Application of a fast neutron activation method to the determination of oxygen in iron and steel

AKALYTICA

146

APPLICATION DETERMINATION

OF

CHIMTCA

A FAST NEUTRON ACTIVATION METHOD OF OXYGEN IN IRON AND STEEL

TO

ACTA

THE

In the iron and steel industry, a rapid and precise method for the determination of oxygen has been in great demand for runny years, since, in the case of the conventional vacuum fusion tcclinique, thcrc have always been difficulties in rapid detcrminntion and complete extraction of oxygen in the sample. IZeccntly, new nuclear mcthodsl-5 for the oxygen determination have been clcvclopcd in order to overcome these clifficulties. The authors have already reported a method 0.7 for the oxygen determination using fast neutron activation and its application to sonic metals with low oxygen contcntG. In this paper, the reproclucihility of the mcthocl is discussed and a comparison is made between the activation and the vacuum fusion methods.

Sal@Jle. In order to investigate cffcctively the precision of the method at various oxygen lcvcls, samples containing oxygen of the orcler of 0.1, 0.01 and o.oo~O/~ were chosen. To keep the geometrical conditions of neutron irradiation and of the mcasurcmcnt of y-rays constant from sample to sample, all samples had the same shape ancl size, 12.0 mm cliamcter and 10.0 mm long. Before irradiation, samples were clcanccl with chloroform. The cleanecl sample was placed in a polycthylcne rabbit and transported from the sample injector to the target arca. After irradiation, the sample in the rabbit was returned to the measuring system. On arrival of the sample at ,the sample injector, the rabbit case was brought to a halt ancl the sample alone reached the detector by momentum. The decapsulation, which eliminates the effect of oxygen contained in the rabbit material, was carried out automatically and instantaneously. Afi/~~t~trs. A Tokyo Shibaura Electric Co., model NAT-200 type “Activac” which is a fast neutron activation analysis unit (117 was used for oxygen analyses. This neutron generator produces 14 McV neutrons by T-D reaction. The neutron output was of tllc order of 1.4 - 1010 neutrons@; this measurement was carried out by a-counting using a silicon semi-conductor. The target usccl was a tritium-loaded target absorbed in a titanium thin layer (prepared by Central Research Laboratory, Tokyo Shibaura Electric Co.). The counting of the irradiated samples was carried out with a single-channel pulse-height analyzer; the window width was 4 MeV and the base line was 6 MeV. It is * Central-Rcscmch Laboratories, Fuji lron and Steel Co., Ltd. A Nd. Clli9ll . -4c/n. 3.1 (I#%)

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IN

IRONS

ETC.

I47

not preferable to USC the multichannel pulse-height analyzer because of its longer resolution time. A pneumatic system was used for rapidly transporting the sample from the target area to the detector (5 x 5” NaI scintillation crystal and Toshiba 7696 photomultiplier tube). Novrnnlization method. In the activation method using accelerator neutrons, it is necessary to normalize the observed count value with regard to fast neutron flux as well as to the decay of activities of nitrogen-x6, because the neutron flux from the accelerator fluctuates during the irradiation of even one sample. The conventional normalization method 1- 4, however, is generally tedious when many samples are analyzed. The C-R neutron monitoring system can automatically normalize the observed count value by a simple integrating circuit wl~osc time constant is ~/jl (A is the decay constant of nitrogen-16). Details of the apparatus have been reported previouslya*7; with this apparatus, it is possible to compare each obscrvcd count value without calculation. Correction for self-absov#ion. Recause part of the irradiating neutrons and the y-photons emitted from the radionuclide produced were absorbed by the sample material, the overall effect of self-absorption under our experimental conditions was determined empirically. The transmittance value obtained for iron and steel samples was 87.90/O,as described later. Cnfibrntio~~ CZIYVC. To prepare the calibration curve, 30 mg of sucrose and about 3 g of graphite powder for spectrochemical analysis were mixed well in a weighing bottle, and the mixture was pressed into a pellet (12 mm diameter and IO mm long) and sealed in a polyethylene rabbit. The samples for calibration were irradiated with neutrons, and counted to prepare the calibration curve The cliscrimination level at which a calibration curve was prepared, depended on the kind of sample matrix. In our experiment, the pulses between G.o and 10.0 McV were counted. Pvocedzrre. The rabbit containing a sample to be analyzed was transferred from the sample injector to the target area by the simple pneumatic system. When the rabbit reached the target area, sample irradiation was automatically started by the neutron beam shutter which was driven by a signal from the arrival indicator for the sample. In the apparatus used, the further operations-namely, stopping of irradiation, transfer of sample to the detector, and starting of the counting--were performed completely automatically by the C-R neutron monitoring system, and the counting time was controlled by a preset timer. The counts were made first for 0-30 set and then for the following 30-60 sec. The latter count helped to eliminate the effect of a slight activity from the matrixmaterial. The calculation of the oxygen content in sample was performed as follows :

