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D~cwnber 26, 19.!&

(PJ@ REACTIONS IN DWJ!ERIUM, SCANDIUM AND VANADIUM

WORK DONE BY : Per REPORT ‘WRITTENBY: . .———

By

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J’. M. Blair R. F. Taschek

J. M. Hush

E. c. Kl(ma

L. W. Seagondollar

R. F. Taschek

C. M. Turner

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ABSTRACT

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The thick target yield curves for D(pn]2p, Sc(pn)Ti, V(pn)Cr have

been investigated to determine whether the yields are lari~eenough to make

these reactions usable neutron sources. The first roac’tionshould giv~ a

collimated neutron beam and the latter two reactions should give neutrons of

very low energy and high symmetry so far as momentum considerations are con.

ccxncdO The yield from D(pn)2p appears to be too anal.?.at threshold for

practical usej Sc(pn)Ti and V(pn)Cr give encmgh neutrons near threshold to make

. them appeafipromising as sources of low energy neutrons.

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[P,N) REACTIOA~SIN DEUTERIUIJ,SCA?iDIUMAND VANADIUM,

This is a preliminary report on the thiak target neutron yield of

the reactions

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~12(p,d2p

45(p,n)Ti22h5

‘c 21

51(p,n)Crti51.

’23

The first of these reactions vas invrmti.gatedwith the purpose of finding a

highly collimated source of neutrons of fairly high energy. The seoond two

reactions were studied to find a source of very low energy neutrons ~.vitkgood

energy resolution &nd perhaps spherical symmetry. The Li~(p,n)Be~ reaction

used with the electrostatic generators is a very strong source of monoenergetic

,

neutrons in the forward

“ 2 Mev. However, due to

light LJ.7,the neutrons

direction over an energy range,from about 150 kv to

the momentum consideration of the proton and

enitted in the forward direction between the threshold

energy of 1.86 Mev and 109 Mev are in two energy groups~ just at thr~shold

only a single group of 30 kev energy is prssentg This necessitates observations in the backward direction for neutrons below 1~0 kev, with q very large

decrease in yield if one wishes to use neutrons of ener~iea below

kewo Further difficulties arise’in this method of taking data in

energy resolution at low energies is very poor and a considerable

about 20

that the

background

of epithermals and scattered neutrons is present because of the high forward

energy and krge yield of these forward neutronso

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D12(P,n)2P

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For certain types of experiments it

source of neutrons which is highly colli-ted.

would be desirable to have a;

Momentum oonsiderstions show

that the reaction D12(p,n)2p should give neutrons of about 370 kev energy in

the forward direction just titthresholds aGsuming that the t~lresholdenergy

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of the protons is s.bout3.3 .Xev. This neutron energy corresponds entirely to

the center of gravity motion of D i-p and means that all the neutrons emitted

say spherically syminetricallyin the center of gravity systemwith zero

energy are emikted with 370 kev energy exactly forwards in the laboratory.

For somewhat above threshold energy the neutrons spread out into a sharply

defined oone within which the energy spread of the neutrons is determined by

the proton energy above the reaction threshold. In this three partiole emission

tho energy spread of the neutrons/apiaybecomes large above threshold GO

it is nece~aary that the yield near thre6hold be large for the reaction to be

useful.

In Fig. 1 is shown the yield of neutrons per proton observed from

a thick tarfletof D P O as determined by a long counters LAMS-66, ~mose

427

efficiency was measured to be 0.9 x 10°5. It will be seen that n large

background is present down to 207 Mev proton energy and that even above the

3.3 Mev threshold for D(p,n)2p there is no marked inoreas~ in yield. If one

asaumes thtittho threshold is 3~3 Mw +nd -thatall the neutrons come from

deuterium, then the target is 200 kv thiok at 3.5 Ma and from the stopping

power of the D~P207 one calculates that the average cross section at 3.4

Ik!evis m greater than a%out 107 x l&=cm2 for D(p,n)2p. If now one — .—.

