Formation of Nitrogen-13, Fluorine-17, And

AE-170 UDC 541.127

o rx LLi Formation of Nitrogen-13, Fluorine-17, and < Fluorine-18 in Reactor-Irradiated H2O and D]0 and Applications to Activation Analysis and Fast Neutron Flux Monitoring

L. Hammer and S. Forsen

This report is intended for publication in a periodical. References may not be published prior to such publication without the consent of the author.

AKTIEBOLAGET ATOMENERGI

STOCKHOLM, SWEDEN 1964 ^>•13°

AE-17Q

FORMATION OF NITROGEN-1 3, FLUORINE-17, AND FLUORINE-1 8 IN REACTOR-IRRADIATED H^O AND O AND APPLICATIONS TO ACTIVATION ANALYSIS AND FAST NEUTRON FLUX MONITORING

Lennart Hammar and Sture Forsen

Abstract ' ’ 235 The equivalent macroscopic U -fission neutron cross-sections with respect to formation of the following nuclei were measured and found to be; N13 in HzO, 13.5+ 2; N^3 in D O, 0.24 + 0. 05g F17 in D-O, 3500 + 700; F18 in H,Q 5.6+ 0.9? and F18 in D_0 4.1 +. 0. 6 units of 1 0~* ^ cm , The yield of N in DÔ refers to DÔ containing no H, and the yield of F* 3 in DÔ to an O* 7 sO* ^ ratio

of 0. 0391 + 0. 0007. . .

Good agreement was obtained in comparing the measured yields 13 18 of N and F in H^O with estimates based on calculations of the neutron ~ induced flux of knock-on protons and on published cross-section data for the reactions O* ^(p, )N* 3 and CV*3(p, n)F* 3. The similarly calculated yield of F* 7 in D^O due to the reaction ^(d, n)F^ 7 was 6 times less than the observed yield. This discrepancy could not be eliminated by taking into account the anisotropy of the neutron-scattering by deuterons.

18 The formation of F in D^O was attributed to the reaction 1T 18 ^ O (d, n)F „ Thi-s reaction may be utilized for the determination of the O* 7;0* ^ ratio in heavy water by neutron activation analysis, e, g» as a ' means of differentiating between heavy water qualities, obtained by diffe rent techniques of enrichment.

. 13 The formation of N in D-O, that cannot be attributed to the 16 ^ O (p,°c ) reaction due to residual H content, is suggested to be caused by the reaction O* ^(d, n ^ )N* 3, not previously reported. The cross-section of this reaction per knock-on deuteron at energies above the estimated reaction threshold (8.4 MeV), corresponding to the yield of N e , is 8 + 2 mb. Application of the 0^(p, ^ )N* 3 reaction for neutron activa tion analysis of H in D-,0 is possible, in spite of the observed extra 13 - yield of N , down to 1 atom per cent (H/D).

The formation of N* 3 and F* ^ in H^O and of F* 7 in D^O may advantageously be utilized for fast- neutron flux monitoring in reactors. The possibility of application for neutron spectrum determinations is restricted, however, because of poor energy resolution.

Printed and distributed in December 1964 CONTENTS

Page

Abstract

Introduction 1

Calculation of radioactivity induced by knock-on nuclei 3

Expe rimental 6

Results and discussior 11

Conclusions 15

Acknowledgements 16

List of symbols 1 7

Referen c e s 18

Figures 22 - 1 —

Introduction Nuclear reactions involving knock-on protons and deuterons arising from neutron-hydrogen and neutron-deuterium collisions constitute a major source of radioactivity in water used as coolant and moderator in nuclear reactors. An important example is the 1 7 radioactivity of react or-irradiated heavy water due to 70 s F , which is of the same order of magnitude as that due to 7.-4 s N*

Furthermore, the radioactivity of the isotopes 10 mih 1 8 and 1.8 h F has been found to be predominant in the coolant of pressurized and boiling light water reactors, within an interval of time from a few minutes to several hours after shutdown ^3

The following nuclear reactions have been assumed to-ac count for the observed radioactivities (Q-values according to reference 6)g

n)F 17 Q * -1627 + 2 keV (1)

oi t(p, «}N13 Q * -5218+ 1 keV B)

o^(p, n)F 18 Q = -2450 + 4 keV (3)

In addition, as shown m this paper, the isotope F* ® is formed in react or-irradiated heavy water to an extent comparable to that in light water. This induced radioactivity may similarly be accounted for, assuming nuclear reactions involving knock-on deuterons, e,gg

O1 6(d, y)F18 Q * 7514 + 4 keV (4)

O1 7(d, n)F 18 Q * 3372 +■ 4 keV (3)

018(d, 2n)F 18 Q * -4675 + 4 keV (6)

Integral cross-sections corresponding to the reactions (1) and (3) were measured in the late thirties by Newsom ' (threshold to 5 MeV) and by DuBridge et al. ^ (threshold to 4 MeV), respectively.

The excitation function of the former reaction, according to New son ^s results and a single measurement at 8 MeV by Brown and Perez- Mendez^^, is shown in Fig. 1. Later the cross-section of reaction

(3) was redetermined by Blaser et al, and the measurements ex tended to proton energies up to 6.9 MeV (Fig, 1), The 8{p, a)N^8 - 2 -

reaction cross-section was measured in 1958 by Whitehead and Foster^ ^ from 6 to 1 5 MeV (Fig. 1) and in 19 61, with improved fl 2) energy resolution, by Hill, Haase and Knuds en'" 7 from 12 to 18 MeV.

