67 Trends in the Study of Light Proton-Rich Nuclei 1
2
1
1
D. M. Moltz, J. Aysto, M. A. C. Hotchkis, and Joseph Cerny 1
Department of Chemistry and Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 Department of Physics, University of Jyväskylä, SF-40100 Jyväskylä, Finland
2
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch067
Recent work on light proton-rich nuclei is reviewed. Evidence for the first T = -5/2 nuclide, are discussed. z
35
Ca, is presented.
Future directions in this field
Introduction Advances in the study of the limits of known nuclei have made possible investigations of nuclear structure in nuclei with unusual neutron to proton ratios. Questions, such as the limits of nuclear stability, the role of charge-dependent effects in nuclear systems and the existence of new radioactive decay modes, have guided the recent research on the most proton-rich light nuclides. Results from these studies have also made possible tests of advanced shell model analyses [WIL 83] far from the valley of beta-stability. Figure 1 shows a portion of the chart of nuclides through the titanium isotopes. It summarizes our present experimental knowledge of light nuclei. The most important advances in the studies of proton-rich nuclei since the last reviews [CER 77, AYS 80] have been the discoveries of the e-delayed proton decays of Ρ [AYS 85b], C1 [AYS 82], and Ca [AYS 81], the mass measurements of the T = -2 nuclides Si [TRI 80], S , Ar and T i [BUR 80] and the discoveries of the new T = -2 nuclides A1 [CAB 82] and P [CAB 83b]. The latter two were found to decay via Β-delayed two-proton emission [CAB 83a, HON 83], a new mode of radioactivity predicted by Goldanskii [GOL 80]. Also, the first T = -5/2 35 nuclide, Ca, has been discovered via its a-delayed 2p-decay |_AYS tfbaj. 32 Most recently a complete e-decay study of the T^ = -2 nucleus Ar by the ISOLDE-group [BJ0 85]has 0097-6156/ been reported. 86/ 0324-0448$06.00/ 0 © 1986 American Chemical Society 27
31
3b
z
28
32
4 0
z
?2
26
7
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
67.
MOLTZ ET AL.
449
Light Proton-Rich Nuclei
«Se
••EU3I
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch067
1 \ΛΆΑ
Γ Κ Ι
I
1 M
I I
F i g u r e 1. The l i g h t n u c l e i t h r o u g h the t i t a n i u m i s o t o p e s . Black squares i n d i c a t e s t a b l e n u c l e i ; shaded s q u a r e s n d i c a t e n u c l e i whose masses ^ave been p r e c i s e l y measured and w h i t e s q u a r e s i n d i c a t e n u c l e i whose e x i s t e n c e has b e e n v e r i f i e d e x p e r i m e n t a l l y . Nuclei I I s u c h as and l % a , w h i c h a r e known t o be nucléon u n s t a b l e , a r e shaded b u t n o t i n c l u d e d i n a framed s q u a r e .
Mechanise j f t w o - p r o t o n e m i s s i o n f o l l o w i n g b e t a decay 2
Two-proton emission c o u l d i n p r i n c i p l e proceed v i a s e q u e n t i a l He e m i s s i o n o r s i m u l t a n e o u s u n c o u p l e d e m i s s i o n .
emission,
A decomposition of
the
o b s e r v e d t w o - p r o t o n s p e c t r a s h o u l d i n d i c a t e w h i c h mechanism d o m i n a t e s . Monte C a r l o s i m u l a t i o n s o f t h e f i r s t two mechanisms have been p e r f o r m e d 22 2 A l [CAB 8 4 b ] and have shown t h a t He e m i s s i o n would y i e l d a p r o t o n
for
e n e r g y c o n t i n u u m , w i t h an a n g u l a r c o r r e l a t i o n peaked a t 4 0 ° , whereas sequential distinct
e m i s s i o n w o u l d be e s s e n t i a l l y
peaks - one c o n s t a n t
isotropic
and w o u l d e x h i b i t
i n energy corresponding t o the f i r s t
and a second one w i t h a p p r o p r i a t e k i n e m a t i c a l
two
proton
shift.
