Radioactivity induced by neutrons

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RADIOACTIVITY

INDUCED BY NEUTRONS

PROEFSCHRIFT TER VERKRIJGING

VAN DEN GRAAD VAN DOCTOR IN

DE TECHNISCHE WETENSCHAP AAN

DE TECHNISCHE HOOGESCHOOL TE

DELFT, OP GEZAG VAN DEN RECTOR

MAGNIFICUS, Dr. Ir. C. B. BIEZENO,

HOOGLEERAAR IN DE AFDEELING

DER WERKTUIGBOUWKUNDE EN

SCHEEPSBOUWKUNDE, VOOR EEN

COMMISSIE UIT DEN SENAAT TE

VERDEDIGEN OP WOENSDAG DEN

15den JUNI 1938, DES NAMIDDAGS

TE 4 UUR, DOOR

FRANS ADRIAAN HEIJN

NATUURKUNDIG INGENIEUR GEBOREN TE DELFT

101X Lj^-f^

^ '

Dit proefschrift is goedgekeurd door den promotor, Prof Dr. H. B. Dorgelo.

Contents.

Chapter I. Introduction and survey.

^^^e-§ 1. Some historical notes; limitation of the subject 11 § 2. Theoretical considerations on the mechanism of nuclear reactions . . . . 12

§ 3. Symbols and notation used 18 § 4. Survey of the types of reaction initiated by neutron bombardment . . . . 20

§ 5. Methods of identification of a special type of reaction 26

Chapter II. Apparatus.

§ 6. General remarks 29 § 7. The first tube (250 kV) 29 § 8. T h e second tube (600 kV) 34 § 9. High tension apparatus 37 § 10. Auxiliary apparatus 38

Chapter III. Results of experiments on the activation of various elements.

§ 11. Lithium, Beryllium, Boron 40

§ 12. Nitrogen 42 § 13. Oxygen 43 § 14. Sulphur 44 § 15. Chlorine 45 § 16. Chromium 46 § 17. Cobalt 47 § 18. Nickel 49 § 19. Copper 50 § 20. Zinc 54 § 21. Selenium 59 § 22. Bromine 60 § 23. Molybdenum 61 § 24. Rhodium 62 § 25. Silver 64 § 26. Cadmium 65 § 27. Antimony 67 § 28. Tellurium 67 §29. Mercury 68 § 30. Thallium 69

Chapter IV. Survey of the reactions leading to radioactive nuclei.

§ 31. Introduction 70 § 3 2 . Tables 72

Chapter I.

Introduction and survey.

§ 1. Some historical notes; limitation of the subject.

According to present views all atomic nuclei are composed of neutrons and protons. T h e number of protons in a nucleus deter-mines the charge, that is the atomic number Z of the nucleus. W i t h a given number Z of protons different numbers N of neutrons may form nuclei, which are called the isotopes of the nucleus with atomic number Z. It has been known for a long time that some nuclei are unstable and disintegrate with the emission of alpha-, beta- or gamma-rays into stable nuclei. This phenomenon is called radioactivity and was discovered in uranium by BECQUEREL

in 1896 and at about the same time in thorium by SCHMIDT and

CURIE. In the years that followed many new radioactive elements were discovered. Besides the three families of radioactive elements, viz. uranium, thorium and actinium, only potassium, rubidium and samarium proved to be radioactive, however. In 1919 it was discovered by RUTHERFORD that light stable nuclei can be trans-formed into other stable nuclei by alpha-particle bombardment. T h e number of these so-called nuclear transmutations or nuclear reactions has increased rapidly of late years. Besides alpha-particles other light particles are now also used as projectiles, e.g. protons, neutrons and deuterons. Such a nuclear transformation may be described by the following scheme: A + a ^ C - ^ B + b . A is the initial nucleus bombarded by the particles a. W h e n such a particle a penetrates into the nucleus A an ,,intermediate" or ,.compound" nucleus C is formed, consisting of the original nucleus A plus the particle a. This system is always in an excited state and disin-tegrates with the emission of light particles b or gamma-rays into the nucleus B which may be stable or unstable. In the latter case, called induced radioactivity, B disintegrates with the emission of one or more negative or positive electrons or by capture of a

K-electron^".'") into a stable nucleus (the emission of alpha-particles has not been observed up till now). CURIE and JOLIOT ^^) were the first to produce artificial radioactive nuclei in this way, using alpha-particles as projectiles (in 1934). Shortly after, FERMI *^> **) succeeded in producing artificial radioactive nuclei by neutron-bombardment. Nowadays, it is possible to induce radioactivity in almost all the elements, using projectiles of various kinds.

Particularly the reactions that give rise to unstable nuclei will be dealt with in this thesis and of these reactions specially those in which the incident particle is a neutron.

§ 2. Theoretical considerations on the mechanism of nuclear reactions. The mechanism of this type of reaction has been elucidated by

BOHR *•), who pointed out that in most cases a nuclear process cannot be treated as a one-body problem, as had previously been done, but that a typical many-body problem is concerned here. It is insufficient to consider the incident particle a as moving in the field of the bombarded nucleus A and eventually transferring its energy to a particle b of this nucleus which may be raised to a sufficiently high level to escape from the nucleus, but owing to the strong interaction between all the particles of the nucleus A the energy of a (composed of kinetic and binding energy) is dis-tributed among all these particles, or at least among a great part of them. This means that in the intermediate nucleus C all the particles or a great part of them have a small excess-energy, well below their dissociation energy, and in order to enable a particle

b to escape from C it is necessary that by some fluctuation a

suffi-cient amount of energy is concentrated on b, which may only occur after a comparatively long time. Hence, the eventual disintegration must be considered as a separate event, independent of the first stage of the collision process, viz. the capture of a. In this second stage a free competition exists between all the various disinte- ;. grations and radiation processes of the compound system *). i So it is e.g. known that the nucleus Al 28, formed by

neutron-capture, may disintegrate with the emission of alpha-particles,

*) The importance of the intermediate nucleus has long before been emphasized by

protons or beta-particles. Particularly when A is a heavy nucleus, that is when the number of particles in the nucleus is large, the energy concentration grows more and more improbable, which causes the probability of gamma-ray emission instead of particle emission to increase. T h e above considerations still hold good for nuclei that are excited by much more than the dissociation energy (about 8 MV) of a particle in the nucleus. T h e only difference is that the disintegration of C into a free particle and the residual nucleus becomes more probable. Furthermore it is obvious from this picture that the number of excited states of heavy nuclei (with high excitation energy) is very large, as the excess-energy can be distributed in numerous ways among all the particles in the nucleus. T h i s provides an explanation of the observed resonance phenomena with slow neutrons. It has been pointed out by BOHR

that the number of these states (the level density) can be calculated on the basis of statistical considerations.

Starting from these considerations BETHE ^^) and LANDAU ^") have

given a closer theoretical discussion of the level distribution. T h e level scheme of nuclei follows from the assumption that the excita-tion energy is shared by the nuclear particles in a way corres-ponding to thermal equilibrium. T h e process of emission of par-ticles by highly excited nuclei has been investigated more closely by FRENKEL *^) and BOHR ^) on the basis of the so-called ,,evapora-tion-model". W E I S S K O P F ^**) has given a more detailed treatment of this model, also using statistical methods.

