Mapping of the proton drip-line up to Z = 20: Observation of the TZ = −52 series 23Si, 27S, 31Ar and 35Ca

Nuclear Physics @ North-Holland

A455 (1986) 149-157 Publishing Company

MAPPING OBSERVATION

OF THE PROTON DRIP-LINE UP TO 2 = 20: OF THE T, = -3 SERIES 23Si, “S, 31Ar AND ‘?a*

M. LANGEVINt’, A. C. MUELLER*, R. ANNE’, M. BERNAS’, S.D. HOATHls3, F. NAULIN’,

D. GUILLEMAUD-MUELLER’, M.G. SAINT-LAURENT’, J. GALIN’, D. GUERREAU*, J.C. JACMART’, F. POUGHEON’, E. QUINIOU’ and C. DETRAZ2

de Physique Nucliaire, BP I, F-91406 Orsay, France ’ GANIL, BP 5027, F-14021 Caen, France 3 Present address: Dept. of Physics, University of Birmingham, PO. Box 363, Birmingham BJS 2lT, ’ Institut

Received

30 December

UK

1985

Abstract: The series of four T, = -3 nuclei, “Si, “S, 3’Ar and %a, predicted to be bound or nearly bound against one- or two-proton emission is observed. The experimental evidence comes from direct identification of the magnetically separated products from interactions of 77 MeV/u ‘%a projectiles with a nickel target.

E

NUCLEAR REACTIONS Ni(%a, X), E = 3097 MeV; measured time-of-flight, magnetic analysis, solid-state detector telescope; deduced A, 2; observation of T, = -2 series ‘%i, 27S, “Ar I 35Ca.

1. Introduction Recently,

we demonstrated

the effectiveness

of intermediate

energy

heavy-ion

fragmentation in the production of nuclei far from stability. The high beam intensity at GANIL, the kinematic properties of the projectile fragmentation process and the performance of the triply focusing magnetic spectrometer LISE allowed the identification of numerous new neutron-rich isotopes 1,2). These results suggest the extension of our experimental technique The limit of nuclear stability against

to a search for very proton-rich one- and two-proton emission

light nuclei. has been a

matter of active investigation for the last few years 3,4). In particular several experiments to study T, = -2 nuclei individually have been reported. These experiments identify nuclei through their radioactive decay mode, p-delayed proton emission in the case of 32Ar [ref. ‘)I and the new mode of P-delayed two-proton emission from 22A1 and 26P [ref. “)I. It may be noted that production of exotic proton-rich nuclei offers the possibility for tests of the renormalization of the axial-vector This work was performed at the national t Deceased on 11 April 1985.

l

laboratory 149

GANIL,

Caen,

France.

150

coupling multiplet

M. Langevin et al. / Mapping

constant in nuclear P-decay ‘). Through the application of the isobaric mass equation, the observation of these exotic nuclei is related to the

identification

of highly excited

analogue

states in more stable nuclei ‘).

A very recent report of the first observation of a T, = -5 nucleus, 35Ca, is based on such a P-delayed activity “). Most updated mass formula ‘-l’) predict that the four T, = -4 nuclei 23Si, 27S, 3’Ar and 35Ca are stable or nearly stable against two-proton emission while they are definitely bound against one-proton emission. In the present work this whole series was produced and identified in flight.

2. EXPERIMENTAL

PROCEDURE

Our study of the T, = -$ nuclei used the interaction of a 17.4 MeV/u 40Ca beam from GANIL with a nickel target and the analyzing features from LISE 12). A four stage telescope, consisting of two 1000 p,rn Si detectors and two 4000 km Si(Li) detectors, located at the final focal point of LISE (fig. l), detected the magnetically analyzed fragments. The incident energy was chosen to approach, within the characteristics of GANIL r3), the relativistic regime for which the fragment velocities are close to the incident beam velocity 14). The slowing down of ions in the 92 mg/cm2 natural Ni target determined a Bp setting of 2.10 T. m to preferentially select fragments with A/Z = 1.75. Proton-rich nuclei were chosen for both the projectile and target (68% 58Ni) to enhance the production rate of proton-rich nuclei. Such a choice is known 15) to be effective at intermediate energy. A 5.3 mg/cm2 Al stripper was mounted at the intermediate focal plane between the two dipole magnets (fig. 1) in order to get rid of (very weakly abundant) incompletely stripped fragments. In comparison with our previous experiments lT2) on the neutron-rich nuclei, the required magnetic rigidity setting lies much closer to that for beam particles. The detector-telescope counting-rate was limited to less than lo3 events per second in order to maintain good energy GANIL beam intensity and the of 1.5 x 10” particles per second, event rate. The second restriction

resolution and to avoid pile-up. To that effect the LISE momentum acceptance were limited. A beam and a 0.5% momentum bite provided an acceptable gave an additional particle identification parameter.

