Discussien

105- CTA

5. Discussien

It was anticipated that the reactien N15 (He 3 , a ) N14 weuld be capable .of pepulating mest if net alIef the levels

in N14 belew abeut 9 Nev excitatien. This hepe was cenfirmed by the fact that seven well-substantiated states leeked fer between 5 and 8.1 Nev

wer~

all clearly visible. In particular, the excitatien range 7-8.1 Mev in N14 was clesely examined

in an' effert te discover the excitatien of pes sible states lying between the knewn levels at 7.03 and 7.97 Mev. Ne evi- dence fer such a state was feund. This negative result inten- sifies the deubt expressed earlier with regard te the incen-

clusive evidence .of ether investigaters. It is always difficult te make positive statements from a negative experiment.

.An

upper limit .of 5

~b/steradian

fer the differential cress sec- tien ef,the reactien N~5' (He 3, a) N14 te any unebserved level in this range may be queted. This upper limit is abeut 10%

.of the .observed group yields. One pessible yield .of such a

-134-

magnitude may be pointed out at Vm =

^.L~4

in figure 24.

This one has been ignored due to a lack of corroborating evidence at 90

^{0 •}

It should be pointed out that the reso- lution employed in this eXperiment demands that any real par- ticle group show a relative yield increase for at least three consecutive spectrometer settings. The experiment could have been improved somewhat by longer bombardments to improve the statistics. It would have been difficult to push the limit of observability below

5~b/steradian,

however, since the

corresponding yield is only about 20% of the continuUll1 back- . ground of alphas.

It should be mentioned that time limitations prohibited an auxiliary investigation of the possibility of strong reso- nant enhancement of the observed yields through possible states

in the compound Fl8 nucleus. Observing the yields of known particle groups as a function of bombarding energy could have provided this irtformation. If the particle yields were found to be strongly dependent on the incident energy, the possi- bility of having overlooked states by using only one bombarding energyiZ, would be greater.

The question of whether this reaction proceeds strongly through compound nucleus formation is not a matter of idle curiosity. By so doing, it could populate two-particle exci-

tatio~ states in N14. The single-particle excitation operators

{}The thickne-ss of the TiN target layer gives the inoi-

dent He3 berum an energy spread of about 20 kev.

-135-

believed to be dominant for inelastic scattering processes and the possible dominance of the stripping mechanism for

(d,n) reactions would lead one to expect that the three re- actions Nl 4

^{(a,a/ )} N

I 4-::·,

N

1 4 (p,

p') N

I 4*, and Cl3 (d,n)

N

1 4*

. will

popula~e

preferentially single-particle

~xcited

states of N ¹ 4. The N ¹ 4 states at 7.03, 7.97, and 8.06 are all

largely single-particle excited states, belonging to the con- figurations s4pIO, s4p9d , and s4p9 (2s) respectively. That these states are the ones well agreed upon by these reactions is therefore not surprising. The state at 6.44 Mev exci-

tation is very strongly populated in both inelastic scatterings and is probably. also a member of s4p9 d (contingent upon

J7T

= 3-). On the other hand, the fact that the 6.23 Mev level is populated only weakly by (p,

^p')

and not at all by

(a,

a

I)

agrees with the configuration s4p8 d (2s) given to it by

Warburton and Pinkston

(41)

on the basis of gamma ray tran-

sitio~

strengths.

These considerations place fUrther doubt on the

^N

1 4 level

at 7.60 Mev reported by Burge and Prowse (37). The intensity

of this inelastic proton group is at least twice as strong as

is that from the .7.03 Mev state, implying that the 7.60 !1ev

state should also be a single-particle excitation. That such

a state does not appear at all in inelastic alpha scattering

could only be easily eXplained by a T = I assignment. But

the lack of any state in c ¹ 4 below 6 Mev excitation is strong

-136-

evidence for the lack of T = ¹ states in N14 below 8 Mev*

(with'the exception of the 2.31 Mev state in N1 4 which is the C 14 ground state).

It seems more likely that the lack of any other visible states in the 7-8 Mev range for N1 4 from inelastic alpha scattering indicates the lack of any other single-particle excited states in this range. vlarburton and Pinkston

(41)

point out the possibility of positive parity states in this excitation range belonging to s4p8 (d,s). Such a state,

al-

though it would certainly possess a proton width much smaller than the single-particle limit, could still provide an ef- fective resonance in C13 (p,

y)

N1 4 at stellar energies.

The reaction N15 (He 3, a ) N14~~ can populate two-particle excited states if it proceeds by compound nucleus formation.

It has aiready been pointed out that the' 7 .03 Mev level is the only one that can be populated by neutron pick-up. The yields to the other excited states in N1 4 f;om this reaction would therefore be substantial evidence of the operation of the compound nucle'u.s mechanism. The negative result from this reaction for states in the 7.03 to 7.97 Mev range of exci-

tation has, therefore, a stronger meaning than the negative ( a, a ) results.

I

*The 6.09 Mev state in c14 is believed to be the analog

of the 8.06 Nev level in N14.

-137-

It may be stated in conclusion that a possible reso-

nance inC1 J (p, r ) N1 4 at stellar energies is quite unlikely.

This result means that the current calculations on the c ¹² /c ¹ 3

abundance ratio produced in the CNO-cycle are correct.

-138a-

Figure 21: Evidence for possible resonances in

c ¹³ I p at stellar energies is shown with the energy level diagram of ~4. Nuclear reactions providing evidence for states in N 14 _{in the} 6-8 Hev range of eAcitation are indicated. The pertinent reactions are N14 (a,a' )N14·:~, N14(p,p' )N14~~, C13 (d,n) N1 4{4-. C13(p,r)N14,

and the present experiment, N15(He3,a)N14~!-. The

eff·ective stellar energy range .for c ¹³ I p is shown

by a horizontal band.

. a, a' (22M ev) STELLAR REGION

3.9 8.06 I-,T

=

I 7.97

2~T=0

J/ ~\\"7.72j· I / 7.60

VZZ/lZ/7Z7771lUll~

~

/18 \\\

(7.50)=1 7.40 if! 7.03 I. /1 (6.60) .1/ Ik.46 16.~3 5.325

13

7.03 (2},T=0 6.44

3~T=

0

~

6.23 I, T = 0 6.23-1-;1----....:..--- C +d-n N I4 14

I

N + X-X

2.8

-.,..-

9.743

f5 3

. N +He-a ~0.55 0.45 7.549

--'-- C13

+p

I ()I (l)

-131.)&-

Figure 22: A typical pulse-height spectrum of the reaction products from the bombardment of the TiN target Hith 500 fLcoul. of He 3 ions. After momentum analysis the particles pass through a 10- 4 cm. Nickel foil before detection in the solid-state particle counter. The proton energy is 3.3 }:ev. The signal from the counter was stored in a laO-channel pulse-height analyzer after

conventional amplification.

-139-

---~---~~---~ ~

o

! ,

_,

'He ³ +,

, and

!

Noise

I I

••••

_•

...

_•

• ·4

. .

I I

a

l

p

!

-

o

0

-

⁰ _~

fi -

^{0 q-}

t

-

0 f'{)

A =

0

C\I

0

I •• ••• ·.1 ••••• J.l\~ ^{I \} f ^I ~.

10 20 30 40 50 60 70 80 90 100