Statistical model analysis of (n, α) reactions

N,~clea~ Physics 51 (1964) 460---469; (~) North-Holland Publishing Co., Amsterdam Not to

be reproduced by photoprint or microfilm without written permission from the publisher

S T A T I S T I C A L M O D E L ANALYSIS OF (n, ~) R E A C T I O N S

01D. Cross-Sections of (n, ~) Reactions Induced by 14 MeV Neutrons U. FACCHINI, E. SAETTA-MENICHELLA, F. TONOLINI and L. TONOLINI-SEVERGNIN I

Laboratori C.I.S.E., Milano and lstituto di Fisica dell' UniversitY, Milano t

Received 17 April 1963 Abstract: The cross-sections of (n, ~) reactions induced by 14 MeV neutrons on nuclei ranging from sodium to uranium, are computed by the statistical model. For nuclei up to A ~ 80 the calculated and experimental a(n, ~) are in good agreement, while in the region between the complete neutron shells the ratio a(n, g)exp/a(n, 00theor may even be greater than 100. It is interesting to remark that the statistical model explains the high values of the cross sections corresponding to the magic neutron shells.

1. Introduction It has been shown in part I (hereafter referred to as (I)) how the statistical model m a y explain the excitation functions, the energy spectra and the angular distributions o f the a-particles emitted in (n, a) reactions for the nuclei of mass number up to A .~80. In this second part we want to calculate the values of the (n, ~) cross-sections at 14 MeV which have been measured for about 70 nuclei, from oxygen to uranium; in some eases the results are very uncertain, and more data would be necessary for a more complete analysis.

2. Experimental Data The (n, ~) total cross-sections have been measured for neutron energies around 14 MeV for about 70 isotopes, mostly by the activation method. The measurements of the (n, ~) cross-sections obtained by the direct method, i.e., from the spectra, are few, and they should be considered only when they are not contaminated by multiple reactions. Fig. 1 shows the (n, ~) total cross section as a function of the neutron number N. Table 1 lists the values o f the experimental cross-sections together with the Q values of the reactions, The general trend is that the cross-sections decrease with increasing A (and therefore increasing Z ) from values of the order of 100 m b down to values of the order of 1 rob. However, values of cross-sections higher even by an order of magnitude than those of neighbouring nuclei are observed in the regions 85 < A < 90 * This work has been supported by the "Consigiio Nazionale delle Rieerche". 460

461

STATISTICAL MODEL ANALYSIS OF (o., oc) REACTIONS (II)

and 140 < A < 150. For these nuclei, the high value of the cross-section is not always associated with a high Q value, as may be seen in table I. It must be observed that the anomalous nuclei Br Hi, Rb 85, Rb a7, Sr as and y a 9 have a number o f neutrons equal to or close to 50, while Ce ~4°, Ce 142, La 139 and N d ~42, have a number o f neutrons close or equal to 82. The measurements for nuclei with a number o f neutrons about N = 126 do not show particularly high values o f the cross-sections; however, the values o f the cross-section in this region a~e all very small. ~(n,~)exp

mb

1o(3~' ~I~i ! ~

I !

,(3 2~

3b

4112

510

I 60

I ~ 70 8(3

I g0

I 100

I '10

'20

130

d0-

m

Fig. 1. The experimental cross-sections a(n, ~) at 14 MeV plotted versus the neutron number N.

4) b) e) a)