The d i s t i n c t peak s t r u c t u r e o f t h e i n d i v i d u a l p r o t o n s p e c t r a i n t h e 22 decay o f A l s u g g e s t e d t h a t t h e m a j o r p a r t o f t h e decay o c c u r r e d s e q u e n t i a l l y [CAB 8 4 b ] , Since t h i s experiment could not c o m p l e t e l y exclude ρ the p o s s i b i l i t y of He e m i s s i o n , an e x p e r i m e n t was p e r f o r m e d u t i l i z i n g p o s i t i o n s e n s i t i v e d e t e c t o r s t o measure t h e r e l a t i v e t h e two e m i t t e d p r o t o n s [JAH 8 5 ] .
The e s s e n t i a l l y
angular c o r r e l a t i o n
isotropic
angular
c o r r e l a t i o n shown i n F i g u r e 2 c o n f i r m s t h a t t h e t w o - p r o t o n decay o f t h e + ?? 4 , Τ = 2 i s o b a r i c analog s t a t e i n Mg t o t h e f i r s t e x c i t e d s t a t e i n 20 Ne i s p r e d o m i n a n t l y a s e q u e n t i a l p r o c e s s . The o b s e r v e d m i n o r
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
of
450
NUCLEI OFF THE LINE OF STABILITY
F i g u r e 2. N o r m a l i z e d a n g u l a r c o r r e l a t i o n o f t h e two p r o t o n s f o l l o w i n g ^ A 1 b e t a d e a c y . The d o t t e d l i n e corresponds t o a 15% a d m i x t u r e o f H e e m i s s i o n t o an otherwise isotropic d i s t r i b u t i o n . See t e x t . 2
2
enhancement a t s m a l l
relative
a n g l e s c a n n o t be i n t e r p r e t e d as p o s i t i v e evidence f o r ρ
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch067
diproton 60
80
100
( He) e m i s s i o n due t o
the poor s t a t i s t i c s
120
Relative Angle of T w o Protons
correlated
yield reaction.
in this
low
However, a 15%
a d m i x t u r e o f t h i s p r o c e s s c a n n o t be The d o t t e d l i n e i n F i g u r e 2 has been c a l c u l a t e d assuming t h e 2 break-up p r o p e r t i e s of He o b s e r v e d i n r e a c t i o n s t u d i e s .
excluded.
35 O b s e r v a t i o n o f t h e T^ = - 5 / 2 n u c l e u s , Ca By e x p l o i t i n g t h e r e l a t i v e l y u n u s u a l decay mode o f
35 = -5/2 nucleus, Ca 35 F i g u r e 3 shows t h e t w o - p r o t o n sum s p e c t r u m o f Ca c o l l e c t e d
t w o - p r o t o n e m i s s i o n , w e have d i s c o v e r e d t h e f i r s t [AYS 8 5 a ] .
beta-delayed
T
7
3
a f t e r bombarding a n a t u r a l Ca t a r g e t w i t h a 135 MeV Lawrence B e r k e l e y L a b o r a t o r y 8 8 - I n c h C y c l o t r o n f o r C and u s i n g h e l i u m - j e t t e c h n i q u e s .
He beam f r o m t h e an i n t e g r a t e d beam o f
The d i s t r i b u t i o n o f
individual
e n e r g i e s s u g g e s t s a s e q u e n t i a l decay p r o c e s s v i a i n t e r m e d i a t e s t a t e s 3 4
Ar.
Since both o f the i n d i v i d u a l
(i)
The
i n d i c a t e s t h a t t h e decays
3 5
33
ground and t h e f i r s t
excited state in
predicted 2p-energies,
(iii)
CI, ( i i )
similar
on t h e agreement
p r e d i c t e d decay modes.
z
= - 2 and - 5 / 2 n u c l i d e s
S*,
P)
on t h e with
A l s o see [AYS 8 5 a ] .