Published cross-section data for the reactions (4)-(6) are scarce and do not suffice as a basis for calculation of the pro- 18 duction rate of F in reactor-irradiated D-O. From the shape (13) ^ of the excitation function Davidson' ' concludes that the forma - 18 tion of F by deuteron irradiation of oxygen is probably due to a (d, n)-reaction, i* e. reaction (b)» The reaction cross-sections at 3. 25 and 3.4b MeV were found to be 6b and 77 mb respectively. By comparing the production rates in deuteron irradiation of two targets of tungsten oxide, one containing ordinary oxygen and the other an equal amount of oxygen enriched in the heavy isotopes, fl 4) 17 Welles' 1} using 3, 7 MeV deuterons, definitely proved the O 18 to be the isotope responsible for the F production. From the (15) absence of capture gamma rays Sinclair' ' estimated the upper limit for the 0^(d, y)F^ cross-section to be 0.5 mb at 1.1 MeV. Butler^ also using oxygen enriched in 7 , estimated this

cross-section to be of the order of 1 0 microbarns at 1, 7 MeV bombarding energy, or about 3000 times less than the cross section of the 7(d, n)F ^ reaction at the same energy.

The first known attempt to estimate the radioactivity induced in water by knock-on protons and deuterons produced in the moderation of neutrons, making use of published reac (1) tion cross-section data, was made by Henderson and Whittier (F1 7 in DzO).

Later similar estimates were made by Blanc et al. ^ ' (F^ 7 in DÔ), Russel and Rider^^(F^ in HÔ), Stehn^^ (N^ in DÔ), and Tagami et al. ^ 7^(N^ and F1 ® in HÔ).

These estimates refer to the total production rates within the cores of particular reactors, and the results were com pared with measured radioactivities in the coolant modera tors of these reactors. Although good agreement was gene rally obtained, this could only be verified within ra.th.er wide limits of probable errors, particularly considering the ip* 3

uncertainty in the assumed relationship between reactor power and the exposure of the coolant to fast neutrons of different ene rgies.

This work was initiated in I960 in an attempt to verify the assumptions regarding the mode of formation in reactor-irradiated water of the isotopes mentioned, particularly by providing reason ably accurate experimental production rates and comparing them with calculations utilizing published cross-section data. Another approach consisted in comparison between production rates obtained in irradiation of samples of different isotopic compositions.

Calculation of radioactivity induced by knock-on nuclei i — ' ' " " g- — The total length of tracks produced per cm and second in a 2 neutron-irradiated medium (1 neutron per cm • s) by knock-on nuclei of the kind in question, having kinetic energies in the interval dE, may be written; P(E)ds 3S-3EL dE (7) /"dE A xds y where P(E) is the number of nuclei per cm and second and per unit neutron flux which during slowing -down passes the energy E Me V, and ds the track length associated with the energy loss dE. Thus assuming the macroscopic reaction cross-section and the threshold energy to be Er(E) and E ^ respectively, the equivalent macroscopic neutron reaction cross-section (£ reactions per cm~y* s and per unit of neu tron flux) may be calculated according to the formula;

2r(E)dE«J §r.(E) Sr(E)dE (8)

E thr

The factor §r(E) * P(E)^(dE/ds) is recognized as the flux of knock- on nuclei per unit of neutron flux.

The calculation of P(E) requires knowledge of the source density of recoiling nuclei, p(E^), which is calculated using the laws of impact and pertinent scattering cross-section data. “* 4 “*

Assuming a macroscopic neutron scattering cross-section per steradian of Z^(E^, cos 8), being the neutron energy in the laboratory system and 8 the scattering angle in the centre-of-mass system, the production of knock-on nuclei of initial energies in the interval dE, , caused by neutrons of energies in the interval dE , can be written;

d( -cos 8 ) p(Et)dEv * 2tt 2e(En , cos 8) dE, 9

3 and integrating, the total production per cm" ° s • MeV and per unit of neutron flux of knock-on nuclei with an initial energy of E^ is found to be;

V(En ) 1 r p(Ek) *---- j 4 TT Ee(En , cos 8) dE (10) n n n Er

If scattering in the centre-of-mass system may be assumed to be isotropic, equation (10) simplifies into 9(En) p(Ek)-_-| 4 „ Ee (EJ dE (11) n T1 E, n

P(E) may now be calculated on account of the fact that every recoiling nucleus with an initial energy E^ exceeding the energy E in question will pass this energy during si owing-down;

* j P dE k (12) E - 5 -

The flux of recoiling nuclei per unit of neutron flux, i. e. the factor $ r(E) a P(E)/( dE/ds) entering into the integrand of equation (8 ), was calculated for energies between 1 and 15 MeV in the cases of re coiling protons and deuterons in neutron-irradiated HÔ and DÔ, respectively (Fig. 2). The energy distribution of the neutrons was assumed to correspond to a fission spectrum according to Cranberg et al, Integral neutron scattering cross-section data and angular distributions were obtained from^ ^ and respectively. In the case of neutron scattering by deuterons only elastic scattering was considered. The energy loss rates, dE/ds (Fig. 3), were obtained (21) from the range-energy tables by Rich and Madey' ' except for the loss rates in the energy ranges 0. 25 - 1 MeV for protons in HÔ and 0. 5 - 2MeV for deuterons in DÔ which were obtained by extrapola tion from higher energies. This was done, utilizing known energy loss rates of protons in air at low energies^^, by extrapolating the ratio of the energy loss rates of protons in air and in HÔ, which ratio varies only slightly with the proton energy, and by making use of the relationship

h2o z-dE (13) Lds E which is based on the Bethe theory of the rate of energy loss of char- (22) ged particles passing through matter'

The agreement between the flux of knock-on protons in H^O thus calculated (Fig. 2) and the analytical expression proposed by Tagami et al. ^ §r(E) * 2 « 10 ^ E^ ^ cp(E^), is not remarkably good, the latter exceeding the former by a factor 1.3 - 3.5 in the energy range 1-15 MeV.

As can be seen from Fig* 4, showing the source function p(E^) for protons in H^O and for deuterons in DpO, calculated according to equation (11) using integral scattering cross-section data, this equation represents a good approximation to the more rigorous equation (10) even in the case of neutron-scattering by deuterons, which is highly anisotropic. The calculations according — 6 — to equation (10) are represented by dots in the diagram of Fig. 4.

The equivalent macroscopic neutron cross-sections of H?0 1 -5 ]7 TO " and DÔ with respect to formation of N , F , and F in HÔ and DÔ were finally calculated using equation (2) in combination with the appropriate excitation functions, referred to above (Fig. 1). The results of the calculations have been included in Table 4.