where PC~is the count value (0-30 set), nc the count value (30-60 set), K the sensitivity (mg of oxygen/count), BG the back-ground, W the weight of sample (mg), and T the transmittance value = 0.879.

Sample.

A neutron activation

test piece of

12

mm diameter

and

IO

mm length

Amal. Chirrr. Acta, 34 (1966) 146-153

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H. MUTO,

K. SHIMURA

was quartered wedge-wise with a hack-saw. (A sample was further halved, if an exccssively hi& gas content was anticipated.) After filing, samples were polished with #280 emery papers. Samples were cleaned with benzene, and weighed after drying. rlfi$~~vatz~s. A Rigosha Manufacturing Co., NRC type vacuum fusion gas analyzer was used for oxygen analysis, The oxygen analyses were performed under the following conditions: degassing tcmpcrature, 2200~-2300~ ; degassing time, over 3 h ; time, 2-4 min; gas extraction gas extraction temperature, 1850”; gas extraction pressure, e 5 * 10-4 mm Hg; weight of sample, 0.5-3.0 6. BZank. The rate of evolution of gases from the system at 1850” was kept in the range of about 0.01-0.03 ml/30 min. Since it normally took only about 3 min to extract the gas specimen from a sample, the rate of evolution mentioned ought to be small enough for the blank to be neglected. Bath. As the samples were steels, the bath naturally consistccl of Fe in its major part. However, if a sample contained over I O/oof such elements as Mn, Si, or Al, or if the total content of Mn+Si in the sample cxcccded I%, the tin-addition technique was adopted. The tin employed for this technique was gg.ggg”/O in purity. In the tinaddition technique, samples and tin-pieces were alternately arranged in the sampleholder, in such a manner that the amount of tin-addition always exceeded 30~7~of the weight of samples dropped. Whenever a piece of tin was dropped into the bath, degassing of the tin itself was carried out for 1.5-2 min before the sample was introcluccd. Procedzcrc. During the fusion of sample in vacuum, the hydrogen and nitrogen contained were liberated in their respective gaseous states, while the oxygen was liberated as carbon monoxide, since the oxygen in steel existccl mainly as oxide inclusions which were reduced by carbon from the graphite crucible. These freed gases were extracted by means of an extraction pump with a capacity of about 60 l/set, and then collected into a constant volume by a collection pump. By measuring the pressure (1’1) at this point, the total quantity of the &as mixture was available. The gas mixture was then passed through a vessel packed with small pieces of copper oxide wire heated at 400’. Hydrogen ancl carbon monoxide were thus oxidized to form water vapor and carbon dioxide respectively, while nitrogen remained unchanged. These gases were passed throu# a column of PcOe to absorb water. By measuring the prcssurc (F’S) of the residual gas, the quantity of hydrogen could be cletermined (PI -I%). Finally, by passing the residual gas through a liquid air cold trap, carbon clioxide was solidified, and the nitrogen pressure was measured (Pa). The difference between the pressures P:, and Pa gave the quantity of oxygen, in terms of carbon dioxide. The calculation of the oxygen content in sample was performed as follows: oxygen “,< = (Pz wherefis RESULTS

the sensitivity, AND

P3)f/7e,

and

70

the weight of sample.

DISCUSSION

Figure I shows a comparison of the analytical results obtained on various steels by the fast neutron activation method and vacuum fusion method. The analytical values for oxygen concentrations between o.zO/~ and O.OOI~/~ by the above methods Anal.

Claim. Acta,

34 (I~GG)

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149

were in good agreement. Table I contains the count values obtained by the fast neutron activation method. AllI the samples were’ decapsulated before the measurements. Figures z and 3 show the reproducibilities of the neutron activation method at oxygen levels of 0.2~/~, o.o~O/~,. and o.oory/,, respectively; the relative standard deviations were 3%. SO/~,and 3o”/;‘,,respectively. For the determination of oxygen, the discriminator level must be such that it is free from interference by the sample matrix; also, a correction for neutron and y-photon absorption by the sample itself must be made.

