Neutron Activation Analysis with the van de Graaff Accelerator Application to the Halogens GEORGE 1. ATCHISON and WILLIAM H. BEAMER Radiochemistry Laboratory, The Dow Chemical Co., M i d l a n d , M i c h .
NEUTRON SOURCE
A rapid physical method of analysis for milligram quantities of fluorine and microgram quantities of the other halogens is based on neutron activation. The using neutron source is the nuclear reaction Beg(d,n)B'o deuterons accelerated at 2 m.e.v. by a van de Graaff accelerator. The available neutron flux is measured by the activation of metal foils. A n apparatus for automatic irradiation and measurement of radioactivities having half lives on the order of 10 seconds is described.
Reaction for Neutron Production. The nuc1e:ar reaction used in this work to produce neutrons Irith the van de Graaff :rccderator is:
Be9
+ D2 -+
Bl0
+ n 1 + 4.3 m.e.v.
.i thick beryllium target \vas bombarded with deutei ons
ttvc
clc 1 -
ated to 2 m.e.v. by the van de Grastff accelerator
A
Beryllium Target. The beryllium target was obtained in the form of a disk '/s inch thirk and 1 inch in diameter (Machlett Laboratories, Springdale, Conn.). I t was mounted by the manufacturer in a stainless steel ring. The mounting ring was soft-soldered into a stainless steel retaining ring which had been silver-soldered t,o the end of the titanium target tube. With a beam of 30 pa. of deuterons a t 2 m.e.v. bombarding the target, 60 watts are dissipated in the target. This makes it necessary t o cool the target. Cooling water was passed through a polystyrene water jacket and across the outer face of the beryllium target. The first beryllium target used was coated with Glyptal varnish on the water-cooled face t o protect the beryllium from corrosion. .-ifter 5 months the target corroded to the point where it began to leak. A second target was installed. This target was coated with an evaporated film of titanium 3.9 microns thick and an evaporated film of aluminum 2.3 microns thick. Thicker coatings are desirable, but with the equipment available the time necessary t o secure a thick film becomes excessive. This target shoived pitting after 3 months of operation and methods of obtaining heavier coatings of titanium or stainless steel are needed. Paraffin Moderator. Seutrons produced by 2-m.e.v. deuterons by the nuclear reaction Beg(d,n)BlO have an energy spectrum extending up to 6 m.e.v. For activation work with thermal neutrons it is necessary t o reduce the energy of the neutron? to energies of the order of 0.002 to 0.5 m.e.v. This was accomplished in the apparatus by surrounding the target with a cube of paraffin (Quaker State 165 11 was cream). The high energy neutrons lose energy by elastic collisions with the hydrogen atoms of the paraffin. On the average after 20 to 25 collisions the neutrons have reached an equilibriuni state with the energy of the paraffin atoms. The moderator for the neutron source is in the form of a cube 2 feet on a face. Samples t o be irradiated are placed a t various positions in the mass of paraffin by a polyethylene stringer. The polyethvlene stringer or slider has a machined cavity into which the sample contai are placed. A photcthylene slider partially gra h of the moderator shoiving the I witRdrawn is shoivn in Figure 1. Equation Relating Neutron Flux and Induced Radioactivity. The thermal neutron flus was measured by irradiating known weights of metal foils. The induced radioacltivity of the foils was measured with a thin mica-window ( 1 , 4 mg. per sq. cm.) Geiger tube. The neutron flus i m e c a l d a t e d from the relationship:
CTIVATION analysis offers many possibilities for rapid and sensitive methods of determining many of the elements in the periodic table. It has been applied to a number of analytical determinations in the past 10 years by investigators, usually connected with the +4merican or British atomic energy programs, who have access to neutron sources. The technique has been used previously by this lahoratorj- for the tletermination of trace impurities ill high purity magnesium nietal ( 1 ) . To do this it w a y necesaar!. to send samples to Oak Ridge, Tenn., or Chalk River, Canatla, for neutron irradi:ttiori requiring Atomir ICnergy Commission :iuthorization. Because of the transit time of the irradiated s:tniplcs, work \vas limited to those c.lenients with isotopes of inore than 12-hour half life. The v:m de Gmaff accelerator provicles a means of ol~taiiiing:i neutron source with it neutron flus of useful magnitude for ai:tivation analysis. Seutroris are produced h ~ .1ionili:mling a t)eryllium target with deuterons zwclerated to 2 n1.e.v. 1)y the :wcelerator. The scneitivity of analysis for :t specific element