Experimental irradiations^ The irradiations of water samples were performed close to the center of the core of the Swedish natural uranium heavy water moderated research reactor Rl^8,

D^O used in the experiments was obtained from two different sources, designated A and B, The isotopic compositions of the water samples used in this investigation are given in Table 1. In the case of D^O the isotopic composition with respect to hydrogen and oxygen isotopes was determined by IR-measurements and by mass spectrometry, respectively. The isotopic composition of the oxygen constituent of light water was assumed according to ref. 25.

TABLE 1

Isotopic composition of H^O and D^O investigated (The percentages given refer to the total number of isotopic atoms.)

Atoms per cent Item H1 o17 o18

H2° 100 0. 042 6 0, 003 0. 198 6 o. 003 D^O source A 0. 38 ± 0. 02 0. 0391& 0. 0007 0. 222 6 0. 001 D^O source B 0.3 0. 0515± 0. 0005 0. 362 6 0. 005 - 7 -

The water to be irradiated was deionized by ion exchange, using a mixed bed ion exchanger, and finally distilled directly into the irradiation vessels. Since dissolved argon would become radio- activated and thus would interfere with the activity measurements, dissolved air was removed by carrying out the distillation under simultaneous bubbling with hydrogen. No degradation of heavy water took place as a consequence of this treatment.

The irradiations were performed in 10 ml polyethylene or silica vessels. Before use, these were carefully cleaned by washing with dilute nitric acid and finally rinsed with water of the same quali- 13 . ty as that to be irradiated. The contribution to the N production in 12 13 water irradiated in polyethylene vessels by the reactions C (p, y)N and C (p, n)N , occuring with carbon atoms in the container mate rial, was subjected to a special test and found to be insignificant.

In most cases the irradiation vessels were covered with 2 mm of cadmium sheets to minimize the radioactive contamination of samples and fast flux detectors by radioactivation of impurities.

Since larger samples of irradiated water were desired when . . . 13 determining the relatively small production rate of N in irradiation of D^O, this determination was partly done by analysing large samples of the reactor D-O moderator (source Aj see Table 1). Since chemical . ^ 13 separation of the N formed in D^O was in any case necessary before measurement, the disadvantage of these samples being considerably . 41 contaminated by A and other radeactivated impurities was minor.

Activity measurements The radioisotopes formed in the nuclear reactions investigated in this work were all positron emitters (Table 2), detectable by the 0. 51 MeV positron-electron annihilation radiation. - 8 -

TABLE 2 ,13 J7 J8 , „ 64 a Nuclear properties of N , F , F , and Cu (according to reference 26)

Isotope Half-life Mode of decay ^transition energy, keV

N13 1 0, 05 m$ 10.08 ± 0. 04 m (3+, 1 00%5 No v 1185 ± 25 F17 66 s* 66 ± 1 s\ (3+, 100%$ No y 1748 ± 6$ 1 760 00 h 1.85 ± 0. 02 hg 1.82 ± 0. 02 h P+, 97%; e 3%$ Noy 644 109. 7 mC ' Cu64 12.80 ± 0. 03 hg 19% 656 12. 90 ± 0, 06 h

ct 64 Cu was used as an absolute standard in the activity measurements. ^Reference 9.

^Reference 27.

The annihilation radiation was identified and measured using scintillation spectrometry with a 100-channel pulse height analyser. By this method the absence of hard gamma rays contributing to the 0. 51 MeV peak by pair creation was at the same time ascertained.

The sample to be counted was transferred into a thin-walled polystyrene container (internal diameter 63.5 mm), inactive water added to a total volume of exactly 25 ml, and the container placed concentrically at the top of a 3 in. x 3 in. Nal (Tl) scintillation cry stal.

The absolute counting efficiency with respect to positron anni- ' . 64 hilation gammas was determined by counting a solution of Cu of known activity obtained by dissolving a weighed copper foil activated in a neutron flux together with a similar foil which was (3-counted using an absolutely calibrated scintillation detector. Again, the calibration of the latter was based on counting a copper foil acti vated in a thermal neutron flux which was measured by absolute counting ((3-y coincidence method) of a simultaneously irradiated 64 gold foil. The branching ratio with respect to the decay of Cu by positron emission was taken to be 19 per cent(^). - 9 -

The possible dependence of the annihilation gamma counting efficiency on positron energy (due to increasing probability with increasing energy of the positrons to escape the solution being counted) was investigated by comparing the relative counting rates of water samp les containing the different positron emitters (Table 2) by use of count ing geometries corresponding to widely different positron escape pro babilities. No differences were found, however, indicating that the effect of positron energy could be assumed to be insignificant.

The contributions of the different positron emitters investigated to the total annihilation radiation emitted from the irradiated samples were in most cases easily determined by analysis of the decay curves. Chemical separation was required, however, when determining the . . 13 . . relatively small production rate of N in D^O. Experiments compri sing ion exchange and distillation, which were carried out to determine 13 13 the chemical state of N formed in water, had shown the N to be partly anionic and nonvolatile, the remainder possessing the properties of ammonia. Depending on whether the water was saturated with air or hydrogen during the irradiation, the anionic fraction was at least 90 % and less than 10 %, respectively. Because of these observations a 13 method for separation of the N in a radiochemically pure condition . 13 was selected, consisting in complete reduction of the N compounds to ammonia and subsequent separation as such by distillation. Thus the analysis of the 250 ml samples of the air-saturated reactor mode rator was carried out by adding a reducing agent (Arndt 's alloy^*^)

and a measured quantity of nitrate carrier to the sample, which was - 13 then rapidly heated and distilled. The N was recovered by adsorp tion in dilute nitric acid contained in the receiver. To increase the ■ '' 41 ' stripping efficiency, and to minimize the amount of A trapped in the receiver, a stream of nitrogen was passed through the solution being distilled* By this method a yield of about 50 % of the total N activity present in a 250 ml sample of the reactor moderator was obtained within 15-25 minutes, the time available for separation ' 13 considering the radioactive decay. The yield of N was estimated by measuring the yield of ammonia based on the amount of nitrate — 10 — carrier added, and assuming these yields to be approximately equal, 13 Strictly, this assumption requires the N to be combined to 100%, in nitrate ions. However, the reliability of this method of determining the yield could be checked by comparing the results of several distilla tions of identical samples, carrying the distillations to different yields;