Oxygen Fig.

I. Conipnrison

laund

of nnalyticnl

P/d

results

Vacuum bctwccn

f uslan method ztctivntion

analysis

l nd

v-LCUUIIl .

fusion

mcthocl.

A disk plate, 12.0 mm in diameter and I mm thick, was fabricated of the same material as the metal to be analyzed. A cellophane paper was cut to the same diamctcr as the metal plate. Such plates and cellophane paper pieces were alternately piled to a height of IO mm and this pile was sealed in a rabbit case, which was then irradiated with neutrons and measured by a 25G-channel pulse-height analyzer. Next, a similar disk was prepared from graphite and a similar operation was performed. The counts obtained over IO, 20,30, . . . channels of they-ray spectra were integrated, respectively. The ratios of each integrated count value for the metal disk to that for the corresponding graphite one were calculated ; the results are shown in Fig. 4, in which the point of lowest energy within the plateau of the curve is taken as the optimum discriminator level for the metal under the investigation. In Fig. 4, the value on the ordinate of the plateau in the curve indicates the overall effect of the absorption of neutron and y-photons by the steel sample with reference to graphite, so this is termed the “Transmittance”. The count value for the sample without absorption could be obtained by Anal.

Clritn. Ada,

34 (1966)

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ANALYTICAL

RICSULTS

._-------

._..__._

OF

OXYGKN

IN

IRON

h?iD

STISISL

DY

K. MIYOSHI,

TllIS

FAST

H. MUTO,

NEUTRON

ACTIVATION

___-_-

_ _..-.----____._-_____.___._-

.

Ncl o-30 nI

._.---_.---__

._

l4.083tl

l-l-5

SCC

. _ _____ _

822

7’$’ I-I -0

8.7778

I-I-H

H..5’~59

871 318 307 34 3 a.tG 381 34 1 5.38 560

1;

X.7722

v-3

8.7840

93 478 188

v-4

8.5700

I.50 162

N-.t

18.3 I fiO.#(’

H.5.)52

I5855

N-5

8.0538

N-C,

X.59oCJ

N-7

8.7284

N-8

I iioRf3 rq1g I3084 lZlf>2 ‘).@) c,ry, errs 4 IO.3 4230 .t.t I CJ 1.3Si I.(28 I393

x.7209

SUCr(>SC StiLntl:lIYl Stxl-I (18.1) mg of osygcn) Sttl-0

‘W47 I WJ5H

(00.0 nig of osygcn)

-_..._Sensitivity Avcragc

19265

=

__

w

of lxxl~groimds

11 IO I l0.i _---.

=

.-

____. -_-_-.-_--.__-.--..-

305 counts/lng =

a.+

counts/30

K. SHIIMURA

hlDTHOD

---.-------

cozcrzfs

OXygEIb

for1nrl

30-60 see 31” -. ..-- -._. _. .-._

(W - --. .-__. -.--_. __-..-

s* 08 .to

0.01

35 33 31 ‘2.1 2 .!

10

o.oo3H

35 3? 3h 30 37

0.007

I

0.0059

2: 3’ 58 799 807 775 (‘57

0.0015

0. 22.3

F :! *I5(J 4 74 494 ,I,> , mm_

0. J73

0.

‘3-l

120

0.0576

217 :: Ho

0.0187

1022 957 056 74

(18 ._..-.----.---....

-_ - .---.

r7ro9

. -------.-.--

of oxygen. see.

dividing the observed count value by the transmittance. In the determination of the transmittance, calculation was made assuming that under our experimental conditions the oxygen in the iron and steel would be uniformly distributed. The transmittance value found for iron and steel samples was S7.9%. The target assembly shown in Fig. 5 was used. The sensitivity of the method, as is well known, is proportional to the neutron flux passing through a sample. In this target assembly, the distance between the center of the tritium-loaded target and the

ACTIVATION

METHOD

FOR 02 IN IRONS ETC.