S i n c e t h r e e members o f t h e A = 3 5 , Τ = 5 / 2 i s o s p i n s e x t e t 3 5
with
on v a r i o u s r e a c t i o n e n e r g e t i c s and ( i v )
e x p e c t e d absence o r n o n - e x i s t e n c e o f n e a r b y T
3 5
proceed
C a decay scheme i s shown i n F i g u r e 4 . 35 The a s s i g n m e n t o f t h e o b s e r v e d 2 p - a c t i v i t y t o Ca i s p r i m a r i l y based on t h e agreement w i t h t h e known e n e r g y d i f f e r e n c e f o r decays t o t h e 3 4
Ar.
in
p r o t o n s p e c t r a c o m p r i s i n g G and X have
a peak a t t h e same e n e r g y , 2 . 2 1 MeV, t h i s v i a t h e same s t a t e i n
2.1
proton
a r e now known, t h e g r o u n d s t a t e mass o f
predicted w i t h the Isobaric M u l t i p l e t
3 5
(
K*,
C a can be
Mass E q u a t i o n , IMME.
The
resulting
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
67.
M O L T Z ET
Light Proton-Rich Nuclei
AL.
451
v a l u e 4453 ± 60 keV i s more t h a n 300 keV b e t t e r bound t h a n p r e d i c t e d by t h e K e l s o n - G a r v e y r e l a t i o n s w i t h t h e most r e c e n t i n p u t masses [KEL 6 6 ] . 1/2
35,Ca
1/2*
9053/
ι 2, 5553V
+
3/2* 1
7
/ Ρ
n
°
5/2
7
4742/
33
1=T
Τ = 5/2
6774 ^
P
V2
15621
+
1
6
2+
2169
0
_78
+
3/2
+
19*16 1
2
3
4
5
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch067
Two-proton energy (MeV)
Figure 4 . Proposed p a r t i a l decay scheme f o r the betadelayed two-proton emission of C a .
Figure 3. Beta-delayed two-proton spectrum of C a . G and X r e f e r to the t r a n s i t i o n s to the ground and f i r s t e x c i t e d states i n 3 5
the 33ci daughter, r e s p e c t i v e l y . Future
3 S
studies
A d d i t i o n a l examples o f $ - d e l a y e d t w o - p r o t o n e m i t t e r s may be s o u g h t among t h e A = 4 n + 2 , T
z
= - 2 n u c l e i such as
4 6
M n and
5 0
C o [CAB 8 4 a ] .
However, o u r p r e l i m i n a r y s e a r c h e s f o r $2p e m i t t e r s among t h e p r o d u c t s o f 14 40 Ν + Ca r e a c t i o n s a t 130 and 180 MeV have p r o v e n i n c o n c l u s i v e . 35 The a - d e l a y e d t w o - p r o t o n decay o f Ca y i e l d s a v a l u e f o r t h e mass 35 f o r the Τ = 5/2 s t a t e i n isospin sextet, unmeasured.
K; h o w e v e r , t h i s o n l y p a r t i a l l y c o m p l e t e s
l e a v i n g t h r e e members (
3 5
Ca,
Use o f t h e i s o s p i n - c o n s e r v i n g
Τ = 5/2 s t a t e i n f o r an i s o s p i n
3 5
3 5
Ar*
(p,t)
and
3 5
C1*)
reaction to locate
C 1 would t h e r e f o r e p r o v i d e t h e f i r s t
the
the
t e s t o f t h e IMME
sextet.
F u r t h e r i s o s p i n s e x t e t s i n t h e sd s h e l l , w i t h A = 2 3 , 27 and perhaps 3 1 , m i g h t be i n v e s t i g a t e d v i a t h e e - d e l a y e d p a r t i c l e e m i s s i o n o f t h e T = 23 27 31 7
-5/2 nuclei
Si,
S and
Ar.
o f t h e lowest Τ = 5/2 s t a t e i n
In t h i s context the recent 2 3
N a [EVE 8 5 ] w i l l
discovery
c o n t r i b u t e towards the A
= 23 s e x t e t . The K e l s o n - G a r v e y mass f o r m u l a [KEL 6 6 ] p r e d i c t s t h e T = - 3 22 nuclide Si t o be e f f e c t i v e l y p a r t i c l e s t a b l e . Studies of t h i s nucleus and i t s decay w o u l d p r o v i d e d a t a f o r p o s s i b l y t h e o n l y s d - s h e l l example o f 22 7
an i s o s p i n s e p t e t , o f w h i c h one member i s a l r e a d y known ( t h e mass o f Also of great i n t e r e s t
i s t h e p o s s i b i l i t y o f ground s t a t e
0).