Yield of ammonia from nitrate, y$ 0*137 0* 210 0.460 Activity of in distillate, A; 274 381 933 Calculated total N13, A/y ; 2000 1810 2030

The identification of the radioactive isotopes investigated was made on the basis of their half-lives and of the energies of gamma rays associated with their radioactive decay, i, e. mainly the 0. 51 MeV annihilation radiation. Furthermore, the expected chemical properties of the isotopes were verified with respect to their be haviour towards anion and cation exchangers, and to their volati- . 13 lity from aqueous solution. In the case of N formed in H^O and D^O, the chemical identity was also confirmed by the successful separation procedure described above.

The measured half-lives of N'*'3 formed in H,0 and in D-O, 17 1 Q ^ ^ F formed in D^O, and F formed in D^O are given in Table 3 (cf. Table 2).

TABLE 3 Measured half-lives of N^3, F* 7 » and F^

Half-life Isotope F ormation minute s

N13 in H2 0 10. 0 ± 0. 2 N13 in D^O 10. 1 ±0,9 F17 inD^O 1. 10 ± 0.02 F18 in D20 115 ±9 - 11

Measurement of fast neutron flux The production rates in H^O and in D^O of N , F , and F , determined in this investigation, were expressed per unit of the equi valent U235-fission neutron flux as calculated from measurements

using the A1 threshold detector. The A1 foils were irradiated together with the water samples under the same Cd cover, i. e, at a distance from the water samples that was less than 1 cm. The average cross section of the threshold reaction Al2?(n, )Na2\ having an effective

threshold energy of 8 MeV, was taken to be 0. 60 mb per fission neutron^ 2^, The spectral deviation of the fast neutron flux in the centre

of the core of the reactor R1 from a flux of fission neutrons corre sponds to variations in the equivalent fission flux with respect to threshold detectors having effective threshold energies between 1, 7 (24) and 8 . 0 MeV, which are within ± 8 %

The aluminium foils were made from aluminium of 99*999% purity, obtained from AB Svenska Metallve rken, Finspang. The . . . 24 induced activity of Na was measured by gamma spectrometry after dissolution of the irradiated aluminium foils in a concentrated solution of NaOH. Determination of counting efficiency was made 24 by counting a solution of known Na activity obtained by dissolving sodium oxalate activated in a neutron flux which was simultaneously measured using the technique of paired Au-Cu foils, as described above.

1 3 The yield of N in D O, as determined by analysis of samp les of the reactor moderator, was related to a calculated average fast neutron flux in the moderator obtained through measurements 17 17 of the F content of the moderator assuming the yield of F per unit of neutron flux (A1 detector) in D_,0 previously measured in the ampoule irradiations#

Results and discussion In Table 4 are given the observed and the calculated yields of 13 17 18 the isotopes N ' , F , and F in react or-irradiated HÔ and DÔ, together with the standard errors in the observed yields relative to . 13 . the yield of N in DÔ. In addition to the errors mentioned there are errors which do not affect the relative yields, viz. errors in neutron flux determination (measurement ± 5%, spectrum shape — 1 2 —

± 8 %), in determination of counting efficiency of annihilation .j. gammas (± 5 %), and in the branching ratio with respect to (3 transition in the decay of Cu^ (assumed to be ± 10 %), adding up to a total of ± 1 5 %,

TABLE 4

13 17 18 Observed and calculated yields of N , F and F in reactor irradiation of H^O and D^O,

Isotope Medium 0 Assumed Yield: equivalent macroscopic fission neutron mode of reaction cross section (E^), cm a produc tion Observed d Previously Calculated published data

N13 0^(p,=) 1-35 " 10"^ . -9 H2° 1.35 10 J

N1? 0l6(p,=) 2.4 - 10-11(± 50%) (0.51 * io~n )g D2° source A 2.9 ’ 10_1,(± 13%)d

P17 0^(d,n) 3-5 * 10"7 (± 12%) 0.59 * 10^ d 2o 5.2 - icr7 ^ source A

F18 0^(p,n) 5-57 * 10-10(± 3%) 10.8. io^°f '7-0 * 10~10 H2° CO O 1 % 0* 7 (d,n) O 44 D2° . source A 00 0I7 (d,n) 5.5 ' 3^)^ D2° source B

Cross-section definedâs reactions/cm 0 s per neutron/cm * s b Estimated errors (standard deviation) in the yields relative to 17 . the yield of N "'in H-O are given between brackets. Estimated ^ 13 total error in the yield of N in HÔi ± 15%, c Density of HÔ and DÔs 1 and 1, 1 cm^/g, respectively. d Measured by analysis of the reactor moderator. e According to Amiel and Peisach^^. f According to Balcarczyk et al. ' ^, g Calculation based on measured yield of N* ^ in HÔ (h/DssO. 0038), 1 8 h Estimated error relative to yield of F in DÔ from source A, - 13 -

The agreement between observed and calculated yields is quite good regarding the formation of N* 3 and F* ^ in H?0 (Table 4), 17 r Regarding the formation of F in D^O, on the other hand, the mea sured yield is by a factor 6 ± 1 in excess of the calculated figure. This disagreement may indicate that the assumed excitation function, depicted in Fig, 1, which is partly based on refs. 7 and 9 partly ob tained by interpolation, gives a poor representation of the true exci tation function in the energy range of the knock-on deuterons.

