J. Rdionml. Chtwr., in press. J. 13. DAHL AND 0. I<. HIRKELUND, l.A.E.A. Confevcme. I~udioisotoprs in tire PkysicdScicnces and I rdttstvy. Copeuhageja. 1960, hoc. Vol. 11, p. 471.
W.
; Solid
IWHNSCH,
Arrrtl.
C/rim.
Actn,
43 (1968) T-II