W e shall give here a short survey of the results (for heavy nuclei) obtained by BETHE and WEISSKOPF, as these results are of special interest for the correct understanding of the reactions to be dis-cussed below.

It is shown in ordinary statistics that when a system can be in a continuous series of states (the number of states with energy between E and E -|- dE being Q ( E ) d E , the probability of finding it in a state with energy between E and E -|- d E is given by:

V — E (1) 77(E)dE = e(E)e "" dE, when the system is in thermal equilibrium *).

•) This equilibrium is not always achieved in nuclear reactions. See BETHE ^').

Here T is a parameter and ip a function of r, while 00

(2) • [ 77(E)dE = 1.

(3) e "" = foiEje "" dE = Z.

The average energy of the system is given by: E

(4) U

/ E . ( E ) e ^ d E

f

e(E)e ^ dE

According to this formula T is that temperature (measured in terms of energy) for which the average energy of the system is U. We use this formula to define the temperature r of the nucleus. As the main contribution to the integral in (3) will come from energies near U, we write instead of (3):

(4a) e "" = e(U)e "^ A(U) , or;

U — y

T

(5) e(U) =

A(U)

Introducing the abbreviation: U —V T we get: e = S(U) S(U) (6) e(U) = ^(U)

S, V and r correspond in ordinary thermodynamics to the entropy divided by k, the free energy and to kT respectively. In formula (6) we have to determine S(U) and /l(U). To this end we rewrite (4), using (3): (7) which gives (7a) U 1 dU dT

dZ

dV

(8)

and using (7a)

(8a) Hence: S we S

u

fd\

J dr^

di

" J

d S _ J . d U

dU T

Thus when U is known as a function of T, S is also known. From (6) and (3) follows:

- ^ ; s ( E ) - ^

(10) e "" = / ^ ^,_ "" dE. o A(E)

We replace A(E) by A(U), as again the main contribution to the integral (10) will come from energies E near U and as A(E) will only

vary slowly with E. Thus using (4a) and (6) to evaluate e ''^ we get:

r S(E) — S(U) + (U — E ) / T

(11) A(U) = / e " dE.

With a TAYLOR expansion for S(E) — S(U) and using (9) we get for the exponent:

and integrating (11) we finally find:

which gives A(U) when U is known as a function of r.

For the average spacing of the nuclear levels we thus obtain:

] _ „.,,, — S(U) _/27rdU\V2 —S(U)

e(U)

^''^ D(U)^--^A(U)e ' - i - ^ j ' - e

a) assuming the interaction between the particles to be small. The nucleus is then comparable to a gas.

b) assuming the interaction to be large. In that case the nucleus is comparable to a drop of liquid.

For the calculations we refer to the papers mentioned above. As the assumption b) seems to come nearer the truth we only give the results obtained with this model:

The total number of normal modes of vibration of the nucleus with energy between e and e -|- de proves to be:

(15) p(e)d(e) = (AfVa + B£'')de.

The first part is due to surface waves, the second to volume waves. According to PLANCK'S formula we have for the total energy of the

nucleus at temperature T:

(16) U = '"'^P^')^^ e ^ / " - l

From this formula one can calculate S and A, using the expressions derived above, and finally D(U). Out of the results obtained by

T A B L E I. N + Z 5 M V 10 20 50 T(MV) 1.1 1.4 1.8 D(U) (volts) 5,900 180 0.55 100 T 0.9 1.2 1.5 D(U) 1,100 16 0.015

T h e spacing is in fair agreement with the data obtained from slow-neutron bombardment, viz. when a slow slow-neutron is captured, the excess energy of the new nucleus is equal to the dissociation energy of the neutron, namely 5—10 MV. T h e observed spacing amounts to 10-20 volts for N + Z = 100 and to a few hundred volts for N 4- Z = 50. T h e temperatures are far below the excitation energy. W e shall recur to them below in connection with the considerations of WEISSKOPF concerning the emission of neutrons, which we shall discuss first.

W E I S S K O P F considers the emission of a neutron an evaporation process. T h e probability of the emission of a neutron with energy £ by a highly excited nucleus C is given by:

Uo*

(17) n{s)de = Const. a(Uc, e) e ''^ ^^"^£ e ''^^^'^ ^ ° ^ (

-m

de.

Here ff(Uc, e) is the average cross-section for the recombination process of B with a neutron with energy e leading to the nucleus C with energy Uc. TC(UC) is the temperature of C when C has an excess energy Uc- W e may put a = qy(e), where q is the actual spatial cross-section of the vapourising nucleus and y(e) ^ 1 gives the part of the colliding particles that is captured by the nucleus. Uo* corresponds to the binding energy Uo of the neutron in the nucleus C, but differs slightly from it. e"^^^) is a correction factor, which appears to be unimportant. T h e relative energy distribution of the neutrons emitted ^s given by:

(18) /7(e) = Const, a (Uc, e) £ e

£

T B

-f(£)

T h i s is a MAXWELL distribution multiplied by e ~ ^^^\ the m u m thus being moved to a slightly higher energy. It has a maxi-m u maxi-m at £ = TB. T h e distribution corresponds to the temaxi-mperature TB(UC — Uo) which is not the temperature of the evaporating nucleus C, but that ofthe final nucleus B, which indicates a consider-able cooling of C during the evaporation. Since the temperatures of nuclei with 20 M V excitation energy are only ofthe order of 2 M V (see Table I), only a small fraction of the available energy will in general be taken u p by the outgoing particle, T h u s , when a fast neutron with energy e, collides with a nucleus B and gives rise to the nucleus C with energy £ + Uo, it will be re-emitted on the average with a much smaller energy ( = 2TB) and thus give the greater part of its energy to B. This represents a very inelastic collision. In some cases the energy remaining in B is large enough to enable the emission of a second particle; in general, however, gamma-radiation will be emitted. Below we shall treat this matter in detail (§ 4).

§ 3. Symbols and notation used.

For a further discussion of the reactions leading to the formation of radioactive nuclei it is most suitable to make use of a diagram in which the number of protons in the nucleus (Z) is plotted against the difference between the number of neutrons (N = A — Z , where A is the ,,mass-number") and the number of protons (the ,,isotopic n u m b e r " I = N — Z = A — 2Z) (fig. 1). T h e number of protons is often plotted against the number of neutrons, this giving rise, however, to impracticably extensive diagrams. T h e stable nuclei are indicated in the diagram by dots, the unstable nuclei by circles. A dot with a circle indicates the stable isotope that is the most abundant one. T h e figure clearly shows that stable isotopes are only possible in a small region of the Z -^ (N—Z) plane. At both sides of this region the unstable isotopes are found, emitters of negative electrons above, emitters of positive electrons below. Nuclear reactions are indicated by arrows. F r o m the direc-tion and the length of an arrow the type of reacdirec-tion can easily be inferred. For these reactions we shall adopt the notation of

FLEISCHMANN and BOTHE and we shall indicate by the scheme

N-Z ISOli isebêtB s 7 6 5 4 3 2 1 0 -1 -2 ~ -• • - ® 1 O 9 • 1 O • • 1 • • • O ® • 1 o • » o < o • • o L.. o • • • o o • o o 1— o • • • o • o o • • • o o • o o • • • o o • o o o • • • • o • o o o • * • o o • • • o o • • • • o 1 o o • o o o o 1__ o • • • • • o • o o o 1 • • • o • o • o o o o • o • • o o • • o •*• z 1 H 2 He 3 Li 4 Be 5 B 6 C 7 N

e

Figure 1. Stable and unstable isotopes of the lighter elements. Types of reaction: 1. (n, g) (d, p) 4. (a, n) 7. (n, 2n) (g, n) 10. (n, a)

atomic number of an element is already uniquely determined by its chemical symbol, we shall only write the mass-number of the isotope in question behind this symbol and at normal height, where this cannot cause any confusion. T h e various light particles are denoted as follows:

n = neutron a = alpha-particle p = proton e~ = beta-particle d = deuteron e+ = positron

g = gamma-ray (photon). So we get e.g. Nl4(a, p ) O l 7 , in words: the nitrogen isotope with mass-number 14 bombarded by alpha-particles is transformed into an oxygen isotope with mass-number 17 under the emission of protons. T h e type of this reaction is indicated by (a, p). W e shall now successively discuss what may happen when nuclei are bombarded with neutrons (cf. fig. 1).

