Nitrogen fixation by the action of ionizing radiations

•

a!mo.

Nitrogen Fixation by the Action of Ionizing Radiations* (Receire'l 16 June 1958) CoN-rx,~tvom.ut~" developments in nuclear engineering raise the question of developing suitable methods of using the power released in nuclear reactions. The direct oxidation of nitrogen by the action of radiations and high energy particles might be visualized as one of these methods. The present investigation is concerned with the study of the main rules governing the oxidation of nitrogen arising from the effect of electron collisions and of y-rays. A 2 kV electron-beam tube, a 200 kV linear accclcrator and a 1400 c Co s° artificial source were used to produce the radiation. Measurements were made of the rate of formation of nitrogen oxides as a function of the duration of the irradiation, the composition of the gaseous nlixture, the pressure, anti the temperature, attd led to the following equation fi~r the reaction rate:

V = Kl'.,q . Pot

(1)

where V is the reaction rate, K a cotraant, ,.~PNt atKI Po, the nitrogen anti oxygen partial pressttres. Fcluation (!) holcL,~good fi)r pressures of ,.--0.1 and ~ T t ; 0 Into tlg. The activation energy of the reaction is 7.5 ~ 1 kcai/mole at low l)ressurcs a n d about 2 kcal/mole at atmospheric pressure. :'kS has been established ct,~} the reaction begins at an electron energy of 16.2 eV which correslmnds to the potential at which molecular nitrogen ions N~ + appear. An appreciable increase of tile reaction rate is observed at an electron energy of 24 cV. This may be due to the formation of atomic ious a n d nitrogen atoms in tile primary process. T h e reaction-rate constant is proportional to the value of tile nitrogen ionization function. The decisive role played by the ionization of nitrogen is also confirmed by experiments in which positive ions were removed from the reaction zone by a probe. Under these conditions the reaction rate decreases. These results, a n d reaction kinetic data, suggest the following mechanism of oxide formation: The main processes associated with the formation

of chemically active particles are: (1) N., . . . . . N~" + e(N~* + e, N~"+ + 2e). (2) N., . . . . N *+N +e(N ++N*+e,N ~*-:+ N* + e). (3) N., . . . . . N~ ~ N* + N(N* + N*). (4) N~" + e ~ N* + N(N~" + [e] 7* N* + N). T h e most important reactions between active particles are: (5) N~" + O 2 ~ NO ~ + NO.

(6) N;* + O 2 ~ NO~ + N. (7) N~" + O 2 ~ N20+ + O.

(8) N + O . , ~ NO + O . (9) N + 02 + M--- NO, + M. (10) N~ + O + M ~ N : O + M. Processes leading to the loss of active particles are: (11) (12) (13) (I-t) (15)

N~" + O f ~ N.: + O.,. N~" + O., ~ N,, + ()~. N~ + NO--- N., + NO% N + N -+ M ~ N,, -I. M. N +- N O , - - N., + O.,.

(it;) N~ +

tel-- N...

O u r resuhs show that fi~r electron collisions (up to 100 eV) tilt: probability of process (l) is about IO times greater th:m that of process (2). The principal reactions between active particles are the processes (5), (6), (8), (9). The interaction and dissociation rcactiom of nitrogen oxides are not considered here. T h e reaction rate constant of equation (I) may be expresscd thus:

Ko • :,.s [ rs l l ~s ~ s

x =

rs00 I'A'maz

~

Jo

(2)

where K 0 is the proportionality factor, J the radiation intensity, E~r the ionization potential of nitrogen molecules, K t the effective constant fi)r the production of cheutically active particles, R the gas constant, T the temperature, ~t the ion recombination cozfticient, 2~I the molecular weight of hydrogen, P the pressure, q(E) the ionization function of nitrogen, F(E) the radiation energy distribution function, Emax the maxin m m energy of the ionizing particles. Using equations (1) and (2), the relation between the oxidation process a n d the conditions and nature of the radiation can be analysed.