p. G. Bizzeti, A. M. Bizzeti-Sona and M. Bocciolini, Nuclear Physics 36 (1961) 38 M. J. Nurmia and R. W. Fink, Nuclear Physics 8 (1958) 139 G. S. Mani, G. J. McCallum and A. T. G. Ferguson, Nuclear Physics 19 (1960) 535 M. Bormann, S. Cierjacks, E. Fretwtmst, K. J. Gieseke, H. Neuert and H. Pollehn, private communication e) L. Allen, Jr., W. A. Biggers, R. T. Prestwood and R. K. Smith, Phys. Rev. 107 (1957) 1363 t) R. W. Fink and R. S. Scanlan, Nuclear Physics 9 (1959) 334 g) E. B. Paul and R.L. Clarke, Can. J. Phys. 31 (1952) 267 h) B. P. Bayrust and R. J. Prestwood, LA-2493 (1961) l) M. Hillman, Nuclear Physics 37 (1962) 80 1) M. Bormann, S. Cierjacks, R. Langkau, H. Neuert and H. Pollehn, J. Phys. et Rad. 22 (1961) 602 m) W. G. Cross and R. L. Clarke, PR-P-53 (1962) n) j. Kantele and D. G. Gardner, Nuclear Physics 35 (1962) 353 o) p. Strohal, N. Cindro and B. Eman, Nuclear Physics 30 (1962) 49 P) H. G. Blosser, C. O. Goodman and T. H. Handley, Phys. Rev. 110 (1957) 531 q) R. F. Coleman, B. E. Hawker, L. P. O'Connor and J. L. Perkin, Proc. Phys. Soc. 73 (1959) 225 r) B. G. Dzantiev, V. N. Levkovskii, A. D. Maliewskii, Doklady Soviet Phys. 2 (1958) 135 8) R. G. Wille and R. W. Fink, Phys. Rev. 118 (1959) 242 t) A. Poularikas and R. W. Fink, Phys. Rev. 115 (1959) 989 The values o f the cross-sections, when chosen from those reported by various authors in agreement between themselves, are those with the smallest error and of more recent data; if in disagreement, those agreed upon by the majority of the authors. A few cases were eliminated, in which the above questions could not be decided, or when ~r(n, ct) were obtained from spectra contaminated by multiple reactions.

U. FACCHINI

462

et

al.

TABLE 1 Experimental a n d calculated (n, u) cross-sections N Target neutron nucleus n u m b e r

°'(n'°~)exp

Ref.

(mb)

Q.,~ (MeV)

"/[n,~t

"dn, nl

tY(rt, 0~)e&le

°'(rl'tX)exl a

(MeV)

(MeV)

(rob)

tr(n, ~t)e~le

N a ta

12

1474-20

a)

--3.86

0

--2.48

137

0.94--1.2

Mg 26

14

894-5

b)

--5.42

--2.25

--4.27

77

1.09--1.2

A1~¢

14

1175:5

c)

--3.14

0

--2.17

125

0.92--1.08

psx

16

1194-14

d)

--1.94

0

--1.74

115

0.91--1.16

S ~'

18

1265:7

e)

--1.32

--2.13

--3.29

123

0.97--1.08

C1s~

20

445:7

t)

--1.29

0

--1.72

75

0.5--0.7

K 41

22

31 "4-l0

g)

0.098

0

1.63

71

0.3 --0.6

S c 46

24

554-3

h)

--0.407

0

--1.41

52

1 --1.1

Ti 5°

28

105:2

l)

--3.41

--1.51

--3.2

V sl

28

434-4

1)

--2.04

0

--1.47

29

Fe 6~

28

955:10

m)

0.84

--1.44

--2.92

86

1--1.22

M n s5

30

525:8

g)

--0.64

0

--1.32

44

1 --1.3

Co 69

32

284- 2

1)

0.31

0

-- 1.46

27

1 -- 1.1

C u 6a

34

235:3

n)

1.72

0

--1.44

40

0.5--0.65

C u e5

36

7.55:2

u)

--0.09

0

--1.46

10.9

0.5--0.87

A s ~5

42

9.85:0.7

h)

1.5

0

--1.47

9.2

1--1.14

Br TM

44

134-3.2

4)

1.86

0

--1.45

12

1.08--1.35

Br el

46

1075:20

o)

0.61

0

--1.28

5

17.4--25.4

R b s5

48

142-4-9

o)

0.91

0

--1.23

70

1.9--2.2

R b a~

50

595:12

o)

--1.22

0

--1.27

40

1.17--1.8

Sr as

50

874-31

o)

--0.70

--1.33

--2.47

20

2.8--5.9

Zr az

52

9.54-0.8

~)

3.4

--1.2

--1.62

2

4.3--5.1

N b °a

52

9.34-0.5

h)

4.97

0

--0.61

5.5

1.6--1.78

Zr °4

54

5.24-0.4

h)

2.26

--1.2

--1.76

0.72

7.7--6.6

M o 1°0

58

14-4-6

o)

2.8

--1

--2.38

2

Pd x°a

62

2.34-0.4

P)

2.51

--1.28

--2.74

0.18

10--15.5

A g 109

62

124-2

q)