§ 4. Surf 631 of the types of reaction initiated by neutron-bombardment. Suppose we are bombarding some substance or other with neutrons. T h e probability that a neutron hits a nucleus is proportional to the geometrical cross-section of the nucleus. It is questionable, however, whether every collision leads to an amalgamation of the neutron and the nucleus and thus to the formation of the inter-mediate nucleus C BETHE has introduced here a ,,sticking prob-ability" (7 ^ 1), that is the probability for combination in a collision. T h e part of the neutrons that is elastically reflected is proportional to 1 — y. For very fast neutrons the levels of the highly excited intermediate nucleus C are very dense and y seems to be almost unity, whereas for slow neutrons the levels ofthe compound nucleus

C are separated by large distances, as the excitation energy is only

small (compare Table I) and thus the capture is often very selective, which reduces y considerably.

Apart from this the following reactions are possible: (n. g).

T h i s is a very frequent process: the capture of the neutron is not followed by the emission of a material particle, but only gamma-rays are emitted. Its existence was proved for the first time by

BJERGE and WESTCOTT ^') for sodium and at about the same time by AMALDI, D ' A G O S T I N O and SEGRÈ ^^) for aluminium. T h e final

nucleus is an isotope of the bombarded element and may be radio-active or stable. In the latter case the existence of the reaction is proved by a high absorption coefficient for neutrons and by the emission of gamma-rays.

It was discovered by AMALDI, D ' A G O S T I N O , F E R M I , PONTECORVO, RASETTI and SEGRÈ " ) that for many elements the activity induced by this process increases very much when the neutrons of the source are slowed down by water, paraffin or any other hydro-genous material. Further, it is striking that by this process even very heavy nuclei can easily be activated. Both particularities can easily be understood with the aid of the considerations of § 2: when the energy of the neutron is small and the number of particles in the nucleus is large, the excitation energy of the intermediate nucleus C (equal to the kinetic energy of the neutron plus about 8 M V binding energy of this neutron in the nucleus C) is distrib-uted among these particles in such a way that every particle receives only a small part of this energy and C will lose its excess energy by gamma-radiation rather than by particle emission, as the energy concentration on a particle necessary to enable its escape becomes most improbable in this case. In other words: for a heavy nucleus and for small neutron energy the temperature of the inter-mediate nucleus is only low and no evaporation will take place. T h e reaction is of special interest because of the very obvious resonance phenomena it shows when slow neutrons are used.

M O O N and T I L L M A N N ^^) and specially A M A L D I and F E R M I *^^)

found that the activation of various elements is caused by different ,,groups" of neutrons, viz. by neutrons of different energy. T h i s is explained as follows: whenever the energy of a neutron coincides with one of the energy levels of the compound nucleus C, the capture cross-section increases very much. Now, for fast neutrons the excitation of the intermediate nucleus is very high and thus the energy levels are very dense and overlap each other (Table I). For slow neutrons, however, the excitation is so small that the levels are wide apart and resonance can clearly be observed. T h e gamma-rays produced in this process have also been observed (see e.g.

F L E I S C H M A N N ' " ) ) . T h e y have to be distinguished well from the gamma-rays produced by inelastic scattering of the neutrons and for that reason the observation can be made best when slow neutrons are used.

(n, n).

In this case an amount of energy sufficient to leave the nucleus is concentrated after some time on one of the neutrons of the inter-mediate nucleus C. Generally it will not take away all the energy of the bombarding neutron and the remaining nucleus B will be left in an excited state. By a subsequent gamma-radiation it will lose its excess energy. So the final nucleus B is identical with the original nucleus A. This process is called ,.inelastic scattering". It seems as though the neutron has suffered a simple scattering with energy-loss. In the rare case that the neutron b has the same energy as the neutron a, the scattering is called ,,elastic". It is predicted by the

BOHR theory that the probability of inelastic collision is far greater than that of elastic collision of fast neutrons with heavy nuclei. Specially for fast neutrons the probability of elastic scattering is very small, as for elastic scattering there is only one final state of

B, the ground state, whereas the number of final states for inelastic

scattering is very large. The amount of energy lost by the neu-tron in an inelastic collision is very large, as may be estimated from the temperature of the nucleus. From the end of § 2 it follows that the average kinetic energy of the scattered neutrons will be only of the order of twice the temperature of the residual nucleus. When the kinetic energy of the incident neutron is 10 MV, this temperature will be of the order of 2 MV for heavy nuclei and consequently the neutron loses about 80% of its energy. Experimental evidence for inelastic scattering was first presented

by DANYSZ, ROTBLAT, WERTENSTEIN and ZYW "'), who observed

the slowing down of neutrons by heavy nuclei, which can only be interpreted by this process. The emission of gamma-rays in this process (which have to be distinguished well from capture gamma-rays) has been observed by LEA ^^') and by KIKUCHI, AOKI and Hu-SIMI *^). No direct evidence exists for the elastic scattering.

in,2n).

This type of reaction was observed for the first time in copper by

HEYN ^^^). It leads to an isotope of the target element of lower mass-number. After the emission of one neutron the nucleus retains in this case enough energy to emit a second neutron, that is the remaining energy must be greater than about 8 MV, the bind-ing energy of a neutron in the nucleus. When the primary neutrons have an energy of 10 MV the first neutron takes up about 2 MV

(see the foregoing section). T h u s the residual nucleus will have an excess energy of 8 M V and the final nucleus no excess energy at all, even when the second neutron is emitted without kinetic energy. Consequently this reaction takes place only with neutrons with an energy greater t h a n about 12 MV, as e.g. emitted by lithium when bombarded with deuterons. T h i s was shown by

H E Y N , who bombarded copper with the neutrons from a (Be + d) source (maximum energy 4.5 M V ) and with those from a (Li + d) source (maximum energy 13 MV). T h e (n,2n) reaction was observ-ed in the latter case only. This technique of using neutrons of different maximum energy provides the most conclusive evidence for the (n, 2n) type reaction. Very recently SAGANE ^^^^) measured

the minimum energy, necessary to cause an (n, 2n) reaction, more accurately and found that in some cases (N, P, Ga, Ge, Ag, Sb) it is considerably smaller than 12 MV, viz. about 6 MV. This may be due to a small neutron-binding energy (see also BETHE ^'), however). It may be emphasized that both neutrons leave the compound nucleus in successive separate disintegration processes.