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“compares the observed threshold$ relative yield as a finction Of enerflyand

flattening of the yield curve above 3.3 Mev with the observations of IWridge,

Barnes. Buok and Strain1) on the reaction 018(pn)F18, it appears that most of

the neutrons come from this 2atter reaction. Assuming that the Vnreshold

is 2P6 Mev and that the neutrons all come from O15(pn)F18 at 2.8 Mevb the

average cross section for the reaction at 207 Mev is no tycater than

.25 2.3 x 10 cm2. This assumes that.200 kv atiove threshold the neutrons emergo

spherically symmetric in laboratory system which is very likely not true so that

the correct cross sc?ctionmy be considerably lower tkn that’given above,

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iindwould compaorefavorably with DuBridge$ et alla value of about

Oe?fj x low2~ CIU2 at 3055 Mev. This make6 it appear that near threshold the

yield from D(pn)2p is extremely small and not usable even with a gas tarflet~

b Charges to a deutt?riumcas target planned for the near future on the long

electrostatic generator will allow a measurement of the thin target yield

from D(pn)2p uncomplicated by 018 neutrons.

v2351(p.4cr2451

The yield of neutrons from this reaction had previously been

found quite large2)

so further studies were made to determine the yield

near threshold. Cr52 is a negative electron emitter so its threshold is

Iow$ about 1.5 Mev proton energyi because of the large mas6 of the compound

nucleus the energy of the neutrons in the laboratory system just at

threshold should be about 0.6 kev from the oenter of gravity motion. Thus

for only very small energies above threshold the neutrons already are emitted

at 811 angles, the energy spread with mgle is small and the yield aa a

1] i)ulhidge,Barnes. Buck and Strain - Phys. Rev. ~$ 447 (1938)

2) UsipublishedWisconsin measurements

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function of angle is not strongly concentrated forward due-to .

the center of

gravity motion.

In Fig. 2 is plotted the thick tar~et

action in terms of neutrons detected by the long

yield curve for this re.

counter (sensitivity

009 x 10-5) at 51 cm per proton ”incidenton a piece of vanadium !Retalo If

the threqhold is ~~tabout I,Y Mev, then at I-.6 Mev the tarfietwill be 100 kev

.27Cm2 thick and the ayerage cros~ section near 106 Kevwill be~l~b =0.8 x 10 8

if one assumes that the yield + the Iuboratory system is roughly spherically

symmetrioo

$inco

thi~ reaction no

due to the small

the long elec%ostatio generator vaa used in the study of

very detailed study of the yield just at threshold was made

proton currents available. 1% appears possible~ however,

to utilize *he larger

accurately locate the

Even with the present

of the Li(pn) and the

which is the point in

ourrent6 from the short electrostatic generator to

threshold and study the yield here in more detail.

inforrmtion it is possible to consider a comparison

V(pn) reaction~s yields at the same neutron energies,

question. Thus under typical conditions in short

JanIs,(Li{p$n) source, fission measurements on rjkv neutrone at 120°6 a

~.~ kv target was used and a long counter of 2.1 times the sensitivity of

tho one used in the present experiment. At a distance of 79 cm from tho

target fjx 105 poouls of protons gave 7761 long counter

reduoed to the conditions in the present

‘7$

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X 7761

=0.0177 Lc

5X105x 2.1

experiment one

CoURt6/~oU~

counts. If this is

,

obtaine

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vhich is to be oompared with the lowF6t value here obtained oi’about 0@02 LC

oountx/ pcoul or roughly a factor of 90 Under’the experimental conditions of

this fission measurement target thioknass and angle subtended by the 9/16”

foil a$ 1-1/2” cauoed the energy from the Li7(pn)Be7 to vary from about 2.5 kv

to about 705 kv across the foil. If one now cdnsid~rs ths.twhen one ha6

5 lcvneutrons in the forward direction froma V(p,n) source thq neutrons at

90° have 205 kev energy it will be seen that the detecting foil can easilybe

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moved close enough to the targat for the approximately ~ variation in

yield to compensate, ~r better, for the factor of 9’&bove and still have at

least as goodan energy resolutions Li(p,n)~ Another advantage is that no

high energy neutrons emerge from the reaction to produce counts from backScattering qpd qpithermalsa Disadvm~tages are that most of the energy spread