1 3 The formation of N in D O was found to be appreciably larger ^ 13. than expected on the basis of the observed yield of N in H^O, assum ing the yield to be proportional to the proton content of the heavy water (cf. Table 1). This applies to the results of the experiments with ir radiation of purified D^O samples as well as to those obtained by analysi of the reactor D O moderator, as described above. The discrepancy, 2 —11 —1 corresponding to A E * (1,9 ±0.7)* 10 cm according to the for- n «12 _2 mer results and (2,4 ± 0. 4) • 10 cm according to the latter, may possibly be due to occurrence of the following nuclear reaction, not previously reported;

016(d, «n)N13 Q * -7443 ± 4 keV ^ (14)

Actually, the cross-section of reaction (14) required to account for the observed discrepancy is quite small. Thus, assuming the most probable macroscopic neutron cross-section of isotopic pure D?0 with respect to formation of N , i. e. (2. 4 ± 0, 4) • 10 cm , the corre sponding average cross-section^r)of reaction (4), defined according to

o $ dE s a f $ dE, (15) J r r r J r ^ ' E E is 8.2 ± 1.9 mb at deuteron energies above 8.4 MeV (threshold energy in the laboratory system).

The ratio between the yields of F in DpO samples obtained from sources B and A, differing with respect to the isotopic composi tion of the oxygen constituent ( Table l), was found to be 1,284 ± 0. 03 6.

1) Calculated from the Q values of the reactions 0^(d, n)F^ ^ and F1 (y, n)N^ ^ according to reference 31. — 14 —

. . 17 . . This is in fair agreement with the O concentration ratio 1 8 (l, 31 ± 0. 027), thus indicating that the formation of F is 17 18 mainly due to the reaction O (d, n)F „ The same conclusion, although limited to deuteron energies below 3, 7 MeV, was previously arrived at by Welles (loc, cit), •

17 18 The average cross-section of the reaction O (d, n)F at deuteron energies above 1 MeV, corresponding to the yields of F* 8 in D^O (sources A and B) is found to be 0„ 27 ± 0, 04 b.

Similar to ordinary neutron-induced threshold reactions, the yields of the nuclear reactions induced by knock-on protons and deuterons in react or-irradiated water can be mainly attri buted to certain energy ranges of the knock-on nuclei, i, e0 the energy ranges within which the product Z^(E) $ r(E) is large?

016(p, «)N13 ? 6. 5 < E < 11 MeV 016(d, n)F 17 ? 2. 5 < Ed <7 MeV 018 (p, n)F 18 g 3 < E < 7 MeV

The energy ranges indicated correspond to more than 90 % of the yields.

On the other hand it can be shown that the energy range (A E^ MeV) of fission neutrons mainly contributing to the knock-on nuclei of a certain energy (E"*") is equivalent to that corresponding to large values of the product cp(E^) Z^(E^)( 1 - E*/T|En), i„ e, at neutron energies between (E^/Tj) and (E / Tj) + 5 MeV.

Thus, even though the reactions considered here are not par ticularly suitable regarding the possible use of water as a threshold detector for neutron spectrum measurements, it may be noted that the O"*' 8(p, ^ )N"*' 3 reaction mainly responds to neutrons in the high energy range, about 7-16 MeV, while the formation of the - 15 - fluorine isotopes is a measure of the neutron flux in the range 3-12 MeV.

Conclusions Good agreement is obtained in comparing measured yields of the reactions O* ^(p, and 0^(p, n)F^ in reactor-irradiated water and predictions based on published cross-section data for these reactions* Regarding the reaction 0^^(drn)F"^ the observed rate exceeds the predicted rate by a factor of 6.

1 8 F is formed in react or-irradiated heavy water by the 17 18 reaction O (d, n)F * The reaction cross-section averaged over the flux spectrum of the knock-on deuterons above 1 MeV is 0, 27 i ± 0o 04 b, This reaction may be utilized for identification of samples of heavy water (e, g, differentiating between qualities obtained by electrolysis, distillation etc* ) by activation analysis, based on the determination of the ^ S O* ° isotopic concentration ratio*

1 3 Small amounts of N are formed in reactor-irradiated heavy water that cannot be attributed to the reaction (p, cc)N^ \ The occurrence of the reaction ^(d, n °=)N^ ^ is suggested as a possible explanation. The cross-section of this reaction required 1 3 to account for the extra yield of N is 8 ± 2 mb per knock-on deu te r on at energies above the estimated reaction threshold. (8,4 MeV), Regarding the possibility of utilizing the Q* (p, oc)N'* ^ reaction for determination of the H content of heavy water, the observed extra 1 3 formation of N represents a quite large correction at 1 % or less of H^O in the heavy water. At higher concentrations this application of neutron activation analysis is possible, the only requirement being that the fluorine isotopes be removed by passing the sample through a strong anion exchanger.

Another method of determining the HÔ content of D^C by radioactivation analysis would be to utilize the reaction O* ^(d, r)F^ ^ as suggested by Amiel et al, ^This method is not suitable, how ever, for the analysis of small concentrations of HÔ in DÔ (less than 20 % or depending on accuracy of the measurements). 16-

The formation of and F^ in H^O and of F* ^ in D^O may advantageously be utilized for fast neutron flux monitoring in reactors. The possibility of application for neutron spectrum determinations is restricted, however, because of poor energy resolution.

Acknowledgements The authors wish to thank Dr, Goran Blomqvist for carrying out the mass-spectrometric determinations and Dr, Ragnar Nilsson for valuable help with the neutron flux measurements. Thanks are also due to the R1 reactor staff for performing the irradiations and for ex cellent cooperation in many other respects, and to Mr. W Sas-Korczynski for operation of the multichannel analyser. We also want to thank Professor T Westermark for reading and commenting upon the manuscript. - 17

List of symbols E Energy of knock-on nuclei (protons or deuterons) during slowing-down, MeV.

Ek Initial energy of knock-on nuclei (protons or deuterons), MeV.

E Neutron energy, MeV, n p(Ek) Production of knock-on nuclei (protons or deuterons) at 3 initial energy E^, particles per cm ♦ s, per MeV, and per unit of neutron flux ($),

P(E) Production of knock-on nuclei (protons or deuterons) at 3 initial energies E^ E, particles per cm • s and per unit of neutron flux ($).

s Length of particle track, cm.