Furthermore, it seems probable that in most cases the neutron emission is followed by gamma-radiation.

(n, 3n).

Evidence for this t j ^ e of reaction was recently obtained by POOL, CORK and T H O R N T O N ^^^) and by W A L K E '^), who both used 20 M V

neutrons produced by a cyclotron. T h e binding energy of the neutrons must have been very small in this case (scandium), as other-wise 20 M V would not have been sufficient. According to W A L K E the probability of the (n, 3n) reaction seems to be even larger in this case than that of the (n, 2n) reaction. This indicates that the highly excited nucleus will rather lose its excess energy by the emission of three slow neutrons than by the emission of two fast neu-trons. It may be expected that a further increase of the primary neutron energy will enable the successive evaporation of still more particles*).

in, p).

T h i s type of reaction was discovered by F E R M I ^^- **). It is quite

different from the foregoing reactions, as in this case a charged

*) Note added in proof. More recently BURCHAM, GOLDHABER and H I L L *3a) entirely disproved the existence of the (n, 3n) reaction in the case of scandium. There remain still some other rather doubtful cases, e.g. fluorine.

particle leaves the nucleus, which has to pass the potential barrier. It goes without saying that in general this reaction is less probable than the (n, n) type of reaction, specially for heavy target elements, for which the potential barrier is high. Up till now it has only been observed with certainty in elements lighter than gallium. The final product B in this reaction is an isobar ofthe original nucleus A, as in this process one proton in A is replaced by a neutron. It is therefore evident that the reaction can never be exotherm, as otherwise A would long since have transformed into B with the emission of positrons. The mass equation of the reaction runs as follows (c^ is omitted):

M^ + n + E n = M^~' + p + Ep,

z

where M is the mass of the nucleus with atomic number Z. As a neutron is about 0.8 MV heavier than a proton plus an electron, slow neutrons can only cause this reaction when the mass of the final nucleus is less than 0.8 MV heavier than the original nucleus, when we suppose Ep to be zero. It seems as though this condition is accidentally fulfilled in the reaction Nl4(n, p)Cl4, which can be caused by slow neutrons. In general, however, in order to enable the proton to penetrate the potential barrier its energy must be comparable with the height of the potential barrier and thus we may expect the reaction to occur for heavier nuclei only when fast neutrons are used.

As it is a general rule that there exist no stable isobars with odd mass-number and no stable isobars with even mass-number, that differ one unit in atomic number, the final nucleus B is always radioactive and the reaction can easily be observed. B transforms back into A by beta-particle emission, that is, a neutron is trans-formed back into a proton. The mass equation for this process runs:

M^~' = M^ + e H E e

-Adding this equation to the preceding one we get:

y^ + n + En =y^ + e- + p + Ep +

Ee-or

En = Ep -F Ee- — 0.8 MV.

This equation enables another estimation of the necessary neutron energy: for heavy nuclei it follows from temperature considerations (§ 2) that Ep is of the order of 2 MV plus the energy due to the

electrostatic repulsion and as for an observable beta activity Ee -is of the order of 1 M V we need neutrons with an energy of several M V for this process.

Here again it may be expected that the proton emission is followed by gamma-radiation, as in general the proton will not take u p all the energy of the intermediate nucleus. Yet experiments seem to indicate the absence of gamma-radiation.

(n, np) and (n, 2p).

These reactions are analogous to the (n, 2n) reaction, although the energy necessary for the escape of protons is much higher. T h u s they might be expected with very fast neutrons and for light nuclei. Specially the reaction (n, np) seems probable in that case. U p till now, however, they have not been observed. T h i s may be partially due to difficulties arising from the fact that for almost all the light nuclei the radioactive isotope that may be expected to be produced by the (n, np) reaction, can also be produced starting from another isotope of the same target element, according to the reaction (n, p), whereas for the (n, 2p) reaction the same difficulty arises from the reaction (n, a). For an attempt to observe the (n, np) reaction compare Chapter I I I , § 15.

(n, a).

Shortly after the discovery of the neutron. FEATHER *') observed for the first time a transmutation by neutrons according to this type of reaction. It was first observed for nitrogen in a W I L S O N

chamber, the final nucleus B being inactive. FERMI and his collab-orators ^*') succeeded in producing radioactive nuclei according to this reaction. As in the (n, p) reaction, enough energy must be released to enable the alpha-particles to penetrate the potential barrier, but the formation of an alpha-particle sets free 28 MV and this causes the reaction to occur rather frequently. It is observed with certainty in elements u p to germanium but also in the very heavy elements thorium and uranium, which may be explained by the small binding energy of the alpha-particles in these elements. For light elements the probability of the reaction is inversely proportional to the velocity of the neutrons, but for fast neutrons with an energy comparable with the height of the potential barrier the probability must increase again, as the higher energy imparted to the alpha-particles makes penetration of the potential barrier easier. Consequently we can distinguish two groups of elements,

those transmuted by slow neutrons (lithium and beryllium; for all other light nuclei the reaction seems energetically impossible with slow neutrons on the basis of the atomic masses) and those trans-muted by fast neutrons (many elements u p to germanium). T h e alpha-particles of the first group have been observed by electrical recording devices and this provides a very useful method for the detection of slow neutrons.

T h e opinions of the various investigators differ as to the gamma-rays produced in this process (compare LIVINGSTON and BETHE '"*)). (n, na).

T h i s type of reaction has not been observed, although it has sometimes been suggested in order to explain some observations. Yet it remains a possibility which must not be excluded.

§ 5. Methods of identification of a special type of reaction. In connection with the chapters to follow, it is of importance to discuss the methods for the identification of a special type of reac-tion. W e shall give a survey of these methods here, supposing that a radioactive substance has been obtained and that we want to know by what process.

a. C h e m i c a l a n a l y s i s o f t h e a c t i v e e l e m e n t .

T h e element irradiated is dissolved and traces of neighbouring ele-ments are added to the solution. After that all the eleele-ments present are successively precipitated and examined for radioactivity. T h e radioactive substance is isotopic with the element showing activity. For long-life substances this method is very convenient, but for short-life substances it is often inapplicable. For some cases special chemical methods have been developed. SZILARD and CHALMERS ^**) have e.g. given a method to separate the radioactive isotope of the target element from the bulk of the bombarded element, the prin-ciple being as follows: when a chemical compound of an element is irradiated by neutrons we may expect those atoms of the element that are struck by a neutron to be removed from the compound. It depends on the nature of the compound, whether the atoms freed in this way will interchange with the other atoms or not. W h e n e.g. iodine is bombarded, bound in the radical JO3 , free iodine atoms are produced which do not interchange with the

bound atoms and which can be precipitated by adding AgNOa to the solution, JO3 remaining in solution. T h e method can also be applied in the cases of chlorine, bromine and manganese. Another useful method is the collection of the active atoms by means of an electric field.

b. D e t e r m i n a t i o n o f t h e s i g n o f t h e p a r

-t i c l e s e m i -t -t e d .

T h i s provides very important information concerning the place of the active isotope, whether it lies above or below the region of the stable nuclei. It can be determined by means of a cloud-chamber with a magnetic field, or by means of magnetic deflection (preferably in vacuo) and observation of the deflected particles by means of a counter tube. In the latter case the trochoide method of THIBAUD is very useful.

c. M e a s u r e m e n t o f t h e p e r i o d .

