β-delayed proton radioactivity of 44Cr, 47Mn, 48,49Fe and 50Co

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A 602 (1996) 167-180

/3-delayed proton radioactivity of 48'49Fe and 5°Co

44Cr,

47Mn,

L. Faux a, S. Andriamonje a, B. Blank a, S. Czajkowski a, R. Del Moral a, J.P. Dufour a, A. Fleury a, T. Josso a, M.S. Pravikoff a, A. Piechaczek b, E. Roeckl b, K.-H. Schmidt b, K. Stimmerer b, W. Trinder b, M. Weber b, T. Brohm c, A. Grewe c, E. Hanelt c, A. Heinz c, A. Junghans c, C. R6hl c, S. Steinh~iuser c, B. Voss c, Z. Janas d, M. P f t i t z n e r d a CEN Bordeaux-Gradignan. BP 120, F-33175 Gradignan Cedex, France b Gesellschaftfiir Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt, Germany c Inst. fEir Kernphysik, TH Darmstadt, Schloflgartenstr. 9, D-64289 Darmstadt, Germany d Inst. of Exp. Physics, University of Warsaw, PL-O0-681 Warsaw, Hoza 69, Poland

Received 18 January 1996

Abstract The proton-rich nuclei 44Cr, 47Mn, 48'49Fe and -S°Co have been produced by fragmentation of a 58Ni beam at 650 MeV/u. The isotopic separation of these nuclei has been achieved with the GSI Projectile-Fragment Separator FRS. The isotopes have been identified in flight by A E - T o F - B p measurements. After implantation in a stack of seven silicon detectors, the signals measured for implantation and radioactive decay were unambiguouslycorrelated in time due to low counting rates. On the basis of the two proton peaks observed for ~Co at (2034±30) keV and (2740-4-41) keV with a half-life of (444-4) ms, a partial decay scheme is proposed for this nucleus. A single proton peak at (959-4-33) keV was observed for 48Fe with a half-life of (44±7) ms. This emission is attributed to the decay of the T = 2 isobaric analog state in 48Mn. No deviation from the quadratic form of the isobaric multiplet mass equation is observed. Additional information on /3-delayed proton branches of 49Fe, 44Cr and 47Mn was also obtained. Keywords: RADIOACTIVITY44Cr, 47Mn, 48,49Fe,~lCo (/3+) lfrom 9Be(58Ni,X), E = 650 MeV/nucleon] ; measured Tl/2, /3-delayed Ep, lp, branchingratio, 5°Fe, 48,49Mn;deduced levels

I. Introduction Precise knowledge of nuclear binding energies in the region of proton-rich fp-shell nuclei is important in the context of the search for ground-state two-proton (2p) decay, 0375-9474/96/$15.00 (~ 1996 Elsevier Science B.V. All rights reserved PII S0375-9474(96) 00109-1

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L. Faux et al./Nuclear Physics A 602 (1996) 167-180

since the two most promising candidates for this decay mode, 45Fe and 48Ni [ 1], lie in this region. Of particular interest is the study of the masses of isobaric analog states (IAS) which are related to each other via the isobaric multiplet mass equation (IMME). The predictive power of this equation is particularly high if its quadratic form holds and higher-order terms are negligible [2]. It is a much less useful tool, however, if the binding energies of proton-unbound nuclear states deviate significantly from the simple mass relations between mirror nuclei underlying the IMME or the Garvey-Kelson mass relation as concluded by Comay et al. [3]. We have shown recently that for the ( J r = 0 + , T = 2) multiplet of mass A = 52 the quadratic form of the IMME is a very good approximation [4]. In the present paper, we will discuss this question for the case of the same quintuplet in A = 48. The production of very proton-rich nuclei in the fp shell is possible via different reaction mechanisms: projectile fragmentation was used at GANIL to discover new isotopes [5] and to study decay properties [4,6,7], while other groups successfully performed decay studies of, e.g., 48Mn [8] and 39Ti [9] with heavy-ion-induced fusionevaporation reactions. The results reported here have been obtained at GSI by using the fragmentation of a 650 MeV/u 58Ni beam. Production rates of nuclei far from stability at the FRS were expected to be comparable to that at other facilities due to a balance of advantages related to the high energy (large target thickness, high transmission through the spectrometer) and the disadvantage of rather low primary-beam intensities ( 108 ions per beam pulse, every 2-3 s). These expectations have been confirmed by our experiment [ 10]. Particularly noticeable is the observed exponential decrease of the cross sections as a function of neutron number, with an average decrease of a factor of about 20 per neutron loss. This exponential trend is markedly different from the empirical Gaussian trend extrapolated from data closer to stability [ 1 l ]. The deviation is particularly pronounced very far from stability with a cross section 400-750 times larger than empirically predicted for instance for 46Fe or 5°Ni. The present paper reports on studies of the decay of 5°Co, 48'49Fe, 47Mn, and 44Cr. The experimental set-up used for this study is described in Section 2, while the results and the discussion form Sections 3 and 4 of the paper.