* Translated by L. C. RONSONfrom Atomnava Energiya 3, No. 10, 350 (1957). 67

68

Technical notes

E q u a t i o n (2) was verified at the pressures 0-1 a n d 760 m m Hg, at electron energies of 20-400 e V anti _'2.10~ eV, a n d at temperatures from 0 to 200°C. T h e reaction rate c o n s t a n t is i n d e p e n d e n t of pressure if ~t , ~ T I P (which applies at fairly low temperatures a n d pressures up to ~ 1 5 atm). T a b l e 1 lists some experimentally measured nitrogen dioxide yields for various r a d i a t i o n energies a n d o t h e r conditions. D a t a for electrical discharges are also given for comparison. T a b l e 1 shows that the greatest yields are o b t a i n e d at fairly high temperatures a n d low pressures. This is connected with the competition between the elem e n t a r y reactions which lead to the formation of nitrogen oxides, processes leading to the annihilation of active particles, a n d dissociation reactions of the oxides. T h e e q u i l i b r i u m NO., c o n c e n t r a t i o n for air irradiated with Fast electrons (at 2 0 - 3 0 ' C ) is 5.5-6 per cent. This concentration is considerably greater t h a n the eqttilibrittm concentration in a thermal reaction. A change to the liquid phase anti to low tentperature does not appreciably affect the reaction yield. ~,Vhcn irradiating nitrogen dissolved in water, or nitrogen above water, the oxide yields per unit of al,s,,rbcd cncrt,,y are rclativt, ly small. Sintilar values were o b t a i n e d by PROSKURNIN et al. (31 attd [[AISSINSKY el al., I'll in oxitlizing ititrogen with the radiolysis products of water in atlucous alkaline sohttions.

It is clear from the table that a c h a n g e from fast electrons to :.,-rays in experiments a t atmospheric pressure does not a p p r e c i a b l y affect the reaction yields. Appro.',dmately similar yields h a v e been o b t a i n e d using a nuclear reactor. V~rRIGIIT *l al. (5~ irradiated air at atmospheric pressure above water a n d o b t a i n e d a yield of approximately one fixed a t o m of nitrogen per 100 eV. PRI.',tAK a n d FucHs (~ i r r a d i a t e d moist air at 1 a r m and. 40 :C, a n d o b t a i n e d a yield of 3 atoms per 100 eV. HARTECK a n d DONDES~7) o b t a i n e d 2.5 NO., molecnles p e r 100 eV at 140~C a n d 1 a t m a n d 5 NO,, molecules p e r 100 e V at 175°C a n d 25 atm. T h e reaction yield can obviously be increased by neutralization of the negative ions ¢sl or by sensitization of the nitrogen a t o m a n d ion forming processes. ;ks yet there is no experimental evidence of yields greater t h a n 6 NO., molecttles per 100 eV. Such a yield corresponds to a c o n s u m p t i o n of 7000 kl, Vh/ton of a n h y d r o u s nitric acid, a b o u t the same as in the anamonia process. T h e radiation oxidation of nitrogen c a n only become economically i m p o r t a n t if use can be m a d e of energy from mtclcar fission, in a nuclear reactor. A COml,licating factor is that ,neutron captttre by nitrogen, i.e. the reaction 7Nt't -~ n ~ 6C I'l -¢- p, has a n adverse effect on the reactor opcrating conditions (tht r effective neutron cal)tttre cross-section of nitrogen is n N ~ 1'5 × It) 21 c n l :I, t h a t o f o x y g e n fit, ~ 2.8 × lO T M cm"). ht l,rincil,lc nitrogen, which ahsorbs

"I'AtlI.E [

Radiation

"ryl)e of r:uliation

Energy (keV)

Ct mlpOSit il)tl of mixture oxygen : nitrogen

7-rad tat itm 7-radiation anti fast electrons

< 133(I 200-1330

1:4 2:1"

Slow electrons Fast electrons

0-02-0.40 240

1:4 1:1

Fast electrons Fast electrons Fast electrons Fast electrons Fast electrons Silent discharge High frequency discharge High frequency dischart~e

200 200 200 200 200 <6 <4

1:11" I :4"I" 0:11" 1:4 1:4 2:1 2:1

<15

1:1

* In saturated aqueous solution,

t Above water.

l'ressurt:

"l'trnl[)erat tire

(ram tlg)

('c:)

l~.cacti,,n yi~.ld, NO s mole per 100 eV

760 71;0 liquid phase <0.1 liquid phase 760 760 760 760 760

15-25 ! 5-25

1.2 0. I-0"3

~5

5--6

-- 183

1 -- 2

1 I

40-50 40-50 40-50 15-25 ,'~200 <50 < 100

2 1.3 0.2 1.5 3 + 3"5 2.5 4.0

760

<200

1.7

Technical notes excess neutrons, can be used to control a nuclear reactor. This negative effect can be to some extent compensated by partly replacing the nitrogen by other neutron absorbers. The proton produced in the above nuclear reaction has an energy of ,--600 keV. This can give about lift ionizing events and, consequently, about I04 nitrogen dioxide molecules for each thermal neutron absorbed by the nitrogen. Neutron capture can be reduced by increasing the proportion ofoxygen in the gaseous mixture. S. YA. PSHEZHE'rSKII ~[. T. DMITRIEV