W h e n the period is known many neighbouring active nuclei may be excluded or the agreement of the period with a known period may provide a strong indication that the activity must be ascribed to the known active nucleus. T h e decay curve obtained is often of a composed type. In that case it may be analysed in two or more logarithmic curves (cf. Fig. 9). By varying the time ofthe irradiation this analysis can be facilitated. W h e n the energies of the particles belonging to two periods are very different, a separation of the periods can also be obtained by covering the activated material with a layer of aluminium that absorbs the less energetic particles.

d. M e a s u r e m e n t o f t h e m a x i m u m o r m e a n

e n e r g y o f t h e p a r t i c l e s e m i t t e d .

T h e maximum or mean energy of the particles emitted as compared with the maximum or mean energy of those emitted by other nuclei provides in many cases strong evidence for the identity of two radioactive substances. These energies can be determined by means of cloud-chamber photographs or by magnetic deflection. A more convenient method consists in the measurement of the half-value thickness of the particles, e.g. in aluminium, this giving the mean energy of the particles. This half-value thickness

depends, however, largely upon the geometrical conditions and must therefore be considered a relative value. Finally, the range of the particles (in aluminium e.g.) permits the calculation of the maximum energy.

e. D e t e r m i n a t i o n o f t h e e n e r g y o f t h e n e u

-t r o n s n e c e s s a r y -t o c a u s e -t h e r e a c -t i o n .

In the preceding pages it has already been pointed out that some types of reaction are possible with fast neutrons only, whereas other reactions are more probable with slow neutrons. In many cases the neutron energy needed thus provides most conclusive evidence for a certain type of reaction. When e.g. an activity is enhanced by slowing down the neutrons with hydrogenated substances, the reaction concerned is most probably a capture reaction.

Besides slow neutrons we have at our disposal the neutrons from the reactions H2 + d (energy 2.4 MV), from Be -(- d (maximum energy 4.5 MV) and from Li + d (maximum energy 13 MV). ƒ. C l o u d - c h a m b e r p h o t o g r a p h s .

These photographs have often given important information, spe-cially concerning the particles emitted in the course ofthe reaction. For further information the number of photographs to be made is very large, however, and the interpretation of the tracks observed is not always conclusive. A critical discussion of this method for the case of neutron bombardment is given by LIVINGSTON and

Chapter II.

Apparatus.

§ 6. General remarks.

In all our investigations the neutrons used were produced by bombarding various targets with deuterons. Various compounds of deuterium (ND4CI, D3PO4), metallic beryllium and metallic lithium were used as targets. T h e neutrons emitted by these targets are quite different in number as well as in energy. Especially is the difference in energy of importance in connection with our investigations.

In the case of deuterium bombarded by deuterons, the neutrons are almost monokinetic as is to be expected from the simple two particle reaction H2(d, n)He3. T h e i r energy is 2.4 M V or probably somewhat lower, when we take the energy of the incident particle as zero. W h e n beryllium is bombarded by deuterons the reaction runs: Be9(d, n)BlO. This reaction is widely used as a source of neutrons in high voltage sets, but it has the disadvantage that the neutron spectrum consists of several lines with about equal intensity. T h e maximum neutron energy is about 4 MV (Ed = 0 ) . By bombarding lithium with deuterons it is possible to produce neutrons of very high energy. According to the reaction Li7(d, n)2He4, the neutron spectrum is continuous*). T h e maximum energy is about 13 M V (Ed = 0 ) . At the end of the spectrum there is a monokinetic group due to the transformation Li7(d, n)Be8. For further details, for data on the neutron yields as well as for literature we refer to the survey given by A M A L D I " ) .

§ 7. The first tube (250 feV).

This tube '*) is a modification of the type described by O L I P H A N T

and RUTHERFORD ^^). In a high voltage discharge tube (ion-source)

*) Note added in proof. Recently a different opinion concerning this reaction has been put forward by STEPHENS ^«sa),

ions are produced which enter through a canal into a second tube, the accelerating tube. The first tube is placed inside the second one. Both lie horizontal. The ion-source S (fig. 2) consists of an aluminium tube B cooled by radiation and a copper tube A with a copper partition L soldered in. The copper tube and the copper partition are chromium-plated, as copper too easily gives rise to an arc-discharge. The partition is cooled by oil. The insulation between the electrodes A and B is formed by a glass cylinder J that is connected to the flanges K and E by a vacuumtight

chro-Ijeaden screen

Figure 2. The first tube.

mium-iron joint. The canal in the electrode B has a length of 5 mm and a diameter of 3 mm. The source has been made wide and the anode not too deep in order to make use of the secondary (reflected) electrons from the anode. The ionisation due to these electrons makes it possible to operate the tube at a lower gas pressure ^^). The shape of the electrodes does not influence the discharge very much, as appears from experiments with electrodes of different shapes. The distance between A and B amounts to about 8 mm. The hydrogen enters the source through a canal in the flange E. The pressure in S can be regulated, whilst the tube is running, with a needle valve V which is operated by means of an insulating shaft. The acceleration of the ions takes place between the electrodes B and D (distance 5 cm) in the glass tube G. Here again chromium-iron joints are applied to connect the glass cylinder G to the flanges E and H. D is of chromium-plated copper. It is shaped in such a way as to make the field strength at its surface

as low as possible in order to avoid field current. T h e part of B opposite to D has been made concave (not shown in the figure) in order to obtain a concentration of the ion beam. A M C L E O D

vacuum-meter is connected with N at M L via a liquid airtrap. T h e oil diffusion p u m p P is connected direct with the metallic part N in order to make as much use as possible from the high pumping speed (2.10* cm^/sec). T h e pressure in G averages 8.10^ m m Hg. F r o m the dimensions ofthe canal and the pumping speed we calculate that the pressure in S must average 2.10~^ m m Hg. After passing through the tap T and the magnetic field M (which field was not applied during the irradiations described in Chapter III) the canal ray enters a FARADAY cage at F, in which the target material has been placed. It is cooled by water. T h e screens at both ends of the tube serve to give a uniform field distribution.

In order to obtain a good canal ray the following points proved to be important:

1. Glass models o f t h e source show that already at a great distance from the canal concentration of the discharge takes place. W e have therefore to take care that the concentrated beam falls exactly on the hole in the cathode. T h i s means that the interior and the exterior electrodes have to be centred well, which is obtained by means of grub screws in the flange K of the interior electrode and verified with an X-ray apparatus. T h e connection between K and the chromium-iron flange at the end of the glass cylinder J is made vacuumtight by means of a rubber ring.

2. T h e purity of the gas is very important. Impurities in the hydrogen decrease the intensity of the canal ray considerably. For that reason the heavy hydrogen obtained by the electrolysis of heavy water was purified by a liquid airtrap and by silicagel. T h e electrodes are freed from gas and impurities on their surface by operating the source without cooling until the anode is redhot. In general the tube operates best when hot. This may be due to the fact that on the hot electrodes no grease or other impurities, which influence the emission of electrons from the electrodes, can be deposited.

3. T h e hydrogen pressure must be optimal and the source tension as high as possible. T h i s is clearly shown by fig. 3, where for three different pressures the source current and the target current 31

have been plotted against the source tension. When the tension increases the target current increases quickly. However, the source current increases too and that the more quickly as the pressure is higher, and this rise of the source current finally limits the use of still higher tensions. It is also shown by fig. 3 that the pressure has to be chosen as low as possible. However, at too low a pressure the discharge becomes unstable.