3.51

0

--1.36

0.9

10--17

Pd aBe

64

144-7

s)

1.32

--1.28

--2.68

3.8" 10 -a

200--700

C d 111

64

2.74-0.2

h)

2.80

--1.38

--2.68

0.15

19.3--16.6

C d 11'

66

0.644-0.2

r)

1.86

--1.38

--2.67

3.5" 10 -z

6.5

1.23--1.84 1.34--1.6

4--10

19--18

STATISTICAL MODEL ANALYSIS OF (n, ~t) REACTIONS (II)

463

TABLE 1 (continued)

Target nucleus

neutronN number

e(n, m)exp (rob)

Ref.

~n, nl (MeV)

. ( n , m)esle (rob)

a(n, ~)e~p cr(n,x)c.xe

In 11s

66

2.9~-0.3

q)

2.75

0

-- 1.29

1.8

1.8-- 1.4

Sn na

68

0.924-0.03

8)

2.08

-- 1.38

--2.56

8.3 • 10 -3

I TM

74

1.394-0.17

a)

4.22

0

--1.20

0.4

3.9--3

Te T M

78

0.374-0.05

q)

0.80

-- 1.32

--2.04

1.6" 10 -a

260--200

Cs Iss

78

P)

4.16

0

--1

0.25

5.2--2.8

La Is'

82

s)

4.66

0

--0.54

1.9

0.7

Ce 14e

82

94-2

s)

5.39

--1.13

--1.75

7.5

1.4--0.9

N d 1'~

82

124-3

s)

6.7

--1.21

--1.97

15

C.e1.2

84

84-2

s)

6.04

--1.13

--1.81

3

3.3--2

N d l~a

88

54-1

e)

5.86

--1.21

--2.18

1

6--4

Sm a~

90

s)

5.30

--1.43

--ZOO

3.4" 10 -3

350--230

Sm T M

92

94-3

B)

3.58

--1.43

--2.12

4.5" 10-'

26000--13000

G d 15e

92

3.2±0.4

q)

6.00

--1.15

--1.96

2.2" 10 -3

163--127

G d leo

96

24-1

s)

4.10

--1.15

--2.04

8" 10 -3

370--120

D y xe~

96

3.6-4-0.4

q)

6.10

--0.99

--1.96

1.5" 10 -3

265--210

D y 164

98

4.54-0.8

s)

5.19

--0.99

--1.97

1.1 • 10 -s

4800--3360

Er lea

100

1.54-0.4

s)

6.28

--0.91

--1.99

2.4. 10 -3

80--45

Er 1~°

102

1±0.2

s)

6.2

--0.91

--1.98

3. 10-3

H f lye

106

24-0.2

q)

7.96

--1

--2.12

0.11

20--16

Re lay

112

0.94±0.14

q)

7.60

0

--0.83

0.23

4.7--3.4

Os 1.0

114 ~

0.574-0.07

q)

6.79

--1.23

--1.59

3.7" 10s

170--130

Ir TM

114

2.44-0.3

q)

8.07

0

--0.74

6.2 • I0 -2

43--33

Pt 1'4

116

q)

7.17

--0.85

--1.64

1.8" 10 -3

86--58

Pt 196

118

0.354-0.1

q)

6.09

--0.85

--1.59

8" 10 -3

5.6--3.1

A u 1'~

118

0.434-0.04

q)

6.89

0

--0.61

0.10

4.7--3.9

H g ~°°

120

1.774-0.34

q)

6.56

--0.98

--1.33

5" 10 -s

420--286

Bi 2°9

126

1.1 4-0.3

t)

9.61

0

--0.81

1

T h ~s°

140

4.64-1.15

q)

9.24

--0.92

--1.48

5" 10 -a

U ~as

146

1.54-0.3

q)

8.3

--0.8

--1.36

1.1 • 10 -~

14-0.3 1.3

104-2

1.34-0.26

Qn, • (MeV)

Jn. • (MeV)

11.4-- 10.7

I--0.6

400--260

1.4--0.8 1150--700 16300--10900

464

u. FACCHINI e t al.