two-proton
radioactivity. K e l s o n - G a r v e y mass p r e d i c t i o n s suggest t h a t two ( t h r e e Z = - 5 / 2 n u c l e i are p o t e n t i a l c a n d i d a t e s f o r t h i s new mode o f
T
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
)
452
NUCLEI OFF THE LINE OF STABILITY
radioactivity.
19
These a r e
Mg, (
31
Ar)
and
39
T i , w i t h p r e d i c t e d 2p
s e p a r a t i o n e n e r g i e s o f - 1 1 7 0 , - 1 9 0 and - 7 9 0 keV, r e s p e c t i v e l y .
However,
such o b s e r v a t i o n s would r e q u i r e s p e c i a l i z e d t e c h n i q u e s due t o t h e i r short predicted h a l f - l i v e s
(< 10 m s ) .
Techniques f o r t h e study o f e x o t i c Traditionally, light
proton-rich
any
very
light-ion
nuclei
i n d u c e d r e a c t i o n s have been used t o
n u c l e i , both f o r
decay s t u d i e s v i a ( l i g h t - i o n ,
r e a c t i o n s and f o r mass measurements f r o m r e a c t i o n Q - v a l u e s
produce xn)
such as ( a ,
ο
He).
A l t h o u g h t h e h e l i u m - j e t t e c h n i q u e has p r o v e n e f f e c t i v e f o r
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch067
f o r m e r , f a s t o n - l i n e mass s e p a r a t i o n w i l l
be u s e f u l
d e v e l o p m e n t s i n t h e a r e a [ARJ 8 5 ] may have an i m p o r t a n t studies.
With t h e o b s e r v a t i o n o f
r e a c t i o n a t e x t r e m e l y low l e v e l s ,
3 5
C a from the
it
limits.
Recent
i m p a c t on such
Ca( He, 3
o4n) Ca 3 5
is possible that t h i s kind of 3
has r e a c h e d i t s
4 0
the
in the f u t u r e .
reaction
ο
While the ( H e ,
He) r e a c t i o n c o u l d be used t o
measure t h e masses o f t h e A = 4 n , T^ = - 5 / 2 s e r i e s , t h e c r o s s - s e c t i o n s l i k e l y t o be f o r b i d d i n g l y
are
small. Highly neutron-rich nuclei
[I
15.5 Mev/A
^Ar^Ca
have
been produced i n h e a v y - i o n deep inelastic
and f r a g m e n t a t i o n
( f o r e x a m p l e , [GUI 8 5 ] ) .
reactions
We a r e
i n v e s t i g a t i n g the p o s s i b i l i t y
of
u s i n g t h i s t e c h n i q u e t o p r o d u c e new proton-rich nuclei. Ar beam f r o m t h e
A 1 5 . 5 MeV/A 88-Inch
C y c l o t r o n i n j e c t e d by t h e ECR i o n s o u r c e has been used t o bombard a thick calcium t a r g e t .
Reaction
p r o d u c t s e m i t t e d a t 5 ° were in the focal spectrometer.
plane of
Measurements
magnetic r i g i d i t y ,
of
time-of-f1ight
and
d i f f e r e n t i a l energy loss allowed F i g u r e 5. a) Charge d i s t r i b u t i o n f o r t h e r e a c t i o n 15.5 MeV/A A r + C a and b) t h e c o r r e s p o n d i n g mass d i s t r i b u t i o n f o r Ζ = 12 (Mg i s o t o p e s ) . 3 6
A
detected
a magnetic
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
4 0
67.
Light Proton-Rich
MOLTZ ET AL.
identification
453
Nuclei
o f the wide range o f n u c l e i
so p r o d u c e d .
p r e l i m i n a r y a n a l y s i s o f a small p o r t i o n of the d a t a . are e n c o u r a g i n g .