700 600 500 400 300 200 100 0 15 20 25 30 35 40 45 50 55 kV Tension J»*'

Figure 3. Source current and target current as a function of the source tension.

The composition of the beam was examined by means of a magnet. In order to obtain a concentrated beam for that purpose we always used an accelerating tension, because the canal ray does not leave the hole as an exactly parallel beam. The intensity of the beam was measured by means of a FARADAY cage, 8 cm deep and 3 cm

wide. The beam enters the cage after passing two diaphragms, the first with an opening of 15 mm, the second, at a distance of 4 cm, with an opening of 20 mm diameter. The opening in the cage is also 20 mm. For the magnetic analysis the second diaphragm is replaced by a slit, 1 mm wide, whilst the opening in the cage is replaced by a slit 2 mm wide. Measured under these

circum-T ^ \ \ r • • • • 0,029mm source pressure y^^yy-0,026 mm 0 00 0 0,021 mm Source current mA

stances the total current was about 1.5 mA. Fig. 4 gives the magnetic spectrum. From the position of the peaks we conclude that the second peak must be ascribed to protons (H,"^), the third to Hj ions, whilst the first (H2-1) is due to particles which have been acceler-ated as H^ ions, but have broken up in the space between the electric field and the magnetic field and have been deflected as

50 ^

n

f

1

u

f\

[

y

V

Figure 4. Magnetic spectrum of the canal ray.

protons (the occurrence of peaks of this type has been discussed in detail by H. D. SMYTH ^''*)). The number of protons is smaller

than the number of H^ ions, which may be expected, the pro-duction of H^ being the primary process. We do not find an indi-cation of the presence of H^ ions. It may be supposed that the pressure is too low for their formation. When we use deuterium instead of ordinary hydrogen we get a similar curve with peaks for D^. 1, Dj*" and D2 . The source pressure and the source tension

in the range indicated in fig. 3 have only a slight influence on the composition of the beam. At a higher pressure the peaks are less prominent. In some cases the intensity of the ion beam was measur-ed, whilst the cage had a negative tension of some hundred volts against earth, in order to prevent the escape of secondary electrons. The results obtained under these circumstances did not differ much from those mentioned above.

In order to get an impression of the number of the neutrons emitted by this tube, we compared the activity of a silver sheet, irradiated in a large block of paraffin with the neutrons emitted by metallic lithium bombarded with about 1 mA deuterons at about 200 kV, with the activity of the same silver sheet, irradiated under the same conditions with a known number of neutrons emitted by 95 mC radium mixed with beryllium powder *). The initial activity of the silver sheet (6 X 9 X 0.03 cm*) irradiated with the (Li + d) source and measured with a GEIGER-MULLER

counter was 130,000 kicks a minute (2.3-minute period), whilst irradiated with the (Ra + Be) source the activity was 4,500 kicks a minute. This shows that this (Li + d) source is roughly equiv-alent to 3 grammes of (Ra + Be). Measured under the same conditions the beryllium target yielded a number of neutrons equivalent to 1.3 grammes of (Ra + Be). The intensity of the neutrons emitted by the deuterium target fluctuated considerably.

§ 8. The second tube {600 feV).

This tube (fig. 5a and 5b) consists of two parts at potentials of-f300 kV and —• 300 kV respectively, whereas the connection between both is earthed. The first part is essentially the same as the first tube. Only the shape of the electrode D has been modified in such a way that it is almost as broad as the aluminium electrode which makes it more difficult for the electrons produced by the impact of the ions on the surface of D to escape from between the two accelerating electrodes. This is very important as it appears from all our experiments that it is of paramount importance for the construction of accelerating tubes to prevent the inevitable secon-dary electrons from striking the walls of the tube or from covering long distances in the tube. For the same reason we extended the

• ) T h e number of neutrons emitted by this radium — beryllium source was

mea-sured by BAKKER 18").

a

TiiF- A

metallic part N till past the accelerating space between the elec-trodes B and D . Further, this part o f t h e tube has been mounted in such a way in the screen in the middle that it can revolve round an axis perpendicular on the plane of the figure. As it rests freely on the insulating support on the left-hand side of the figure, the glass remains entirely free from mechanical stress.

T h e second part of this tube is formed by another accelerating tube connected with the first by means of a piece of flexible tube. By way of trial we made this tube of a kind of bakelite as used for the P H I L I P S high tension condensers. It proved to be absolutely vacuumtight and the amount of gas given off was so small that it did not cause any trouble. T h a t is why we continued to utilise this bakelite tube, but care should be taken that the bakelite does not grow hot. T h e electrodes are of chromium-plated copper. T h e hole in the first electrode is slightly smaller than that in the second in order to prevent the ions as much as possible from striking the front of the second electrode. It proved to be necessary to cool the first electrode by water, as otherwise it becomes very hot. T h i s heating is due to the impact of electrons as appears from the X-rays produced by them (X-rays produced by ions are very soft and of very small intensity). These electrons are most probably created by the impact of ions on the right-hand electrode. T h e movable connection makes it possible to place the second tube exactly in the direction of the beam. T h i s is checked by the fluorescence of a piece of quartz, mounted in place of the target, making it possible to observe the point of impact of the beam. Between the tap and target and the tube a piece of flexible tube has also been inserted in order to be able to alter the point of impact of the ion beam on the target. As soon as the beam has deposited too much carbon on the target, tap and target are moved in such a way that the beam strikes a clean place on the target. As the target is at high potential it is cooled by oil. Nevertheless, we experienced much trouble by the fact that our lithium target melted repeatedly (in this tube we did not use deuterium targets, whereas the beryllium target did not cause any trouble). T h i s was avoided by improving the heat-contact between the lithium and the cooled copper disc behind it by providing this disc with some ribs on which the metallic lithium was pressed with great force. T h e target is cooled at the sides in order to be able to bring the

substance to be irradiated nearer to the source of the neutrons. T h e neutron yield of this tube, measured in the same way as de-scribed above, proved to be equivalent to 300 grammes of radium mixed with beryllium, when lithium was bombarded with 0.5 m A 600 kV deuterons. Some data on the number of neutrons emitted by our tubes are summarized in the table to follow:

T A B L E I I . N e u t r o n o u t p u t o f o u r t u b e s . (Li + d) r e a c t i o n . Current (mA) 1 1 0.3 0.5 Tension (kV) 200 250 450 600 (Ra + Be) equivalent (grammes) 2.5 10 70 300 Total number 5 X 1 0 ' 2x10» 1.4X10» 6x10» Number per fiA 5 X 1 0 ' 2 x 1 0 ' 4.7 X10» 1 . 2 x 1 0 ' Data of Amaldi.etc"). Number p e r ^ A — 8x10» 2 X 1 0 '

T h e ions were magnetically separated in the first and second case only, whereas A M A L D I and his collaborators always used a magnet *).

§ 9. High tension apparatus.