The flight time of the collected fragments is measured between the initial and final foci of LISE, i.e. at the target and detector positions, respectively (see fig. 1): this ensures a practically constant (* 1 mm) flight path length of 17 945 mm, independent of their angle of entry into the spectrometer. The time of flight is determined from the time signal of either of the 1000 pm detectors and the radio frequency (r.f.) signal of the last cyclotron. The constancy of the correlation between the r.f. and the time of impact on the target is monitored by a neutron detector mounted close to the primary beam catcher at the exit of the first dipole magnet. The time resolution obtained for the beam used in this experiment is 1% of the typical flight time of 170 ns through LISE. The atomic number 2 of the detected fragments is basically determined by the energy loss in the first detector and the time of flight through LISE. Likewise, the

M. Langevin et al. / Mapping

Telescope achromatic

151

at the focal point

Fig. 1. Schematical layout of LISE. The telescope, mounted at the achromatic focal point, four solid state detectors (1000~ AE,, 1000~ AE,, 4000~ E, 4000~ 8).

consists

of

ratio number between the mass A to atomic number 2 is given by the time of flight (t.o.f.) corresponding to the restricted momentum bite of LISE. The bidimensional plot (see fig. 2) of m/t.o.f. (i.e. 2) versus t.o.f. (i.e. A/Z) was inspected on-line in order to calibrate the particle identification. This plot exhibits characteristic curves which are labelled by their isospin projection T,; the line of constant flight time, for example, corresponds to T, = 0, N = Z self-conjugate nuclei. The Z calibration is readily assured by presence or absence of well-known isotopes in these T, lines: the absence of ‘Be (T, = 0), 9B (T, = -4) and r6F (T, = -1) correlates with the presence of 9Be (T, = +), *B (T, = -1) and l”B (T, = 0).

M. Langevin et al. / Mapping

152

T,= +I U

Fig. 2. Two-dimensional

T,= +1/2

I .~

100 representation

T,=-l/2 T,=-1

T,=O

200 of events in a diagram

300

411~cnanrds Tof

of Z versus time of flight (see also text).

Fig. 3 shows the same bidimensional representation after 14 hours of integration time. The T’ = -$ series 23Si, 27S, 31Ar and “Ca clearly becomes visible. The four-stage telescope ensures in principle a double identification in 2 (AI?, , AE2 and t.o.f) and a single identification in A (total energy and t.o.f.) applied to the off-line analysis in our previous experiments. This time, however, malfunction of the AE2 detector was observed which results in poor resolution for the calculated mass spectra above A = 25. Therefore, the mass spectra of fig. 4 have been calculated by means of the AEl and the time of flight signals for each event and its known magnetic rigidity. This non-redundant identification ensures a good mass resolution (AA/A = 1.5%) but features remaining background at a level of 1 or 2 counts per mass unit. 3. Discussions The question of whether the proton drip-line has been reached by the present experiment should be handled with some precaution. The steepness of the valley of p stability on the proton-rich side and some remaining background events do preclude any definite statement as to the non-observation of a given isotope. Table 1 is a compilation of the lp and 2p separation energies, for proton-rich nuclei along the drip-line, calculated by applying the charge-symmetry formula of Kelson and Garvey 16) as done by Janecke in ref. ‘I) but using the most recent experimental

153

M. Langevin et al. / Mapping

z 20.

15. -

-

23s

i

IO-

w_!

S .

--

5I

180

I

170

I

160

60 T.O.F.

NS

Fig. 3. Part of the previous figure (fig. 2) after an integration time of 14 hours at a magnetic rigidity setting of Bp = 2.10 T. m. Events from the level of two upwards are indicated in the color code given on the top of the figure.

M. Langevin et al. / Mapping lo*

”

‘.