T| Defined by equation (9), Normalized flux of ^-fission neutrons, $(E^)s§, MeV *.

®r(E) Flux of knock-on nuclei (protons or deuterons), particles 2 per cm * s, per MeV of particle energy, and per unit of neutron flux ($).

Z^(E^, cos 9) Macroscopic cross-section with respect to elastic scattering of neutrons by protons or deuterons, cm per steradian.

£r(E) Macroscopic cross-section with respect to a proton or deuteron induced reaction, cm ^„ s Equivalent macroscopic neutron cross-section with respect n to a proton or deuteron induced nuclear reaction, i. e. defined by $ ’ £ a reaction rate per unit volume, ' n n 9 Neutron scattering angle in the centre-of-mass system. — 18 —

References 1. Henderson, W J and A C Whittier, Handbook of Shielding and Heat Production Calculations for the N R U Reactor 1955. Repr. 1957. (CRR-578) (AECL-403) p. 107-111.

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27. Carlson, C H, L Singer, D H Service, W D Armstrong. Preparation of Carrier Free Radiofluoride with a New Estimate of the Half Life of F Intern, J. Appl. Radia tion and Isotopes 4. (1958/59) 210-13,

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31. Balcarczyk, L and K H Kim and E Lanzel. 18 Messung der F -Aktivierung im Kern eines wassergektihlten Reaktors. Nukleonik 4 (1962) 105-108, Fig. 1

mb(a) mb (b) mb (c) 250 - - 600

200 500

-300

-200

Reaction cross-sections for a) ^(d, n)F* b) 0* 3(p, n)F* 3 (9,10)^

and c) 016 (p,a)N13 ^ll\ - Abscissae: Energy of incident particle, MeV, Ordinate: Reaction cross-section, mb. The single measurement of the O*^(d, n) reac tions cross-section according to ^ represented by a dot. Fig. 2

0 2 4 6 8 10 12 14 E, MeV

Calculated differential and integral fluxes of knock-on . 235 protons in I^O and deuterons in D^O per unit of U - fission neutron flux. - Abscissae! E» energy of protons or deuterons, MeV, Ordinate: $ (E), MeV * (broken n oo * curve) and I $ (E^dE '(solid curve). JE r dE/ds, in Differential 1.0 M eV/cm (I)

DgO. and

energy 1. -

Abscissae: 1

g/cm\ energy

loss,

loss respectively.)

MeV/cm. E,

particle

MeV protons

10 (Density

energy,

12 in

H^O

14 of

MeV. and HgO 16

and

Ordinate: deuterons

D^O;

MeV cm neutron-scattering neutron-scattering per sent Ordinate! deuterons Differential -

Abscissae!

neutron/cm calculations

p(E, in

source

D^O ^ E,

), *

the taking • initial per

by in s. density

number

the unit deuterons. -

into Curves energy

centre-of-mass

account of U

knock-on

235

knock-ons ' are of

-fission knock-ons,

calculated the

protons anisotropy

system. per neutron

cm MeV. assuming

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Dots •

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the *

Fig.