T h e high tension sets which supply the tension to accelerate the ions in both tubes are of the cascade type, as has been described in detail several times already *"- ^^'). Figure 6 shows the source tension and the accelerating tension circuit for the first tube. For the second tube another high tension set of the same type was used for the second acceleration, but with a tension negative with respect to earth. W e applied a modification o f t h e cascade principle to obtain the ion-source tension. T h e figure (right-hand side) demonstrates this new method. T h e condenser of the first stage is superseded by the two condensers C4 and Cs in series. Each of these two condensers can withstand the entire accelerating tension

•) More particulars on this tube and a description of a new 1 MV tube will be

In order to be able to use normal condensers (C4, Cg and Cg) it is profitable to apply a high frequency. In that case the ripple on Ce is only small, even when the condensers are not very large. We used 500 cycles. As compared with usual methods (insulated

AAAAAA-tnnnp-(gT^

Figure 6. Source tension and accelerating tension circuit.

generator, high-insulated transformer), this method is much cheaper and simpler. The tension was measured by means of a spark-gap or a resistance.

§ 10. Auxiliary apparatus.

C o u n t i n g d e v i c e s .

The activity of the substances investigated is observed by means of a GEIGER-MÜLLER counter. It has an aluminium wall with a

thickness of 0.15—0.20 mm and a diameter of 20 mm and is filled with a mixture of neon and hydrogen (in equal parts) at a pressure of 15 cm Hg. Connected with a circuit as described by

NEHER and HARPER "*) and a WYNN-WILLIAMS ^°^) 3-stage

scale of two, the counting device works linearly up to 4,000 kicks a minute. The number of natural counts amounts to 20 per minute. For the measurement of long periods we used a filming apparatus and automatically made a photograph of the mechanical counter every minute.

M a g n e t i c d e f l e c t i o n .

The sign of the particles emitted by the radioactive substances was investigated by means of magnetic deflection. With an electro-magnet we deflected the particles over an angle of 180° in a flat evacuated box. The deflected particles were observed by means

of a GEIGER-MÜLLER counter, screened with lead against the direct

radiation from the active substance. The apparatus was tested with some thorium. Without the magnetic field the counter gave its natural counts only. By regulating the magnetic field a part of the particles emitted was counted (about 10%). This method is not very sensitive, but suffices to establish the sign of the par-ticles unambiguously.

Chapter III.

Results of experiments on the activation of

various elements.

In this chapter an account will be given of all the experiments we carried out on the activation of elements by slow- and fast-neutron bombardment. T h e substances investigated will be dis-cussed in the order of their atomic number. Of the results obtained by other investigators only the data that are important in connec-tion with our own work will be summarized here; for full particu-lars we refer to Chapter IV. For the stable and active isotopes of the elements u p to and including cobalt compare fig. 1.

§ 11. Lithium, Beryllium, Boron. L i t h i u m .

Although FERMI and his collaborators ^") did not observe any activity in lithium bombarded with neutrons, a marked absorp-tion was found, which, according to them and to CHADWICK and

GOLDHABER *^), is caused by the reaction Li6(n, a)H3. K N O L and

VELDKAMP ^*'' ^^^), however, observed a period of about 0.8 seconds when lithium was bombarded with mainly slow neutrons. About the same period (0.5 seconds) had previously been observed by

CRANE, DELSASSO, FOWLER and LAURITSEN ^*) and by others (cf. chapter IV), who bombarded lithium with deuterons ((d, p) reaction). K N O L and VELDKAMP therefore suppose that the carrier of the 0.8-second period is Li8 produced according to the reaction (n, g). NAHMIAS and W A L E N ^**) reported a 0.7-second activity induced in lithium when bombarded with slow as well as with fast neutrons. They suggest that the fast-neutron activity may be due to the reaction Li6(n, p)He6, as He6 is known to have a period of about 1 second (see beryllium), whereas the slow neutrons may give rise to the reaction (n, g). T h e capture reaction has also been

observed by L E W I S , BURCHAM and CHANG ^^*), who gave a period of 0.88 seconds. According to these investigators the Be8 nucleus, produced by the disintegration of Li8, is unstable and disinte-grates itself into 2 alpha-particles. This was confirmed by FOWLER

and LAURITSEN '*). A n activity produced according to the reaction (a, n) and ascribed to B9 has been reported by M E I T N E R ^**), but has not been confirmed by other investigators ^^®).

B e r y l l i u m .

A period of about 1 second has been found by BJERGE '^- ^*' ^°) in

beryllium bombarded with neutrons. T h e carrier of this activity proved to be He6 produced according to the reaction Be9(n, a)He6. T h i s was confirmed by NAHMIAS and W A L E N ^**). N o other activity, but for a dubious one induced by deuterons, has been reported. B o r o n .

As in the case of lithium, slow neutrons are strongly absorbed by boron as observed by FERMI and his collaborators ^") and by

CHADWICK and GOLDHABER **). T h e reaction that causes this ab-sorption is Bl0(n, a)Li7. A radioactivity induced by neutrons (from Rn + Be) in boron has been reported by NAHMIAS and

W A L E N i®*), who found a period of about 1 second. T h e y suppose that this activity is due to the reaction B l l ( n , a)Li8.

W i t h deuterons a period of 0.02 seconds has been reported, due to the reaction B l l ( d , p)Bl2, as well as a period of about 20 min-utes, due to BlO(d, n ) C l l (cf. chapter IV). It is doubtful whether an activity is produced by protons.

W i t h alpha-particles an activity induced in boron according to the reaction BlO(a, n ) N l 3 has been discovered by CURIE and

JOLIOT *^). T h i s reaction has been investigated by many others (cf. chapter IV). T h e most probable period seems to be 10.5 ± 0.5 minutes.

As results from this survey and as fig. 1 shows, there are still several unoccupied places for radioactive nuclei left near these three elements. Reactions of the type (n, 2n), (n, p), (n, 2p) and (n, np) may be expected. Hence, we attempted to activate them by bombarding them with the fast neutrons from our (Li + d) source (second tube). T h e pure elements as well as various com-pounds were bombarded. Although all samples became radioactive,

often very strongly, chemical separations of the carriers of t h e activity showed that these activities were due to impurities, perhaps with the exception of boron. Besides other activities, we observed in this element a 2.5-minute period, which is probably not entirely due to impurities. This activity was too weak, however, to examine it more closely. W e did not look for half-lives shorter than 2 seconds. Bombarding boron with deuterons, an activity with a period of about 20 minutes was readily obtained in accordance with the observations of other investigators. This activity must be ascribed to C l l .

§ 12. Nitrogen.

FEATHER *') was the first to observe the transformation of nitro-gen when bombarded with neutrons. T h e observations were made with a cloud-chamber and the reaction proved to be N l 4 ( n , a ) B l l . T h i s was confirmed by HLARKINS, G A N S and N E W S O N ^^), by

Ku-RIE "^) and later on by BONNER and BRUBAKER **). CHADWICK and

GOLDHABER *°) stated that this reaction also occurs with slow neutrons, b u t from the masses of the nuclei in question it follows that the minimum energy of the neutrons necessary to cause this

reaction amounts to 0.34 M V . According to BONNER and

BRUBA-KER ^^' ^^) the particles emitted under slow-neutron bombardment are protons. These particles were observed already before with fast neutrons by KURIE ^*'') and by BONNER and BRUBAKER ^^). T h e fact that slow neutrons give rise t o this reaction was confirmed by BURCHAM and GOLDHABER*^), who applied the photographic method to this end.