will come from target thickness

pf the protcme,

generator

Thin

will be used to

short tank, and

which9 hoiveverB

‘argeks ‘f ‘2*9

and the nqces6ity ~or

seems fcaqible oq the

have Deen prepared by

very good energy control

short electrostatic

Corporal Miller which

study the reaction ylald near thre6hold moro carefully on the

to determine the protionenergy at which the neutrons are no

longer monoenergetioo

Since protons on vanadi~lm~ave a very low yield near threbhold

,

due to t~ him barrier for 1.5 ?ds!vprotons$ ;he reaction S02145 (pn)Ti=45

I

was casidered for the follawing reaeona: It i6 a single isotope, the product

nucleus is a positron emitter such that the threphold should be near j~()WV

‘T proton energy, tihebarrier is slight y lower ”due to the lower charges and the

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mass is still sufficiently hi[~hto give low energy neutrons near threshold, i.e.s

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1.4 b at threshold. The Srinci.i>alra~son for choosing scandium is that at

the high threshold cnargy the protons are up to about 50 per cent of the top

of the 6 Mev barrier maicin~the barrier penetration rough’iy100 ti!neeas large

at -thresholdas fc}rthe Y(pn) reaction- This would mdce it tippctira much

better source of low energy noutron6 than vanadiumo

The targets were made by heating scandium oxalate on quartz in an

oxygen flame to form fairly uniform fused So O O? the surface. 23 Ueasuremerlts

of the yield in the forward direction were made ‘with the same long counter

previously used at 50 cm. The-yield curves shown ~n Fig. 3 were obtain6d

with the LC at 50 cm and with fission foil @B at 106 cm. It will be seen

that theIlong counter data has a rather slowly decreasing yield down to

2.6 E4ev,whereas the ~is~ion foil shows a sharp thre6hold at ab~at 2095 Mev.

It ?3Qemevery likely that 018(pn) is causing the tuil in ttL6 loag cOunter

meaeuremente sinae it is approximately uniformly sensitive to the high

energy neutrons from 019 and to the low energy ones fr’omSc~ in the fission

foil measuremen’tsnhowever, the Sc neutrons are much more heavily weighted

than the fast ones from O

18

~ thus giving a more nearly correct value for

the threshold and shape of the yield curve of SC. Background measurements

with the proton beam falling on a quartz platieindicate rhat the low energy

tail iG due to 018 but that at 3.05 Mev proton energy the oxygsn yield is

only about 5 per cent of the obser-redyield from Sc O

2 3* The yield from

SC203 is about 17 times that for metallic vauadium; reducing this to Ss

metal, the yield is roughly ~0 timo6 the yield.I’romvanadium and Lhe cros8

section about 3 x 10 .26 2

cm at 3.C.)~Mev proton energy. Thin targets of

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SCF5 on Ta have been prepared by Corporal Mil]er which will allow more accurate

detertiation of the cross section without confusion by tho neutrons from O1ti.

Tho u80 of So as a tar~et material for low energy neutrons depends

in part upon being able to obtain tr!i.ntargets with a maximum amount of metal

and no oxygen~ upon obtaining

generator and upon a study of

%0 the firsk excitation lerOl

good energy control of the Iong electrostatic

energy distribution of the emerging neutrons up

above threshold. It should be possible to

determine the bombarding energy at which the neutrons art?no longer monoenergetic by ob8erving the ratio of fission6 ‘~ hydrogen.recoils as a function

of energy~ the appearance of a slow group should fliverise to a marked change

in ratio if its intensity iG at all appreciable compared to the main groupo

Th9 Iarflecross section of 018(pn)F18 would make it appear inter.

esting a6 a target material if it were not for its low e.bundunce(about

0.5 per cent). The energy of the neutrons at threshold in this case is about

6kv and they are liksly to remain monoener~etic over a ]ar~er energy interval

than from the heavier e16mentr3Sc and V. A rough comparison of [p,n) cross

sections g~vee

D(pn)2p< 2 x 10°29cm2

.27 ~m2 V51(pn)Cr51~ 10

Se45(pn)Ti4~ j x 10=2Gcm2

018(pn)F18#10-25 CIIA2

Li7(p,n)Be7 3 x 10-25cm2

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