Isotopic assignment of slow neutron resonances in germanium, rubidium, neodymium and tungsten

Nuclear Physics 3 (1957) 553--560; North-Holland Publishing Co., Amsterdam

ISOTOPIC A S S I G N M E N T OF SLOW N E U T R O N R E S O N A N C E S IN GERMANIUM, RUBIDIUM, N E O D Y M I U M AND T U N G S T E N E L E A N O R M. B O W E Y Atomic Energy Research Establishment, Nuclear Physics Division, Harwell near Didcot (Berks.) Received 20 F e b r u a r y 1957

The uses of a scintillation detector in time-of-flight experiments with the 15 MeV Linear accelerator at Harwell are described more fully elsewhere 1). The detector measured the yield of gamma rays due to neutron capture in a sample placed in the neutron beam, and it was shown t h a t even when only very small samples of the capturing material are available this provides a sensitive method for the detection of neutron resonances. In order to make a detailed determination of resonance parameters, it is necessary to know the isotopic assignment of the resonances. For the determination of the positions of resonances in small samples the capture gamma ray detector is particularly suitable, since it gives rise to positive peaks in a yield curve rather than small dips as in a transmission experiment. In the measurements described below the isotopic samples were all mounted in thin walled aluminium cans 1" in diameter. The neutron beam was collimated and had a diameter of 1" at the sample position. The sample was suspended normal to the axis of the beam in the centre of a 2" diameter hole through a 6" cubical aluminium box, containing a mixture of B4C chips and water. Sodium iodide crystal detectors were recessed into the faces of this cube not traversed by the neutron beam so t h a t these surrounded the sample. The minimum thickness of B4C + water shielding between the sample and the crystal was ~ inch. The whole assembly was surrounded by lead shielding to reduce the background, with an outer shield of boron and wax (see fig. 1). The output of the photomultipliers (up to four in number -- E.M.I. type 6260) were connected in parallel to the input of the main amplifier, and thence through a discriminator to the timing channels. The bias level of the discriminator was set to correspond to an energy dissipation of about 100 keV in the crystals. In this condition the detector responds not only to gamma rays due to neutron capture in the sample, but also to those due to neutron capture in the B4C from neutrons scattered by the sample. Thus scattering resonances can also be identified, though with a lower efficiency than for the detection of capture rdsonances. The probability 553

5~4

ELEANOR M. EOWEY

of detection of a neutron captured in the sample is of the order of 30 %, and the background counting rate is sufficiently low to enable even weak resonances to be identified quickly with such small samples. In most cases the counting time per isotope was only 1½--2 hours. The resolution of the apparatus was 0.17 #sec/metre; and the neutron burst had a duration of

NEUTRON BEAM

END

II

I

~

I:'VACUATI[D & COLLIMATED FklGHT PATH

L r A D sHIrLDINQ c PHOTOMULTIPLIER

,,TAL

Fig. 1

1/~sec, the timing channels being 2/~sec wide. The recurrence frequency of the spectrometer is 400 cycles/sec. The separated isotopes of the elements Germanium, Rubidium, Neodymium and Tungsten have been studied, and the results are shown as gamma ray yield curves. In every case the yield is defined as the number of counts recorded in a given channel times the delay in/~sec of that channel per 50 000 monitor counts. The multiplication b y delay time is to eliminate the effect of the neutron spectrum from the accelerator, which is approximately inversely proportional to delay. The monitor counts are those recorded from a B F a counter continuously monitoring the neutron output of the linear accelerator. GERMANIUM

Samples of the oxide GeO~ were used. With this element an attempt was made to define the resonances better b y repeating the experiment with the initial delay increased b y 1/~sec, so that the two runs on each isotope interlaced, and points were obtained at 1/~sec instead of 2/~sec intervals. These runs are plotted on an expanded scale, b u t it is obvious that the resolution is still not good enough to separate the higher energy resonances which are probably due to two or more isotopes in some cases.

ISOTOPIC ASSIGNMENT OF SLOW NEUTRON RESONANCES

~'~A I

55~

'70"Ogmgcm' I

I

I

I

I

~

=

,

J

~

61~°

n_rl_

0

G¢ -72

~

,

1 '°

,ool

C~-73

33.40mgcm2

_nil° 60

/O

I

I0 ~0 lO !0

~-76

S

li Do

I

i

2

Iobo ~ o

4

sbo

I

6

sOo

2bo

e~ tdo

?~

~n_N see/metre "1072?s

~

eV

Fig. 2. Yields of ~-rays from n e u t r o n capture i n G e r m a n i u m samples. The h i s t o g r a m s show t h e sample composi~on in % m g / c m !