and

repre MeV isotropic

and

LIST OF PUBLISHED AE-REPORTS 135. A Monte Carlo method for the analysis of gamma radiation transport from distributed sources in laminated shields. By M. Leimdorfer. 1964. 1—90. (See the back cover earlier reports.) 28 p. Sw. cr. 8: —. 136. Ejection of uranium atoms from UO2 by fission fragments. By G. Nilsson. 91. The energy variation of the sensitivity of a polyethylene moderated BF; 1964. 38 p. Sw. cr. 8; —. proportional counter. By R. Fraki, M. Leimdorfer and S. Malmskog. 1962. 12. Sw. cr. 6:—. 137. Personnel neutron monitoring at AB Atomenergi. By S. Haqsqard and C-O. Widell. 1964. 11 p. Sw. cr. 8: —. 92. The backscattering of gamma radiation from plane concrete walls. By M. Leimdorfer. 1962. 20 p. Sw. cr. 6:—. 138. Radiation induced precipitation in iron. By B. Solly. 1964. 8 p. Sw. cr. 93. The backscattering of gamma radiation from spherical concrete walls. By M. Leimdorfer. 1962. 16 p. Sw. cr. 6:—. 139. Angular distributions of neutrons from (p, n)-reactions in some mirror nuclei. By L. G. Stromberg, T. Wiedling and B. Holmqvist. 1964. 28 p. 94. Multiple scattering of gamma radiation in a spherical concrete wall Sw. cr. 8:. room. By M. Leimdorfer. 1962. 18 p. Sw. cr. 6:—. 140. An extended Greuling-Goertzel approximation with a Pn-approximation 95. The paramagnetism of Mn dissolved in n and fi brasses. By H. P. Myers and R. Westin. 1962. 13 p. Sw. cr. 6:—. “ in the angular dependence. By R. Hakansson. 1964. 21 p. Sw. cr. 8: —. 96. Isomorphic substitutions of calcium by strontium in calcium hydroxy 141. Heat transfer and pressure drop with rough surfaces, a literature survey. apatite. By H. Christensen. 1962. 9 p. Sw. cr. 6:—. By A. Bhattachayya. 1964. 78 p. Sw. cr. 8: —. 142. Radiolysis of aqueous benzene solutions. By H. Christensen. 1964. 40 p. 97. A fast time-to-pulse height converter. By O. Aspelund. 1962. 21 p. Sw. cr. Sw. cr. 8: —. 6:—. 143. Cross section measurements for some elements suited as thermal spect 98. Neutron streaming in D2O pipes. By J. Braun and K. Randen. 1962 rum indicators: Cd, Sm, Gd and Lu. By E. Sokalowski, H. Pekarek and 41 p. Sw. cr. 6:— E. Jonsson. 1964. 27 p. Sw. cr. 8: —. 99. The effective resonance integral of thorium oxide rads. By J. Weitman. 144. A direction sensitive fast neutron monitor. By B. Antolkovic, B. Holm 1962. 41 p. Sw. cr. 6s—. qvist and T. Wiedling. 1964. 14 p. Sw. cr. 8: —. 100. Measurements of burnout conditions for flow of boiling water in vertical 145. A user's manual for the NRN shield design method. By L. Hjarne. 1964. annuli. By K. M. Becker and G. Hernborg. 1962. 41 p. Sw. cr. 6;—. 107 p. Sw. cr. 10:—. 101. Solid angle computations for a circular radiator and a circular detector. 146. Concentration of 24 trace elements in human heart tissue determined By J. Konijn ona B. Tollander. 1963. 6 p. Sw. cr. 8: —. by neutron activation analysis. By P.O. Wester. 1964. 33 p. Sw. cr. 8: —. 102. A selective neutron detector in the keV region utilizing the 19F(n, y )12°F 147. Report on the personnel Dosimetry at AB Atomenergi during 1963. By reaction. By J. Konijn. 1963. 21 p. Sw. cr. 8: —. ' K.-A. Edvardsson and 5. Hagsgdrd. 1964. 16 p. Sw. cr. 8: —. 103. Anion-exchange studies of radioactive trace elements in sulphuric acid 148. A calculation of the angular moments of the kernel for a monatomic gas solutions. By K. Samsahl. 1963. 12 p. Sw. cr. 8: —. scatterer. By R. Hdkansson. 1964. 16 p. Sw. cr. 8: —. 104. Problems in pressure vessel design and manufacture. By O. Hetlstrom 149. An anion-exchange method for the separation of P-32 activity in neu* and R. Nilson. 1963. 44 p. Sw. cr. 8: —. tron-irradited biological material. By K. Samsahl. 1964. 10 p. Sw. cr. 8: —. 105. Flame photometric determination of lithium contents down to 10-3 4ppm 5 * * * 150. Inelastic neutron scattering crass sections of Cu6: and Cu6$ in the energy in water samples. By G. Jonsson. 1963. 9 p. Sw. cr. 8: —. region 0.7 to 1.4 MeV. By B. Holmqvist and T. Wiedling. 1964. 30 p. 106. Measurements of void fractions for flow of boiling heavy water in a Sw. cr. 8: —. vertical round duct. By S. Z. Rouhani and K. M. Becker. 1963. 2nd rev. 151. Determination of magnesium in needle biopsy samples of muscle tissue ed. 32 p. Sw. cr. 8: —. by means of neutron activation analysis. By D. Brune and H. E. Sjoberg. 107. Measurements of convective heat transfer from a horizontal cylinder 1964. 8 p. Sw. cr. 8: —. rotating in a pool of water. K. M. Becker. 1963. 20 p. Sw. cr. 8: —. 152. Absolute El transition probabilities In the dofermed nuclei Yb177 and 108. Two-group analysis of xenon stability in slab geometry by modal expan Hf179 . By Sven G. Malmskog. 1964 . 21 p. Sw. cr. 8: —. sion. O. Norinder. 1963. 50 p. Sw. cr. 8: —. 153. Measurements of burnout conditions for flow of boiling water in vertical 109. The properties of CaSOjMn thermoluminescence dosimeters. B. Bjarn- 3-rod and 7-rod clusters. By K. M. Becker, G. Hernborg and J. E. Flinta. gard. 1963. 27 p. Sw. cr. 8: —. 1964 . 54 p. Sw. cr. 8: —. 110. Semtanalytical and seminumerical calculations of optimum material 154. Integral parameters af the thermal neutron scattering law. By S. N. distributions. By C. I. G. Andersson. 1963. 26 p. Sw. cr. 8: —. Purohit. 1964. 48 p. Sw. cr. 8:- —. 111. The paramagnetism of small amounts of Mn dissolved in Cu-AI and 155. Tests of neutron spectrum calculations with the help af foil measurments in a D2O and in an HzO-moderated reactor and in reactor shields of Cu-Ge alloys. By H. P. Myers and R. Westin. 1963. 7 p. Sw. cr. 8: —. concrete and iron. By R. Nilsson and E. Aalto. 1964. 23 p. Sw. cr. 8: —. 112. Determination of the absolute disintegration rate of Cs137 -sources by the 156. Hydrodynamic instability and dynamic burnout in natural circulation tracer method. S. Hellstrom and D. Brune. 1963. 17 p. Sw. cr. 8:— . two-phase flow. An experimental and theoretical study. By K. M. Beck 113. An analysis of burnout conditions for flow of boiling water in vertical er, S. Jahnberg, 1. Haga, P. T. Hansson and R. P. Mathisten. 1964. 41 p. round ducts. By K. M. Becker and P. Persson. 