The energy spectra, angular distributions and excitation functions referring to (n, g) reactions for medium and light weight nuclei have been presented and discussed in (I). Here we shah discuss the few available results for the nuclei heavier than Br. The energy spectra of the (n, g) reactions on Ho 165, L u 175, T a l a t and A u 197 have been obtained by means of silicon detectors 2) ; in using a crystal both as target and detector, the energy spectrum of the emitted s-particles in the (n, g) reaction on CsI(T1) at 14 MeV was obtained by Marcazzan et al. 3) and independently by Bormann 4) for neutron energies from 12 to 19 MeV. Recently, Sen 5) has obtained the spectrum from the (n, g) reaction on indium by the nuclear emulsion method. Data on the angular distribution of the emitted s-particles are also rather scarce. Facchini et al. 6) have found that in the case of Au and Ta, the ratio between the forward and backward emitted s-particles is about 5. For (n, g) on In 11s, however, Sen 6) found for the s-particles emitted a symmetrical distribution with respect to 90°. The (n, g) excitation functions for heavy nuclei have been obtained in some cases (Sn and Au) by Bayrust and Prestwood 7) using activation methods, and by M. Bormann et al. 4) for CsI(TI) from measurement of the spectrum. A characteristic of these excitation functions is the increase of the cross-section as a function of neutron energy up to the energy values for which the cross-sections themselves were obtained.

3. Analysis of Energy Spectra Even though the measurements of the s-particles for heavy nuclei are few, they are quite significant 6). /

(U ÷t~l

F n(E)

x

In

F~n(e)- (U~~i)! L ~Occ({£)

x,/,

4~

34

/

2-

/

•_;

x

x/ -3

/ 181 ITB Ta (n,er]Lu

a= 3MeV I

-4

lnllS(n,~,) Agllz

-5

/

X

Q=19 NeV4

-6

1

Fig. 2. Values o f p a r a m e t e r a f r o m energy spectrum o f In, ct) o n T a TM at 14 MeV.

2

3

~(M,V)

Fig. 3. Value o f p a r a m e t e r a f r o m the b a c k w a r d s p e c t r u m o f (n, e) o n In ~15 at 14 MeV.

465

STATISTICAL MODEL ANALYSIS OF (n, ~) REACTIONS (II)

In figs. 2 and 3 the logarithms of the reduced spectra for TalSl(n, a)Lu 17a and InXlS(n, a)Ag 112 are shown as functions of x / ~ - In the first case the curve is not a straight line and the average a value deduced is ~ 3 MeV -1. This value is very small, indicating that in this case the reaction proceeds by a non-evaporative mechanism. Analogous conclusions may be drawn from the (n, ~) reaction induced on CsI(TI) by 12 and 14 MeV neutrons. In the case of InaXS(n, a)Ag 112, however, the deduced a value is ~ 19 MeV - l in good agreement with the one obtained from slow neutrons. The peak position of the spectra also in agreement with the statistical model and the angular distribution o f the emitted a-particles with an energy less than 13 MeV is symmetrical with respect to 90 °. 4. Computation of (n, ~) Cross-Sections at 14 MeV The cross-sections at 14 MeV have been compared with the statistical model predictions, using the formula that gives directly the ratio between the emission of aparticles and the emission of inelastic neutrons

.(n,n,)

_

_

.a., tA+3)

- o

.

.

.

.

.

(1)

Here a(n, ni) represents the total cross-section for inelastic emission of neutrons. It may be computed from experimental data as o-i,-a(n, p ) - a ( n , a), where (ri, is the total inelastic cross-section. The other symbols are defined in (I). For a~, we have used the values of Huizenga et aL 8), while for # ~ we have used the optical model estimates of Campbell et al. 9) as in (I), for nuclei up to Co sg. For heavier nuclei we have used the values 1o) calculated from a square well with a radius r o = 1.4 fm. Test computations were made for light and medium nuclei both with optical model tr~, values and values derived from a square well with r o = 1.4 fm and the results in most cases differ only by 10~o. The Q values of the (n, a) reactions have been obtained from the values of the masses 11, a2), and the pairing energies are those given by Cameron 13). For all nuclei the parameter 6 has been taken equal to 1 MeV. 5. The a Values in the Level Density Formula The values of parameter a used in the formula of the level density are those determined by Erba et aL ~4). In they fig. 4 are plotted versus the neutron number N. The prominent feature of this curve is the presence of shell effects for N = 50, N = 82 and N = 126. Corresponding with N = 50 and N = 126 the values of Z are Z = 40 and Z --- 82, respectively. Generally we must use an interpolation procedure for a values, which becomes quite critical in the shell filling regions, because of the strong variations. In these regions it happens that the value of the parameter a may be differ-