Sulphur

”

1 25

21

29

31

A

Calcium

1 29

31

33

35 A

-lri 33

35

37

39 A

Fig. 4. Mass distributions for the elements Si, S, Ar and Ca. They have been obtained by summing up the data from 14 hours integration time at a magnetic rigidity setting of Bp = 2.10 T. m and 6 hours at Bp = 2.15 T. m. Thus 74, 112, 315 and 246 events are observed for ‘%i, “S, “Ar and %Za respectively.

masses “) for the conjugate neutron-rich nuclei. The standard deviation of this estimate should not exceed 300 keV. It is clearly seen that all nuclei which are beyond the ones seen in the experiment are largely unbound with the possible exception of 22Si. This isotope is predicted to be bound against 2p emission by 16 keV. For the experimental limitations mentioned above, its non-observation certainly does not demonstrate its particle-unstable character. The observation of 3’Ar, however, predicted to be 2p unbound by 180 keV may point to a contingent case of direct two proton emission. The possibility of such a decay from nuclei bound to lp, but slightly unbound to 2p emission was discussed long ago by Goldansky ‘*), and later on by Janecke 19). The increased probability for a nearly equal partition of the available energy E,, between the two protons was demonstrated. An upper limit of the energy width of the emitting nucleus r(max) is the product of two factors: (i) the penetrability of the Coulomb barrier by a 2He of energy E,,, which is in first order equivalent ‘8*‘9) to the case of two protons, each with energy :E,,; and (ii) the Wigner width which reflects the probability of finding the emitted fragment at the surface of the nucleus. The minimum value of the partial half-life T,,,(min) for 2p emission is deduced from r(max). r(max) hence T,,,(min), depend on the angular momentum of the emitted 2p system since the centrifugal barrier may add to the Coulomb one. The decay

155

M. Langevin et al. / Mapping TABLE

Predictions

for mass-excess

1

values and lp and 2p separation proton-rich isotopes

energies

for very

Mass excess Isotope

TZ

3oC1

-2

4842*40 -1467*1

-f

32233120 26014*50 23282*170 19478*100 17 837*40 11 607570 4675*80

-3

32 134590 27474*110 21 875*210 14 700*300

34K i9Mg =A1 *sSi =P 27S “Ar %a **Si ?S 30Ar Wa

(keV)

S,, NV)

S,, WV)

-713*65 -624* 30

2575 f 40 2715*8

L ) -1153*60 2047 f 190 -1434* 100 712*300 b, 522*310 b, 11341310s)

-1175*55 -1493 f 50 22105 170 -1867* 100 568 f 40 -189*85 523 * 85

“)

16*95 -2141*75 -3167*270 -2302 zt 300

1; “)

The mass excess has been obtained by applying the charge symmetry formula of Kelson and Garvey 16) as done by Jaenecke ‘I) but using most recent experimental masses “) for the conjugate new-neutron rich nuclei. The indicated errors are the ones of these experimental masses. The procedure itself is believed to be accurate at a level of 100 keV I’). “) Decay to an unbound nucleus, value therefore insignificant. b, Mass excess of the daughter nucleus experimentally not known and itself determined by systematics I’). 31Ar+

29S + 2p is most probably compatible with a Al = 0 transition since the whole series of N = 13 isotones have a ground state spin 2’ with an increasing energy of

the first excited 4’ state when 2 increases. In the following discussion T,,,(min) is calculated under that very plausible Al = 0 hypothesis. A realistic value of T,,* could be substituted to T,,,(min) through complete analysis of the structure factors which allow the formation of a 2p system with proper quantum numbers in 31Ar. For instance, to this effect, two protons in a Id orbit must be coupled to an internal 1s state with I = 0 relative angular momentum in 3’Ar to result in a Al =0 transition, through a (0202)0020) Talmi coefficient. Furthermore a 9j-coefficient must account for the coupling of 29S and the 2p fragment into 31Ar. The exact derivation of a theoretical estimate of T,,,, through detailed spectroscopic calculation, is not necessary at this point. Indeed, for a qualitative discussion of the possibility of 2p radioactivity this is in fact dominated by another factor, the extremely sensitive dependence of T,,,(min) with EPP. On the one hand, for E,, greater than about 800 keV, T,,,(min) becomes shorter than the flight time of 31Ar from the target to the detectors. In this case, the two protons are emitted before 31Ar can be detected. On the other hand, for E,, smaller

M. Langeuin et al. / Mapping

156

than about 500 keV, T,,,(min) /3 emission. ity against

Thus, although weak-interaction

this experiment

confirms

becomes

2p emission

much longer than the expected remains

decay becomes

energetically

negligible.