1963. 28 p. Sw. cr 8:- —. Sw. cr. 8: —. 114. Measurements of burnout conditions for flow of bailing wafer in vertical 157. Measurements of neutron and gamma attenuation in massive laminated round ducts (Part 2). By K. M. Becker, et al. 1963. 29 p. Sw. cr. 8: —. shields of concrete and a study of the accuracy of some methods of 115. Cross section measurements of the 58 Ni(n, p)s8 Ca and 39Si(n,a m)2&Mg reac calculation. By E. Aalto and R. Nilsson. 1964. 110 p. Sw. cr. 10:—. tions in the energy range 22. to 3.8 MeV. By J. Konijn and A. Lauber 158. A study of the angular distributions of neutrons from the Be9 (p,n) B9 1963. 30 p. Sw. cr. 8: —. reaction at low proton energies. By B. Antolkovic ’, B. Holmqvist and T. Wiedling. 1964. 19 p. Sw. cr. 8:— . 116. Calculations of total and differential solid angles for a proton recoil solid state defector. By J. Konijn, A. Lauber and B. Tollander, 1963. 31 p. 159. A simple apparatus for fast ion exchange separations. By K. Samsahl. Sw. cr. 8: —. 1964. 15 p. Sw. cr. 8: —. 117. Neutron cross sections for aluminium. By L. Forsberg. 1963. 32 p. 160. Measurements of the Fe54 (n, p) Mn 54 reaction cross section in the neutron Sw. cr. 8: —. energy range 2.3—3.8 MeV. By A. Lauber and S. Malmskog. 1964. 13 p. Sw. cr. 8: —. 118. Measurements of small exposures of gamma radiation with CaSO^Mn radiothermoluminescence. By B. Bjarngard. 1963. 18 p. Sw. cr. 8: —. 161. Comparisons of measured and calculated neutron fluxes in laminated 119. Measurement of gamma radioactivity in a group of control subjects from Iron and heavy water. By E. Aalto. 1964. 15 p. Sw. cr. 8: —. the Stockholm area during 1959—1963. By I. D. Andersson, I. Nilsson 162. A needle-type p-i-n junction semiconductor detector for in-vivo measure and Eckerstig. 1963. 19 p, Sw. cr. 8: —. ment of beta tracer activity. By A. Lauber and B. Rosencrantz. 1964.12 p. 120. The thermox process. By O. Tjolldin. 1963. 38 p. Sw. cr. 8: —. Sw. cr. 8: —. 163. Flame spectro photometric determination of strontium in water and 121. The transistor as low level switch. By A. Lyden. 1963. 47 p. Sw. cr. 8: —. biological material. By G. Jonsson. 1964. 12 p. Sw. cr. 8; —. 122. The planning of a small pilot plant for development work on aqueous 164. The solution of a velocity-dependent slowing-dawn problem using reprocessing of nuclear fuels. By T. U. Sjoborg, E. Haeffner and Hu It- case's eigenfunction expansion. By A. Claesson. 1964. 16 p. Sw. cr. 8: —. gren. 1963. 20 p. Sw. cr. 8: —*. 165. Measurements of the effects of spacers on the burnout conditions for 123. The neutron spectrum in a uranium tube. By E. Johansson, E. Jonsson, flaw af boiling water in a vertical annulus and a vertical 7 —rod cluster. M. Lindberg and J. Mednis. 1963. 36 p. Sw. cr. 8: —. By. K. M. Becker end G. Hernberg. 1964. 15 p. Sw. cr. 8: —. 124. Simultaneous determination of 30 trace elements in cancerous and non- 166. The transmission of thermal and fast neutrons in air filled annular ducts cancerous human tissue samples with gamma-ray spectrometry. K, Sam through slabs of iron and heavy water. By J. Nilsson and R. Sandlin. sahl, D. Brune and P. O. Wester. 1963. 23 p. Sw. cr. 8: —. 1964. 33 p. Sw. cr. 8: —. 125. Measurement of the slowing-down and thermalization time of neutrons 167. The radio-thermoluminescense of CaSOj: Sm and its use in dosimetry. in water. By E. Moller and N. G. Sjostrand. 1963. 42 p. Sw. cr. 8: —. By B. Bjarngard. 1964. 31 p. Sw. cr. 8: —. 126. Report on the personnel dosimetry at AB Atomenerg? during 1962. By 168. A fast radiochemical method for the determination of some essential K-A. Edvardsson and 5. Hagsgdrd. 1963. 12 p. Sw. cr. 8: —. trace elements in biology and medicine. By K. Samsahl. 1964. 12 p. Sw. 127. A gas target with a tritium gas handling system. By B. Holmqvist and cr. 8: —. T. Wiedling. 1963. 12 p. Sw. cr. 8: —. 169. Concentration of 17 elements in subcellular fractions of beef heart, tissue determined by neutron activation analysis. By P. O. Wester. 1964. 29 p. 128. Optimization in activation analysis by means of epithermal neutrons. Determination of molybdenum in steel. By D. Brune and K. Jirlow. 1963. Sw. cr. 8: —. 11 p. Sw. cr. 8s —. 170. Formation of nitrogen-13, fluorine-17, and fluorine-18 in reactor-irradiated HjO and D2O ana applications to activation analysis and fast neutron 129. The Pi-approximation for the distribution of neutrons from a pulsed flux monitoring. By L. Hammer and S. Forsen. 1964. 25 p. Sw. cr. 8: —. source in hydrogen. By A. Claessan. 1963. 18 p. Sw, cr. 8: —. Forteckning over publicerade AES-ropporter 130. Dislocation arrangements in deformed and neutron irradiated zirconium and zircaloy-2. By R. B. Roy. 1963 18 p. Sw. cr. 8s —. 1. Analys medelst gamma-spektrometri. Av D. Brune. 1961. 10 s. Kr 6:—. 131. Measurements of hydrodynamic instabilities, flow oscillations ond bur 2. Bestralningsforandringar och neutrorotmosfar i reaktorlrycktankar — nout in a natural circulation loop. By K. M. Becker, R. P. Mathisen, O. nagra synpunkter. Av M. Grounes. 1962. 33 s. Kr 6:—. Eklind and B. Norman. 1964. 21 p. Sw. cr. 8: —. 3 Studium av strackgronsen i mjukt stdl. Av G. Ostberg och R. Attermo. 132. A neutron rem counter. By I. O. Andersson and J. Braun. 1964. 14 p. 1963. 17 s. Kr 6—. Sw. cr. 8: —. 4. Teknisk upphandling inom reaktoromrddet. Av Erik Jonson. 1963. 64 s. 133. Studies af water by scattering of slow neutrons. By K. Skald, E. Pilcher Kr 8 •— and K. E. Larssan. 1964. 17 p. Sw. cr. 8: —. 5. Agesta Kraftvarmeverk. Sammanstdllning av tekniska data, beskrivningar m. m. for reaktordelen. Av B. Lilliehodk. 1964. 336 s. Kr. 15:—. 134. The amounts of As, Au, Br, Cu, Fe, Mo, Se, and Zn in normal and urae mic human whole blood. A comparison by means of neutron activation Additional copies available at the library of AB Atomenergi, Studsvik, analysis. By D. Brune, K. Samsahl and P. O. Wester. 1964. 10 p. Sw. cr. Nykoping, Sweden. Transparent microcards of the reports are obtainable through the international Documentation Center, Tumba, Sweden.

EOS-tryckerierna, Stockholm 1965