466

u. FACCHrNI et aL

ent in the numerator and the denominator of formula (1) and the emission of gparticles may be particularly favoured or unfavoured relatively to the emission of neutrons. 313.-]

XX~x

x

a (MeV)-X

XX

x o

25"

x

x

x

X~xx X x o

xx xx

x x

x

x x° x

o

x

x

20-

xx x x

o

15o o

~

fO-

x

x

~×x

x

x

x

x

~ X

x

~

o

x

x

xx

o x

x

x

x o

x

Xx

o

~x

x

x

x

x

x x

x

5-

x x Xo Y'o

~;

.

:;o ab .~

~b ~

7'0 io

gb ~o ~o 1~o ~o ~.~o 1~o 1;o ,.~.o..~r.b..

Fig. 4. Values o f 1¢v¢1 density p a r a m e t e r a f r o m resonances for n e u t r o n s ( x ) and from energy spectra (O) plotted versus t h e n e u t r o n n u m b e r N.

6"(n,~.) ~(n,nl)

16

I

I

I

I

I

a ~ ( = a ~ ) M e V -1 Fig. 5. T h e a(n, ~)/a(n, n 1) ratio sensitivity with respect to value o f p a r a m e t e r a. • - - Z == 38, N = 50; x - - Z = 4 9 , N = 66; o - - Z = 5 8 , N = 82, A - - Z = 60, N - - 8 2 .

STATISTICAL MODEL ANALYSIS OF (n, ~t) REACTIONS (II)

467

TABLE 2 Values of parameter Residual nucleus

a

F s°

4.5

N e 2s

Residual nucleus

a u s e d i n (n, ~) c o m p u t a t i o n s

a t 14 M e V

a

Residual nucleus

a

Residual nucleus

G a Ts

13

C d :14

17.5

D y les

23.6

4.5

A s 75

13

C d t15

18.6

D y 16'

22.8

N a ss

4. 5

A s 7e

13

I n 115

16

D y 18s

20.2

H a s4

4.9

A s TM

13

S n 11s

16.4

D y ls~

23.5

M g 26

4

B r ~'

12.6

S n TM

17.5

E r les

21.4

A I 2~

4.9

B r 81

12

S b TM

18

E r 1~°

24

A1 ss

4.6

B r ss

14

T e xs°

17

Y b 1T5

22

Si 81

5

B r s4

14

1ls~

18

H f 1~s

23.5

pal

5

K r a5

14

1la°

17.5

Ta

TM

30.6

pat

5

R b a~

10

C s 1as

17.4

W

TM

23.5

S a4

5.5

R b s7

8

Cs TM

15.5

R e ls7

27

C I s~

6

St"as

8

B a TM

15.5

R C sa

23.5

a ss

6

S r ss

10.5

B a TM

17

O s 1'°

27.7

K 4x

7.2

S r sl

13

L a lss

13.5

O s TM

24

K 4s

7

ygo

10

C C as

13.5

Os l's

25

C a 4v

8

Z r s~

11.8

C e 1~°

12.5

I r TM

23.7

Sc 4~

7

Z r s4

13.5

C e 142

15

Ir i s '

24

Sc 's

8

Z r 9~

15.7

C e t45

25

P t 1s4

23

T i 6°

7

N b 'a

11.8

Nd m

11.5

P t ls6

20

V 51

7.3

M o is°

15.7

Nd m

21

P t xs7

20

V 5s

8.5

R u t°s

16.5

N d 14s

24

A u l°~

20

C r 61

7.5

R u 1°~

16.5

N d 151

23

H g ~°°

19

M n 55

8

R h x°s

17.6

Sm t~

23

Tl~°S

10.3

M n 68

9

P d ~°s

17

S m l~a

23

Bi ~°'

11.2

Fe ~

7

Pd t°'