ran Iir (loinputed from the neutron activutioii cross sei,tion, isotope abuntlariw, :tiid half life. The 1-oniplicntion of Senftle :tiid Leavitt (4)has ]wen put into a form \\.liicli, wing the neutron flus ohtiLiiiah1e from the author$' source, give. the analytical sensitivity for :L purticaulnr element iindrr v:ti,iuiis 1)omb:wdmeiit timw and countiiip eWitienries. Thr. revicv artirl(j of 13oytl ( 2 ) 113s r~q)laineclthe nicthotl of :wtiv:itioii :tii:tlysis, Subsequent papers, such us those of Lctldicotte and Reynolds ( Y ) , have described the use of the nuclear reactor for this method. The general procedure is to irradiate the analytical sample along u.ith pure Ptandards of each element to be determined as flus monitors. The several samples are then given the same cheniical treatment, and measurements are made of the radioactivity indured in each separate elenient. Using the accelerator neutron source the neutron flus is estahlished b>- measurement with standard detector foils, and then changes in flux are determined by relative neutron measurement,s made with counters. 1;niploying short-lived activities is advantageous, since the required irradiation time is short and the use of differential counting methods can often permit determining a qiecific element without chemical separations. This is dependent upon the elements present in the sample and the interferences obtained. This report describes the method of producing thermal neutrons with the van de Graaff uwlerator, the results of neutron flux measurements, and tlie methods and results obtained for the determination of chlorine, bromine, iodine, and fluorine by iieutron activation using the van de Graaff as a neutron source.
S * X = S u f k (1 -
e-At)
where .\-*X u
S f
k X t
= = = = = = =
activity, disintegrations per wcond :it zero decay time isotopic activation cross section number of atoms in the foil flux in neutrons per sq. em. per second isotopic abundance of target nuclide decay constant of radioactive nurlide t'olmetl time of irradiation, seconds
The foils were irradiated both with and without a '/,-inch caclmium filter to correct for activatiod by neutrons of resonance energy. 231
238
ANALYTICAL CHEMISTRY
Foils for Absolute Measurements. The foils were circular evaporated silver and indium films mounted on 1-mil polystyrene film in DOh'StVCne rings. The rings were of I-inch inner diam-
beryllium targets are given in Table I. These data were obtained with silver and indium foils at, t.hr ont,imnm difit,nncn from the target B energy were USI
weight. Three difierent count&k efficieniies were usea a t various times in the thermal neutron flux measurements. T h e counting efficiency values were 3.92, 5.51, and 18.81Yo. Disinteaation rates a t zero deeav time were determined bv counting &radiated foils over a Deribd of time and Dlotting thk results On semilog paper. A s t r a k h t line of proper sldpe was-then drawn through the experimental points and extrapolated t o zero decay time.
\
Table I. Thermal N e u t r o n Flux, a t O p t i m u m Distance from Target Thermal Neutron Flus.
,
: 60
3
4
5
2
I
POLYETHYLENE, CM. lllc.yyyD
I.C.PLllr
Illr.-.LILYnll=.I.D.
Lluululull
uu
metbod of absolute flux measurement, two methods of relative
the c h n t e r w&e fed t o . m &plifieer (Nuclear Instpument and Chemical Corp., Model 1061, plus one additional s t a e ) and then through a cable from the accelerator room to rate meter (Nuolear Instrument and Chemical Corp., Model 1615-B) in the control room. A Brown circular chart recorder v a s used to record the counting rate from the rat,
Figure 2.
Thermal neutron Eux for various d i s t a n c e s a l o n g target axis
These results me in agreement with the value of 5.8 X 106 thermal neutrons per sq. em. per second per pa. determined earlier in experiments a t the High Voltage Engineering Corp. laboratory. Flux Distribution in Moderator. Flux distribution in a moderator is generally shown by a curve of activity m 2ns. r, where r is the distance from the source to the point of measurement in the moderator. Far this purpose the plot of thermal neutron induced activity us. T is more useful. The point of maximum thermal neutron flux was determined by measuring the flux distribution along an extension of the target tube axis with varying thicknesses of polyethylene hetneen the target and the test fail. Results obtained with silver foils itre tabulated in Table I1 and plotted in Figure 2. With 0.75 cm. of polyethylene between the target and the sample holder, the effect of displacing the center of the sample
Table 11. Thermal N e u t r o n Flux Distribution i n M o d e r a t o r Along T a r g e t Axis Distanae between Polyethylene Tarzet and Boil, C",.