F i g u r e 5 shows a
These i n i t i a l
The c o l l e c t i o n f o r decay s t u d i e s o f new
results
proton-rich
n u c l i d e s f r o m such r e a c t i o n s p r e s e n t s a f u r t h e r e x p e r i m e n t a l
challenge.
M u l t i n u c l e o n t r a n s f e r r e a c t i o n s w i t h heavy i o n s m i g n t be used t o measure masses o f v e r y p r o t o n - r i c h n u c l e i .
Recent r e s u l t s f o r
[FIF 82] i n d i c a t e t h a t s i m i l a r r e a c t i o n s f o r substantially
neutron-rich
proton-rich nuclei
greater c r o s s - s e c t i o n s than the l i g h t
the
nuclei
may have
i o n r e a c t i o n s used
in
the past. One m i g h t e n v i s a g e , f o r e x a m p l e , u s i n g t h e 4 0 ~ / 2 8 . 28 %40. 40 /28 . 27^41. Ca( Si, S) Ar and Ca( Si, S) Ar r e a c t i o n s t o 28 27 remeasure t h e mass o f S and t h e n t o i n v e s t i g a t e S.
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch067
ç
ς
r a
c
Acknowledgment T h i s work was s u p p o r t e d by t h e D.O.E. under C o n t r a c t
DE-AC03-76SF0009tf.
References [ARJ 85] J. Arje, et al., Phys. Rev. Lett. 54, 99 (1985). [AYS 80] J. Aysto and J. Cerny, in Future Directions in Studies of Nuclei Far from Stability, J.H. Hamilton, et al. (eds.), North-Holland, Amsterdam, 1980, p.247. [AYS 81] J. Aysto, et al., Phys. Rev. C 23, 879 (1981). [AYS 82] J. Aysto, et al., Phys. Lett. 110B, 437 (1982). [AYS 85a] J. Aysto, D.M. Moltz, X.J. Xu, J.E. Reiff and J. Cerny, Phys. Rev. Lett. in press, Report No. LBL-19732. [AYS 85b] J. Aysto, X.J. Xu, D.M. Moltz, J.E. Reiff, J. Cerny and B.H. Wildenthal, submitted to Phys. Rev. C, Report No. LBL-19757. [BJO 85] T. Bjornstad, et al., CERN Report EP/85-23, February 1985, to be published in Nucl. Phys. A. [BUR 80] G.R. Burleson, et al., Phys. Rev. C 22, 1180 (1980). [CAB 82] M.D. Cable, et al., Phys. Rev. C 26, 1978 (1982). [CAB 83a] M.D. Cable, et al., Phys. Rev. Lett. 50, 404 (1983). [CAB 83b] M.D. Cable, et al., Phys. Lett. 123B, 25 (1983). [CAB 84a] M.D. Cable, et al., in Proc. of the 5th Nordic Meeting on Nuclear Physics, Jyvaskyla, Finland, March 1984, p. 119. [CAB 84b] M.D. Cable, et al., Phys. Rev. C 30, 1276 (1984). [CER 77] J. Cerny and J.C. Hardy, Ann. Rev. Nucl. Sci. 27, 333 (1977). [EVE 85] D.D. Eversheim, et al., Phys. Lett. 153B, 25 (1985). [FIF 82] L.K. Fifield, et al., Nucl. Phys. A 385, 505 (1982). [GOL 80] V.I. Goldanskii, PTsma Zh. Eksp. Teor. Fiz. 32, 572 (1980); JETP Lett. 32, 554 (1980). [GUI 85] D. Guillemaud-Mueller, et al., submitted to Z. Phys. [HON 83] J. Honkanen, et al., Phys. Lett. 133B, 146 (1983). [JAH 85] R. Jahn, et al., Phys. Rev. C 31, 1576 (1985). [KEL 66] I. Kelson and G.T. Garvey, Phys. Lett. 23, 689 (1966). [TRI 80] R.E. Tribble, et al., Phys. Rev C 22, 17 (1980). [WIL 83] B.H. Wildenthal, et al., Phys. Rev. C 28, 1343 (1983). RECEIVED July 21, 1986
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