F E R M I and his collaborators ^'') bombarded nitrogen with the neu-trons from 600 m C (Rn + Be) and looked for activity, of which they found no trace, however. Neither did NAHMIAS and W A L E N ^**), who specially looked for an activity produced according to the reac-tion (n, p). T h e y irradiated liquid nitrogen for one hour and chemi-cally separated carbon from it. M C M I L L A N "''), however, observed

a period of 3 months in nitrogen bombarded with neutrons, which he ascribed to C14. T h e same active nucleus was obtained by him^'*) according to the reaction Cl3(d, p ) C l 4 . T h e experiments of

BURCHAM and GOLDHABER *^) seem to provide some evidence,

W i t h the fast neutrons from (Li -f- d) POOL, CORK and T H O R N

-TON *^*) observed a weak activity with a period of 10.5 minutes. T h e y suppose that N 1 3 is formed here according to the reac-tion (n, 2n).

W e bombarded nitrogen in the compounds N4H4C2 and NaNs several times with the fast neutrons from (Li -|- d), each time for about 15 minutes. These compounds were chosen, as they do not contain elements that give rise to interfering periods under neutron bombardment. A n activity with a period of about 10 minutes was observed in all cases. No chemistry was performed, b u t it seems most probable that this activity is due to N13 ((n, 2n) process), as it is known from other reactions that N13 has indeed a period of 10.5 minutes.

§ 13. Oxygen.

W i t h deuterons oxygen can readily be activated ( N E W S O N ^^), K U R I E , RICHARDSON and PAXTON "^)). T h e carrier o f t h e activity is a fluorine isotope, produced according to the reaction 0 1 6 ( d , n ) F l 7 (1.2-minute period). DUBRIDGE, BARNES and BUCK '^' ''^^' ''^^) ob-tained evidence for an activation by protons according to the reactions (p, g) and (p, n) leading to the radioactive nuclei F l 7 and F18. W i t h the neutrons from (Rn -|- Be) F E R M I and his collaborators") could not observe any activity. M E I T N E R and

P H I L I P P ""' 1*^) found the reaction O l 6 ( n , a ) C l 3 leading to the stable C13. T h i s process has also been observed by FEATHER **). T h e reaction (n, p) was reported by CHANG, GOLDHABER and

SAGANE *'), who measured the period of active N I 6 and found 8 seconds. W i t h the fast neutrons from (Li -|- d) POOL, CORK and

T H O R N T O N ^'*) observed a period of 2.1 minutes, which they ascribed to 0 1 5 , produced according to the reaction (n, 2n). In order to check the neutron reactions we bombarded water and several other oxygen compounds with the neutrons from (Li + d), using the second tube. A strong activity with a period of 8 seconds was observed, but no trace of the 2.1-minute period was found. T h i s is in accordance with the results of C H A N G , GOLDHABER and

SAGANE, who neither found this longer period.

F r o m the atomic masses as calculated by LIVINGSTON and BETHE ^**) it follows that the binding energy of a neutron in 0 1 6 amounts

to 16 MV. As the energy of our neutrons and of those of C H A N G , GOLDHABER and SAGANE is smaller than 14 MV, the activity with

a period of 2.1 minutes is not to be expected (the mass of O l 5 we

used is not quite reliable, however). T h e neutrons used by POOL, CORK and THORNTON are of a much higher energy.

§ 14. Sulphur.

Sulphur bombarded with the neutrons from (Rn + Be) transforms, according to F E R M I and his collaborators ^"' **), into active phos-phorus (P32), the period of which is 14 days. This has been confirmed by several other investigators (cf. chapter IV). POOL, CORK and

T H O R N T O N ^^*) bombarded sulphur with the fast neutrons from (Li + d) and observed another activity as well, with a period of 26 minutes. They report, however, that this activity is very weak. A thorough investigation of sulphur has been carried out by SAGANE *'^). Besides the activity with a period of 14 days men-tioned above, he observed an activity with a period of 2.6 hours in sulphur bombarded with the neutrons emitted by (Be + d) due to Si31. W i t h slow neutrons he could not observe any activity, neither with deuterons according to the reaction (d, p). This seems very surprising, as an activity with an 80-day period produced out of chlorine according to the reaction (n, p) and due to S35 has been reported ^^) and as SAGANE irradiated sulphur long enough to make the observation of this long-life activity possible. As

SAGANE used only rather slow neutrons, he could not have observed the 26-minute period mentioned above.

In connection with the observation of an activity in chlorine bom-barded with fast neutrons (5-minute period), which proved to be due to a sulphur isotope, we attempted to activate sulphur with slow as well as with fast neutrons (from (Be + d + P) and from (Li + d) resp.). W e used the second tube for this purpose. W i t h slow neutrons practically no activity was observed in pure sulphur after several 15-minute irradiations. W i t h fast neutrons we irradi-ated (NH4)2S04 powder repeirradi-atedly for two hours, but we could not observe any activity in the chemically separated sulphur. In phosphorus a rather strong long-life activity was induced (P32). As S34 is present in sulphur to an amount of 2 % it seems

§ 15. Chlorine.

Bombarding chlorine with the neutrons from (Rn + Be), F E R M I *")

observed a long-life activity. Afterwards he and his collabora-tors ^°' **) showed by chemical separation that this activity is due to P32; this was also suggested by AMBROSEN ^*). It has a period of 14 days. Further a second, shorter, period was observed by these investigators. A chemical analysis showed that this period (35 min-utes) is due to a chlorine isotope. HURST and W A L K E ^^®), using the neutrons from a cyclotron, state that the period of the active chlorine produced according to the reaction Cl(n, g)Cl is 37.5 minutes. ANDERSEN ^^) observed a period of 80 ± 10 days in sulphur chemically separated from chlorine that had been irradiated with the neutrons from (Rn + Be). H e supposes that this activity is produced according to the reaction Cl35(n, p)S35. Finally, POOL,

CORK and T H O R N T O N ^^^), bombarding chlorine with the fast neu-trons from (Li -|- d), observed, besides the well-known 14-day period, a period of 33 minutes, which they ascribed to C134 (chemically verified; positive electrons).

According to several authors ^^°' ^^*' ^^') chlorine bombarded by alpha-particles yields a radioactive potassium isotope with a period of 7.5 minutes. Bombarding chlorine with deuterons, KURIE, RICHARDSON and PAXTON ^*^) as well as VAN VOORHIS ^ ) observed

a period of 37 minutes. According to KURIE, RICHARDSON and

PAXTON "^) the beta-spectrum of the radioactive isotope thus obtained is of a composed type. Finally, we may remark that G R I F -FITHS and SZILARD ^°^) infer from their experiments on the neutron

capture radiation from chlorine that a second, long-life, radioactive chlorine isotope most probably exists.

F r o m these data and from the fact that a 37.5-minute period, due to a chlorine isotope, has been observed by HURST and W A L K E ^^®), who bombarded potassium with neutrons ((n, a)reaction), it may be inferred that the carrier of the 37.5-minute period is C136. C138 may have either a very long period ( G R I F F I T H S and SZILARD)

or a period of about 37 minutes ( K U R I E , RICHARDSON, PAXTON) or a very short one.

T h e 33-minute period obtained with fast neutrons may indeed be due to C134. This would be in accordance with RIDENOUR and

HENDERSON ^*'), who produced C134 out of phosphorus and gave