Thermal Neutron Flux. Neutrons/Sq. Cm./Seo./ra. ( x 10-6) Foil 1
Foil 2
Boil 3
Foil 4
Average
Figure 1. Paraffin m o d e r a t o r o u h e
&de co&d dn t.hd phototude fach.~The current dutput from the phototube was passed to ground through a high resistance.
of the moderator in Figure 1.
.
Thermal Neutron Flux 'at Optimum Distance from Target. The results of thermal neutron flux measurements for the two
from Target Axis, Cm. 0 2 4 6
NeutrondSq. C m . / S e ~ . / ~ a(. x 10-8) Foil 3 Foil 4 Average
V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6
239 The sum of these errors in one direction could make the measured value lorn by as much as 55%. A better estimate would be 30%. ACTIVATION ANALYSIS OF HALOGENS
Determination of Chlorine. NUCLEAR RE~CTION TO PRODUCE The nuclear reaction C13' ( n ,y ) C P R A D I O ~ C T ICHLORINE. ~E as used for a method of determining chlorine by activation. Chlorine-38 decays with a half life of 37.29 minutes. The radiatioii emitted in the decay process consists of three p particles and two y rays. The energies of the p part,icles are as follows: 53% 1.81 m.e.v., 16% 2.77 m.e.v., 31% 1.11 1n.e.v.; 47% of the disintegrations result in y rays of 2.13-m.e.v. energy, and 31% result in y rays of 1.60-m.e.v. energy.
B I ~
c
5
6
I
I
4
2
I 0
I
I
J
2
4
6
POLYETHYLENE, CM.
Figure 3.
Thermal neutron flux for various distances normal to target axis ,
laterally from the target tube axis was investigated. Results obtained with silver foils are tabulated in Table I11 and plotted in Figure 3. Effect of Deuteron Energy. The voltmeter of the accelerator was calibrated by accelerating protons onto a lithium fluoride target. The nuclear reaction Li7(p,n)Be7has a reaction threshold a t 1.88 m.e.v. By slowly increasing the accelerator voltage R hile bombarding the lithium fluoride target n ith protons, the voltmeter was set a t 1.88 m.e.v. a t the point where neutron production began. The effect of deuteron energy on neutron production with a thick beryllium target was investigated over the energy range 1.8 to 2.2 m.e.v. Results obtained with silver foils are tabulated in Table IV and plotted in Figure 4.
IRRADI.4TIOX AXD C O C S T I S G PROCEDURE. .%qUeOUS SanlpleS ivere placed in a 1-ml. polyethylene cup to be irradiated. The pup was made to fit a two-piece polystyrene irradiation cell. .I piece of 0,001-inch polyethylene film \vas laced over the polyethylene cup and held in place by pushing t i e ring portion of the polystyrene irradiation cell down over the film, cup, and bottom pmt of the irradiation cell. The cup is 2.5 cm. in diameter and 0.2 cm. thick. Samples were irradiated for 30 minutes at a thermal neutron flux of 2.5 x 108 neutrons per sq. cm. per second. This was ohtained by using a deuteron beam of 30 pa. a t 2 m.e.v. At the end of the irradiation period the sample solution was transferred from the irradiation cup to a polyethylene counting dish. h 1-nil. hypodermic syringe was used to make the transfer. The needle was pushed through the thin polyethylene film, and the content? of the irradiation cell were drawn into the syringe and then espelled into the counting dish. The radioactivity was measured placing the open counting dish directly under the window of a thin mica-window (1.4 mg. per sq. cm.) Geiger counter. The total number of counts above background observed during the interval of 3 to 18 minutes after the end of the irradiation was (letermined.
I
I
1
I
I
I
I
I
18
19
20
21
22
Table IV. Voltage Effect on Thermal Neutron Fluu Voltage. Millions of Volts 1.8 1.9 2.0 2.1
2.2
Thermal Xeutron Flux, Neutronfl/Sq. Cm./Sec./pa. ( X 10-6) Foil 3 Foil 4 Average 6.3 7.7 7.0 8.0 9.4 8 7 8.4 Q.1 8.8 9.1 10 9 10.0 9.9 11.1 10.5
Estimation of Errors in Thermal Neutron Flux Measurement. The absolute thermal neutron flux values are believed to be good to about 30y0. Errors arise from the following causes: S o t all the measured ion current is expended in the target. This does not produce an error in the neutron measurement, but could make an error in the neutrons per microampere of a s much as 25%. The Ra D-E 6 has an E,,, of 1.2 m.e.v.; for indium it is 0.8 and for silver it is 2.0. The flux calculated from the silver data is high. That from the indium data is low. An average will be off by not more than & l o % . This error could be eliminated by extrapolating aluminum absorption curves for the standard and test foils to zero absorber. The back-scattering of the samples is not so large as for the standard, because of the thin polystyrene layer between the samples and the silver disk. This will make the measured flux lower than the true value by 10 to 15%. The preparation of the evaporated metal foils involve handling and weighing foils of 0.4 to 0.8 mg. mass. The measured weight of the foils used in this work may be in error by &5%.