17.6

Sm~S~

24.5

R a a~7

24.8

Co ~

9

Pd ~°

17.5

Sm

TM

22.5

T h sa°

27.3

C o s°

9

Pd m

17.8

Gd

TM

24.5

T h ~a~

30

C o ~s

9.5

A g t°~

17

G d l~s

21.6

U ~ss

29

C u ~a

9.5

A g tls

19

G d le°

21.4

Cu ~

9.5

Cd ns

17.5

Gd

18.2

TM

a

468

U. FACCm2~ et aL

One point is very interesting: near the magic neutron shell the decrease in a both in the numerator and in the denominator favours the emission of e-particles relatively to neutrons, since the relative reduction of the level density is smaller for higher energies of the outgoing particles. This effect is shown in fig. 5 where the variations of the ratio a(n, e)/a(n, n 0 as a function of a are shown in some typical cases. The a values used for the computations are presented in table 2. They may be compared with the values of Erba et al. shown in fig. 4. 6. Results and Discussion

The experimental and theoretical (n, ~) cross-section values are reported in table 1. We may divide the analysed nuclei into four groups. 1. Nuclei with A < 80. The calculated (n, e) cross-section values of these nuclei are in good agreement with the experimental ones. This shows the validity of the evaporative model for these reactions, in accord with the conclusions of (I). The chosen a parameter values are in excellent agreement with those deduced earlier by Erba et al. ~ ) . 2. Nuclei with N ~ 50 and N ~ 82. As already mentioned, the (n, e) experimental cross-sections for these nuclei are rather high, about 8-10 times as high as those of neighbouring nuclei. The statistical model may explain this effect, as due to the smaller a parameter values in these magic shell regions which favour the e-particle emission. There is good agreement between experimental and calculated cross-sections. An accurate systematic analysis of the energy and angular distributions of the eparticles could tell us, however, whether there is an appreciable contribution of nonevaporative effects to these reactions. 3. Nuclei with 55 < N < 78. For these nuclei, the agreement between the computations and the experimental results is not good; often the experimental results are 5-10 times greater than the theoretical ones. A different choice of values for the parameter a may give better agreement in some cases. We have already seen that the analysis of the energy spectra shows that for CsI strong non-evaporative effects are present, while for indium the e-particle emission may be of evaporative type. In this region we can say that the direct and evaporative effects may be comparable and a more precise systematic analysis of energy spectra and angular distributions must be carried out. 4. Nuclei with N > 82. In this region, th~ (n, e) experimental cross-sections are of the order of 1 mb and the disagreement between the (n, e) calculated cross-sections and the experimental ones is considerable; cr(n, e)e~p./a(n, e)tb.... is greater than 100-1000. Another choice of values for the parameter a around the values of fig. 4 cannot eliminate the disagreement. The energy spectra and the angular distributions in this region indicate direct effects, and therefore it should be concluded that these effects are the principal cause of e-particle emission. In the region N = 126, the small

STATISTICAL MODEL ANALYSIS OF (n, ~) REACTIONS (ll)

469

values of the parameter a produce evaporative emissions comparable with the direct effects, i.e., of the order of 1 rob. An analysis of this situation for Bi might give interesting results. We thank the staff of I.B.M. Italia for their collaboration. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)

U. Facchini et aI., Padua Conference, 1962 M. G. Marcazzan, F. Tonolini and L. Zetta, Nuclear Physics 46 (1963) 51 M. G. Marcazzan, E. Saetta-Menichella and F. Tonolini, Nuovo Cim. 20 (1962) 903 M. Bormann, Z. Naturf. 17a (1962) 479 B. Sen, Nuclear Physics 38 (1962) 601 U. Facchini, M. G. Marcazzan, (3. Merzari and F. Tonolini, Phys. Lctt. 1 (1962) 6 B. P. Bayrust and R. J. Prestwood, LA 2493 (1960) J. R. Huizcnga and G. J. Igo, A N L 6373 (1961) E. J. Campbell, H. Feshbach, C. E. Porter and V. F. Weisskopf, Technical Report No. 73 (1960) J. M. Blatt and V. E. Weisskopf, Theoretical nuclear physics (Wiley, New York 1952) L. A. Koning, J. H. E. Mattauch and A. H. Wapstra, Nuclear Physics 31 (1962) 1 P.A. Seeger, Nuclear Physics 25 (1961) 1 A. G. W. Cameron, Can. J. Phys. 36 (1958) 1040 E. Erba, U. Facchini and E. Saetta-MenicheUa, Nuovo Cim. 22 (1961) 1237