1s

front of the sample was 3 mm, and the number of effective neutrons in the irradiation of sample was ~3% of the total neutrons emitted. The thickness of the cooling water layer was I mm in which the number of neutrons lost was theoretically about 15%. The allowable total number of neutrons, however, had a limiting value for the following reason. When the neutron flux is greater than a certain value, the functioning of the measuring system may not be normal because of the pile-up phenomena of

eto,25/

N-0

3

“02

&“oroi’ ,,J’+--7

‘--.‘+J\“’

2nd

J-kJv-

3ra

doy

' i ?3n(,i,7/c~ioliijjriji,7cir41bilijj~;’~i.Bboi,iij6iBi~j~i, Order 01 ,ne.- mearurrmcnts --._-c _..----..1st

Fig.

2.

Dcvintion

_._..._

2nd cloy

cloy

of nndyticol

_._.\.,, _,,.\, _,C,_.

day

4th

day

_._.__-_--

-__.

---....-a.

4 th

3ra day

results by neutron activation

1.0

mcthocl.

~

\

------l_.

2

I

s

5 E

0.020 K-2

2

2

e

Iron

&- 0.5

0.0051

I_--123456789 Order

10 O?

Fig.

3. Deviation

Fig.

4. Thnsmittancc

the

measurement5

of analytical

and steel

5.0

11

Oiscrim~notor

-

results by neutron activation

value vs. discriminator

day

__-._-

---

10.0 IPVC;

(Me’/)

method.

level. Awd.

Ciiim. Acta, 34

(1966)

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I<. MIYOSHI,

H. MUTO,

I<. SHIMURA

input pulses even if the ol~crvecl count value is small. Since the resolution time of the measuring system used was 2 psec, the neutron flux might have already reached the limiting value if the samples were of copper. The sensitivity obtained in our cxperiment was about 1000 counts (6-10 MeV)/mg of oxygen in the iron and steel sample. Ammeter

Tritium

Cooling

water

I-

1Omm Fig.

5,

Vertical

tylx

target

asscn~l~ly.

In the case of activation analysis with a nuclear reactor, the sensitivity cxpresscd as a concentration may increase when the dimension of sample is increased. Howcvcr, as the neutron source is a point in the present method, the sensitivity dots not increase proportionally to the sample size. Particularly, in the “vertical type target assembly” used, the sensitivity decrcascd if the sample was too long. In general, it was concluded that the analytical results by the vacuum fusion method were in good agreement with those of the fast neutron activation method, although the former involves difficulties in rapid determination and complete cxtraction of oxygen in the sample. Since the fast neutron activation method is non-destructive, it has a significant advantngc over the conventional method in that repeated analyses of the same sample arc possible. Therefore, it might hc suggested that the activation analysis could 1~ utilized as a means of checking analyses if the results obtainccl by the conventional methods wcrc inconsistcut. The authors wish to express their appreciation to Prof. T. SOMIYA, Dr. Ii. and Dr. T. UNO for their interest and helpful suggestions during the course of this research. The authors also thank Mr. M. A~IANO for performing vacuum fusion gas analysis, and Mr. M. SLTO and Mr. I<. HAMANAICA for their experimental assistance. KAMOGAWI~,

The determination of oxygen (o.oo1-o.2~/~) in iron and steel by the fast neutron activation method was investigated. The relative standard deviations of the

Anal. CJrim. Ada,

34 (1966) I&-153

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153

ETC.

method at oxygen levels of 0.1, 0.01, and o.ooI~/~, were 3, 5, and 300!~, respectively. The analytical results of the method were in good agreement with those of the conventional method in the range stated.

Les auteurs ont examine le dosage de l’oxygene (O.OOI-0.2%) dans fer et acier. par activation au moyen de neutrons rapides. Les deviations relatives, pour o. I, 0.01 ct 0.001 o/o d’oxygene sont respectivement 3,s et ~oO/~.Les rdsultats obtenus par cette methode correspondent h ceux de la methode conventionnelle. %USAM XIENFASSUNG

Es wurde die Aktivierungsanalyse stoffgchalten von 0.1, 3, 5 bzw. 30%. Die iiberein.

I :! 3 4 5 G 7

Bestimmung von o.oo1-o.2~/~ Sauerstoff im Stahl mittels der unter Anwendung schneller Ncutronen untersucht. Bei Sauer0.01 und o.oor"/b betrugcn die relativen Standardabweichungen Brgebnisse stimmen gut mit denen konventionellcr Methoden

I<. 1:. cOI.I:hlAN ANDJ. L. l'rSRKIN,/f1lUIySI,8.) (1059) 233. R. F. ~OLIIhlAN,~~tUdy.St, 87 (rgGz) 178. I-1. J. I
32.