I .7
I
DEUTERON ENERGY, M I L L I O N S OF V O L T S
Figure 4.
Thermal neutron flux for various deuteron voltages
The sample transfer to a nonirradiatcd counting dish was found to be necessary in order to reduce the background to a low and constant value. Twenty-two polymers and copolymers were irradiated in an effort to find a material which could serve as a sample cell but which would not become radioactive when irradiated with neutrons. S o such material was found so the sample transfer procedure was used. RESULTS. A calibration curve TVas prepared by activating and counting ammonium chloride solutions of known chlorine content. A typical calibration curve in the range 0 to 5.0 mg.
240
ANALYTICAL CHEMISTRY
of chlorine irradiated is a straight line. The slope of the plat of total counts collected from 3 to 18 minutes after the end of the irradiation minus the background is 8600 counts per mg. Known samples prepared from 0.1000N hydrochloric acid were analyzed, with the results shown in Table V. The minimum amount of chlorine that can he detected by this procedure i8 0.02 mg. with an.uncertainty of +15%.
Unknown samples of dilute aqueous solutions of a brominated polymer were analyzed for bromine by this method. The results obtained are tabulated in Table VII.
Table VI. RI,
Results
of
Chlorine Analysis
Cl, Mg.
Found bv activation Sample 3.545
o.m
9.035
8,"dJrsiS
Error,
Error,
setivstion a"alY*i* 40.2 19.8 19.5 2o.o
Sample 40.2 24.1 20.1 2o.1
7'
0.0
-3.0 -3.0
-o.s
%
o.400
+C12.7 0.3
o.030
-14.3
3.555
7
Found by
T a b l e V.
R e s u l t s of B r o m i n e Analysis
Table VII.
R e s u l t s of B r o m i n a t e d Polymer Analysis Bromine, P.P.M. 13.5,13.5,11.3
Sample 47A 47B 47c 47D 1-1 1-2 1-3 1-4
Y
4.0.3.5
Determination of Bromine. SUCLEAR REACTION TO PRODUCE 2.8, 3 . 0 4 . 4 , 4.5 RADIOACTIVE I~RODIIXE. The nuclear reaction Br" (n,y)Brm 5.2, 4 . 7 5.0. 5 . 0 was used to produce riidioeetive hromine. The half life of 3.5.3.G bromine-80 is 18.0 minutes. The radiation emitted in the decay 1-5 8.2, 7.9 1-0 5 . 4 , 5.8 process consists of two fl particles and one y m y . The energies of the B particles are as follorrs: 85% 1.99 m.e.v. and 15% 1.1 me.". A y m y of ahout O.%m.e.v. enerEy occurs in 15% of the disintegrations. Xeotran capture by- hromine-79 also re. The minimum amount of hrominc detectable by this method suits in the formation of i ~ nietast;ihle . statc of bromino80 which is 1 y with an uncertainty of 1 1 5 % . decays by isomeric transition wit,h n half life of 4.4 hours. The Determination of Iodine. ~ U C L E . ~ REACTIOS R TO PRODUCE decay of the activit.v ahscrved in this work ~ m falloivxl s for GO RAnIoAcTIVE IODINE. The nuclear reaction I 1 s ~ ( ~ , y ) Iwits ~s~ minutes. The artivit>-dpc:~ycdn-ith :L half life of 18.5 mimitea. used to produce radioactive iodino. The half life of iodine-128 is 24.99 minutes. The radiation emitted in the decay process consists of a B particle of 2.02-m.e.v. energy plus shout 7y0of the disintegrations resulbing in the production of 5 y ray of 0.43 m.L?.v. IRRADIATION .AND C a u a ~ r s oI'ROCEDURE. The irradiation and counting procedures \vere the same as those used for chlorine determinations. RESULTS. A calibration curve was prepared by activating and counting lithium iodide solutions of. known iodine content, The calibration curve is a straight line over a weight range of 0 to 100 y of iodine irradiated. The dope is 307 counts per y . Aqueous solutions of ammonium iodide and calcium iadidc mere checked against the calibration curve with the results tabulated in Table VIII.
T a b l e VIII.
I
