NEUTRON AND DEUTERON YIELDS

FROM ¹⁶ 0 + ¹⁶ 0

.01

.001 ____

^~

____

^~

__

^~

____

^~

__

^~

____

^~

__

^~

7 8 9 10 II 12 (MeV)

eM ENERGY

- 139 -

losses in the target foils. Errors on all three sets of cross sections consist of:

A) Errors on the beta spectrum corrections B) Tirning correction uncertainties

C) Efficiency correction errors

D) Meas urement errors on the monito r solid angle

E) Elastic scattering cross section errors F) Statistical uncertainty on NMON

G) Errors on background subtraction from an incorrect arnount of beam during the background run or errors resulting frorn changes in bearn intensity during the run caus ing a change in equilibriurn concentrations of 30p or 31 8 (estirnated)

H) Least squares fit uncertainties for . NCOUNT8

:I: 10 - 20 %

± 12 %

:I: 10 %

:I: 5%

:I:

2%

<3%

:I: 5 - 20

%

At higher energies the errors are generally larger for 31 8 than for 30 p because it was difficult to separate the 31

8 and 278i in the decay curves. The yield of " 30 P" did not change as a function of discrbninator cutoff above E

=

9 MeV, so it was concluded that

cm

there was no contaminant activity with a halflife of about 2 to 3 minutes with a sizeable yield cornpared to the actual 30p _{yield at} these energies. The longer lived activity was studied with very high discriminator cutoffs, as well as the usual cutoffs, and with long bombardrnent (T ^0< 3 min) and counting times (T

=

10 rnin) at

o c

E

=

8.45 and 7.95 MeV.

crn At the former energy the cross sections for forming 30 P agreed at larger cutoffs. but increased with lower

cutoffs, thus indicating the presence of a contaminant activity with a 2 to 3 minute halflife but lower

f3

endpoint energy (such as 15 0 ).

At E = 7.95 MeV only a limit on the crOBS section could be cm

obtained (8 ee Table 11). The 31 S eros B sections did not exhibit variations with discriminator cutoff. Average values of the cross sections at each energy are also given in Table 11. Note that the 30 P crOBS section falls much faster with decreasing energy than the 31 S cross section.

- 141 -

NEUTRONS

Introduction

The cross sections for the production of 30 P as measured by counting

P

particles. and for production of deuterons as

measured with the counter telescope, differ by a factor of 11 at E

=

12 MeV and a factor of

9

at 10 MeV. The difference is

cm

attributed to three body reactions. and as a check, a measurement of the neutron yield was performed at E _cm

=

^{12 MeV.}

Dis regarding exit channels with large negative Q values on the basis of Coulomb barriers to be penetrated, the most important modes of neutron production should be

__ 30 p

+

p

+

n

-- 27 Si

+

a

+

n

The '( ray spectra taken indicated that other exit channels are negligible and that the 27 Si yield was probably smaller than the 31 S yield. Since a certain amount of Carbon contamination cannot

16 12 27

be avoided when using foil targets ^t the reaction 0

+

C - 8i

+

n must also be taken into account. The activation data showed that Carbon buildup on the target was sufficiently slow to keep the neutron yield from this reaction lower than the yield from 31 8

+

n at

E

=

¹² MeV for many hours. In addition, a liquid Nitrogen trap cm

was used at the entrance to the target chamber to reduce Carbon buildup.

If the cross section determinations from either the radio- activity measurements or from the deuteron (charged particle) data

30 30

were incorrect, and all P were formed in the P

+

d exit channel, then the nuznber of neutrons would be at znost 2 or 3 tiznes the nUInber of 31S forzned at E = 12 MeV. However, if 30p

czn

16 16

was produced znai.n1y by 0 ( O,pn) the nUInber of neutrons should be 11 to 13 tiznes the 31 S yield. Even with the usual uncertainties

in measuring absolute neutron cross sections. the large factor between the two alternatives made the experiment feasible. It was concluded frozn the results of the neutron zneasureznents that both eros s section deterzninations are consistent and that 30 P is znainly produced by three body breakups.

Experiznental

A SiO foU target. siznilar to those eznployed in the activation zneasureznents. was boznbarded with a 24 MeV

16

0 beazn. The target thickness was 210 ± 60 keV (lab) (see the Activation Method Section, page 112). The low detector efficiency and difficulties in localizing the target region for taking angular distributions prevented the use of the gas target. The target chaznber was 10 czn I. D. with 3.2 mm thick brass walls. As usual, the cross section was nor- znalized to the Mott scattering at Slab = 45⁰ using a znonitor counter with solid angle

- 143 -

dn -4

MON

=

^{(3.6 ±} 0.3) X 10 sr.

This value was obtained from purely geometrical measurelnents.

A correction for the finite beam spot size « 2 rnrn square) to the monitor counts was estimated to be ~ 10

%.

Precautions were taken to reduce the neutron background as much as possible. In addition to shields of boron loaded paraffl.n and cadmium against neutrons produced upstream from the target chamber, there were no slits or collimators near the target. Instead, the beam was first focused on slits 10 m from the target.. Passing through a magnetic quadrupole about 4.7 m from the target, the beam was then focused to a 1 X magnified image on a piece of quartz at the back of the chamber. The beam stop during the actual measure- ments was not the quartz, but a piece of tantalum with a Jayer of gold evaporated on it.

The choice of detectors was limited by the large y flux. A standard long counter, similar to the shielded counter described by Hanson and McKibben (1947), was chosen because of its low y sensi- tivity and fairly flat neutron response (see Figure 36). The angular distribution of the neutrons was taken at a constant distanc e of 26 cm from the target to the front face of the long counter's inner wax

cylinder.

The neutron counter efficiency was determined with a "cali- brated" (to about ± 10%) Pu-a-Be source in the place of the target.

- 144 -

Figure

36

"Long Counter" Response and the Predicted Neutron Spectrum.

The response of a standard "long counter ^II for neutron

energies up to En

= 9

MeV was taken from unpublished data in Allen (1960). An extrapolation up to E = 18 MeV was made

n

using an equation fitted to this data (see page 150). The neutron spectrum calculated on the basis of a compound nucleus model is also shown (see the text page 150). Both were used to estj~ate a correction to the measured neutron production cross section to allow for a non-flat response for the "long counter".

>-

u

z

w

U I.J...

I.J...

W

W

>

- 145 -

LONG COUNTER RESPONSE

- _- --- _{- ---}

- - - _{- -}

~

0.5 w -.J 0:::

w

>

i= «

...J

w Q:

TRUE EFFICIENCY RELATIVE TO EFFICIENCY FROM A

Pu - a - Be SOURCE

°O~---~5---1~0---~15~---~20 Eneutron (MeV)