Academy of Sdences of the U.S.S.R. 3loscow, U.S.S.R. References

1. PSHEZHETSKIi S. YA., Academy of Sciences AIoscow

Conference on Peaceful Uses of Nuclear Science (Chem. Sci. Dept.) p. 64 Izd. Akad. Nauk SSSR Moscow (1955). 2. PSHEZHETSKliS. YA. and DMtTRtEV M. T., Dokl. Akad. Nauk SSSR 103, No. 4, 647 (1955). 3. PROSKURNIN~[. A., Ogrmtov V. D. and BARELKO

E. V., Academy of Sciences /tloscow Conference on Peaceful Uses of Nuclear Science (Chem. Sd. Dept.) p. 4 ! Izd. Akad. Nauk SSSR, Moscow (1955). 4. Vr.a.~mt. C., Co'rr1N M. anti tlAlsstNs~v M. .I. Chem. Phys. 49, 437 (1952). 5. ~VRIGI|TJ., LINACRF.J. K., M,,,as~I W. R. anti BATEST. H., Conferenceon the Peaceful Uses of Atomic Energy, New York. 1956. Paper 445, p. 560. 6. PaIMAKW. and Fuc.us L. I[., Nucleonics 13 (3), 38 (1955). 7. HAwrrcg P. and Dom~:s S. Nucleonics 14, (7), 22 0956). 8. DMITRIEV M. T. and I'S,mZHETSKtIS. YA. Papers

given at the First All-Soviet Cbnference on Radiation Chemistry p. 5 Izd. Akad. Nauk SSSR, Moscow (1957).

A Simple Apparatus for the Production of Radiatlon-induced Mutations in Seed*

(Received 29 September 1958; in revisedform 21 October 1958) Introduction PLANT breeders are essentially concerned with exploiting the variability which occurs in our cultivated varieties of crop plants or in their closely related varieties and species. They do this either by a process of selection from existing populations or by * Read before the Congress of the South African Genetic Society held in Pretoria from 22 to 24July, 1958. 5

69

recombining and reshuffling characters by hybridization and selecting from the progeny. Among other things radiation provides an increased source of variability in plants exposed to it, and work on radiation induced mutation breeding has been going on in various institutes for several years. Promising varieties of cereals, peas, soyabean etc. with increased resistance to disease or higher yielding ability have been produced, of which some have already been released on the market. The rate of occurrence of the so-called mutations can be increased from the spontaneous frequency of 1 in 105-10 ¢ to as high as 1 in 30. However, only one useful mutant may be produced for every 700-800 inferior ones. Hence a large number of plants must be handled in any program of mutation breeding. Up to the present time, seeds have been the part of the plants to be most frequently treated. They are easy to handle, are not easily damaged and large numbers can be irradiated at once. The value of pollen irradiation or even whole plant irradiations is still in doubt. There are many different types of ionizing radiation which can be used to induce mutations in plants viz. alpha-, beta-, gamma- and X-rays, neutrons and various charged particlcs. The difference in types of mutations obtained with the various radiations is remarkably small considering their totally different natures. Practical considerations The l)epartmcnt of Agriculture at the University of l'retorla has recently initiated a groundnut breeding program in which specilically immunity is lacing sought against the dreaded Rosette virus disease which is ravaging the groundnut crops in South Africa. Suitable facilities exist in several overseas institutes for such irradiations. However, the cost of transporting such seed is considerable and there are serious difficulties connected with the import and export of agricultural products. Hence it was decided to execute these irradiations at the local National Physical Research Laboratory. Quantities of seed of the order of 15 Ib each, had to be irradiated to give a suitable number of individual seeds. The doses decided on by the geneticist ranged from 8000 to 21,000 tad. Only neutrons, gamma- and X-rays are sufficiently penetrating to make the uniform irradiations of this quantity of seed feasible. In South Africa the only apparatus which can possibly deliver a big enough neutron flux, is the G.S.I.R. cyclotron. However, considerable practical problems are presented in this case as regards dosimetry and uniform, reproducible