556

ELEANOR M. BOWEY TABLE 1

(fig. 2)

O b s e r v e d E n e r g y in eV

M a s s n u m b e r of I s o t o p e

1 0 0 . 6 ± 2.8 111.1± 3.3 199.64- 7.6 221.74- 9.0 3 1 5 . 2 + 14.0 3 5 1 . 7 ± 18 471.04-4- 26 524.44-4- 31 7 5 3 . 0 t 53 1000.04- 80 1039.04- 85 1457.04-140

73 70 73 73 73 73, 74 73 76 76 76 70 70

RUBIDIUM

Rubidium chloride samples were used. i

Ru-B5

BO

70 5 mg cm a

60

4O ZO I

I

I

I

•

Ru-87

O0

30.25 mg crn=

$0 40 20

n

I

i

!

I

I

2

4

6

8

I0

I

I000

~usec/metre

I

n

•

S00

200

I00

i

cV

Fig. 3. Yields of 7 - r a y s f r o m n e u t r o n c a p t u r e b y R u b i d i u m .

85

0 87

ISOTOPIC ASSIGNMENT OF SLOW NEUTRON RESONANCES TABLE 2

557

(fig. 3)

O b s e r v e d E n e r g y in eV

Mass n u m b e r of I s o t o p e

37O4- 19 5204- 30 1420~136

87 85 85

NEODYMIUM

Neodymium oxide samples were used. With this element there was insufficient time available to do the experiment with interlaced runs, so only those resonances already reported b y the Brookhaven Laboratory 2) have been assigned. It is hoped to repeat the experiment with improved resolution with the 25 MeV linear accelerator at a later date. TABLE 3

(fig. 4)

O b s e r v e d E n e r g y in eV

Mass n u m b e r of I s o t o p e

43.54- 0.9 56 4- 1.2 83 ~: 2.2 105 4- 2.9 128 -t- 4.1 158 4- 5.4 180 4- 6.5 236 4- 9.4 290 4-12.8 375 ± 1 8 . 7

145 t 143 ¢ 145 145 143 143, 148 143 145, 150 148 150

t Already assigned by Brookhaven Laboratory. TUNGSTEN

Tungsten trioxide samples were used. The resonances already assigned b y Brookhaven workers were confirmed 2). It was only possible to do interlaced runs on two isotopes Tungsten 183 and Tungsten 184. TABLE 4

(fig. 5)

O b s e r v e d E n e r g y in eV

M a s s n u m b e r of I s o t o p e

1 8 . 7 ± 0.3 21.14- 0.32 27.04- 0.5 40.04- 0.76 4 6 . 0 3 ± 0.92 100.6 4- 2.8 112.5 4- 3.4 155.8 4- 5.5 180.4 -4- 6.5 219.7 4- 8.8 259.8 4-11.2

186 182 183 183 183 183 182 183 184 186 182

558

5:

ELEANOR

M. BOWEY

i''

+"'1+

f.o

10

.

.

.

Nd-143

.

211.sSm~,¢.~

80 ,

60

5

\,,+ i

i

•

.

.

.

.

.

.

.

Nd-145

+o..,,,~I

i

i

.

'~++'-+, ~, _ . _ ~ o

10

,

,~ S

"

~

L

+

~ ~ ~

~

"

+,

10.99.V~m21

i

l

l

I

I

I

I

I

20 ~

--

~

''

--

40 ~

&

S .

12"lSr~cm2

i 2

i ~ i

,ooo s~o

i 6

a~o

i 8 I

ioo

i 10 i

so

i 12

i 14

i ,ll 16 tp ,xJ s e e / m e t . r e I

=s

ev

142 I'14 146 IS0 143 14S 146

'

Fig. 4. Yields of y-rays from neutron capture by Neodymium.

ISOTOPIC ASSIGNMENT OF SLOW NEUTRON RESONANCES

559

IC

i

W-183 IS

i

i

i

l

~1 r~ r ~ j o

I

l

a

92 .$9 mll/cm

60 40 $ I

tS

i

i

i

I

t

r'~

~"~

W-184 127-16 m!l/cm 2

I

I0

60 40 ~; i

,o

I

i

i

i

i

20 0

i

W-186 9S.41mg/¢m 2

4@

2 ll

i ,

Iooo ~

i

:~o

.u Sec/me~re

i~o

SiO

F i g . 5. Y i e l d s o f 7 - r a y s f r o m n e u t r o n

2~

eV

capture by Tungsten.

(m

1015

5~0

E L E A N O R M, B O W E Y

I would like to thank Dr. E. R. Rae for his continued interest and advice and also Dr. M. L. Smith, of the E. M. Separator Group at Harwell, for the loan of Isotopic samples. References 1) E. R. Rae and E. M. Bowey, Journal of Nuclear Energy 4 (1957) 179 2) D. J. Hughes and J. A. Harvey, Neutron Cross Sections, BNL 325