that 3’Ar is not unbound

possible,

The observation

by more than

half life for its probabilof 3’Ar in

800 keV, which

agrees with current mass predictions. But it says nothing as to the possibility that it may lie in the narrow window 500 keV< E,, s 800 keV which would make it a good candidate for observing 2p radioactivity. 4. Final remarks The present study certainly marks an important step towards mapping the proton drip-line for light nuclei. The fragmentation of a **Ni beam which now has become available at GANIL should allow an extension of our experiment to still higher 2 values, i.e. possibly up to 2 = 28. The observed production rates of exotic proton-rich nuclei should permit further study of their P-delayed proton emission. The present observation of a series of T, = -2 isotopes underlines that the study of a sextet of isobaric states, an opportunity of great interest 4), might become feasible. Finally, it shows that the production of potential emitters for the two-proton radioactivity is within present experimental capability. We like to acknowledge the technical assistance of F. Geoffroy, Y. Georget and D. Plessis. Further thanks go to the GANIL operating crew for delivering the beam and M.L. Marie for careful editing of the manuscript. References 1) M. Langevin, E. Quiniou, M. Bernas, J. Galin, J.C. Jacmart, F. Naulin, F. Pougheon, R. Anne, C. Dttraz, D. Guerreau, D. Guillemaud-Mueller and A.C. Mueller, Phys. Lett. 150B (1985) 71 2) D. Guillemaud-Mueller, A.C. Mueller, D. Guerreau, F. Pougheon, R. Anne, M. Bernas, J. Galin, J.C. Jacmart, M. Langevin, F. Naulin, E. Quiniou and C. Detraz, Z. Phys. A322 (1985) 415 3) M.D. Cable, J. Honkanen, R.F. Parry, S.H. Zhou, Z.Y. Zhou and J. Cerny, Phys. Lett. 123B (1983) 25 4) J. Aystii and J. Cerny, Future directions in studies of nuclei far from stability, ed. J.H. Hamilton (North-Holland, Amsterdam, 1980) p. 257 5) T. Bjornstad, M.J.G. Borge, P. Dessagne, R.D. von Dinklage, G.T. Ewan, P.G. Hansen, A. Huck, B. Jonson, G. Klotz, A. Knipper, P.O. Larsson, G. Nyman, H.L. Ravn, C. Richard-Serre, K. Riisagner, D. Schardt and G. Walter, Nucl. Phys. A443 (1985) 283 6) M.D. Cable, J. Honkanen, E.C. Schloemer, M. Ahmed, J.E. Reif, Z.Y. Zhou and J. Cerny, Phys. Rev. C30 (1984) 1276 7) P.D. Eversheim, F. Hinterberger, S. Kuhn, P. van Rossen, J. Rdmer and R.P. Trelle, Phys. Lett. 153B (1985) 25 8) J. Aystii, D.M. Moltz, X.J. Xu, J.E. Reiff and J. Cerny, Phys. Rev. Lett. 55 (1985) 1384 9) M. Uno and M. Yamada, INS Report NUMA 40 (1982) 10) P. Miiller and J.R. Nix, At. data and nucl. data tables 26 (1981) 165 11) S. Maripuu, special ed., At. data nucl. data tables 17 (1976) 12) M. Langevin and R. Anne, in Instrumentation for heavy ion nuclear research, ed. D. Shapira, vol. 7 (Harwood Academic Publishers, Chur, 1985) p. 191

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13) A. Joubert, Proc. Xth Int. conf. on cyclotrons and their applications, East Lansing, 1984, ed. F. Marti (IEEE, New York, 1984) p. 3 14) T.J.M. symons, Y.P. Viyogi, G.D. Westfall, P. Doll, D.E. Greiner, H. Faraggi, P.J. Lindstrom, D.K. Scott, H.J. Crawford and C. McParland, Phys. Rev. Lett. 42 (1979) 40 15) D. Guerreau, V. Barrel, D. Jacquet, J. Galin, B. Catty and X. Tarrago, Phys. Lett. 131B (1983) 293 16) I. Kelson and G.T. Garvey, Phys. Lett. 23 (1966) 689 17) A.H. Wapstra and G. Audi, Nucl. Phys. A432 (1985) 1 18) V.I. Goldansky, Nucl. Phys. 19 (1960) 482; Nucl. Phys. 27 (1961) 648 19) J. Janecke, Nucl. Phys. 61 (1965) 326