2. Experimental set-up The nuclei to be studied were produced by fragmentation of a 58Ni beam at an energy of 650A MeV delivered by the SIS synchrotron. Projectile fragments produced from the 5SNi beam were isotopically separated with the fragment separator FRS [ 12] operated with an achromatic degrader [13]. The beryllium target had a thickness of 6019 m g / c m 2. Both target thickness and beam energy were chosen as a compromise to maximize production of exotic nuclei in the primary target and to minimize their destruction in the slowing-down material necessary for the implantation. A schematic lay-out of the FRS and the detectors is shown in Fig. 1. The in-flight

L. Faux et al./Nuclear Physics A 602 (1996) 167-180 ~SNi beam

Degrader

\ rSEETR Fragments Target

Scintittator

169

MUSIC (AE)

A Tetescope

Fig. 1. Schematic drawing of the FRS and its detectors used for the A and Z identification the ions and for the study of their radioactive decay. The quantities given in parentheses denote the physical observables measured with the respective detectors.

identification of the nuclei transmitted was achieved by magnetic-rigidity (Bp), energyloss (AE) and time-of-flight (ToF) measurements. A first plastic scintillator [ 14] placed in the intermediate focal plane, next to the degrader, provided both a start signal for the ToF and a position measurement along the dispersive focal plane yielding the magnetic rigidity Bpl of the ions in the first half of the spectrometer. A second plastic scintillator installed at the exit of the FRS provided correspondingly a stop time signal and a Bp2 measurement. Two ionization chambers (MUSIC) [15], which were placed further downstream at the exit of the FRS, allowed us to determine AE as well as position. After correction for the position dependence of the AE signals, these detectors yielded a Z resolution good enough to separate neighbouring elements. Behind the MUSIC detectors, an adjustable energy degrader allowed to tune the implantation depth of the selected nuclei. Down to that position, it was possible to detect all nuclei dispersed over the full width of the exit focal plane of about 20 cm. Behind the energy degrader, a silicon-detector telescope was mounted on the optical axis of the FRS which counted nuclei only within a radius of 2 cm. The telescope consisted of two 1 mm thick entrance and exit detectors on either side of a stack of five 0.5 mm thick detectors. An additional 200 # m thick tantalum foil was placed at the entrance of the stack of the 0.5 mm detectors to minimize losses due to angular straggling. 12 /zm thick aluminium foils separated the other detectors. The data acquisition was triggered by two types of events: (i) implantations of transmitted nuclei, (ii) radioactive decays occurring in the silicon detector telescope. However, all detector signals were recorded for both types of events.

3. Experimental results

3.1. Calibration with 41Ti The AE-ToF calibration of the heavy-ion detectors was performed with the primary beam at different magnetic rigidities. A typical AE-ToF spectrum corrected using the Bp measurement is shown in Fig. 2 for the tuning on 41Ti. The observation of this characteristic/3-delayed proton emitter provided a check of the A and Z determination. Due to the smaller active surface of the silicon telescope compared to the MUSIC detectors, only 54% of the 41Ti ions delivered by the FRS entered the telescope. Among

170

L. Faux et al./Nuclear Physics A 602 (1996) 167-180 950 t,¢:

900

..¢: 0

850

I,LI

800

C~

"'$c

4°Sc

750 700 650

600 550

SOOo

' 2~o'

go

600

' 8~o'

'1o'oo'

~oo

ToF(channels) Fig. 2. AE-ToF spectrum obtained with the MUSIC detector (AE) and the scintillators (ToF) for the FRS being tuned on 41Ti. The energy loss has been corrected by using the magnetic-rigidity (Bp) information.

those the actual 41Ti activity was reduced by another 22% due to secondary nuclear reactions in the slowing-down material placed upstream of the telescope. The wellknown [ 16] /3-delayed proton properties of 41Ti were used for an energy calibration of the silicon detector telescope. Since the production rate of 4lTi was rather low, the accuracy of calibration is limited by counting statistics. The proton-energy spectrum obtained after individual calibration and summing over the five inner silicon detectors is shown in Fig. 3. The 4JTi activity also provided a calibration of the detection efficiency. Since the implantation profile was approximately uniform both in and perpendicular to the beam "E

350

41Ti

o

o

25O

2OO

lOO

5o 1

2

3

4

5

6

Energy(MeV)

Fig. 3. /3-delayed proton spectrum of 41Ti obtained by adding the calibrated signals from the five central detectors.

L. Faux et aL/Nuclear Physics A 602 (1996) 167-180

171

direction, protons may not deposit their full energy in the detectors. The resulting detection efficiency can in principle be rigorously calculated using the measured implantation profile and the known proton ranges. Depending on the proton energy, this efficiency is about 98% at 1 MeV and about 90% at 4 MeV. Under the conditions of the experiment, with beam-on periods of about 1 s during which both implantations and radioactive decays occur, another source of efficiency loss stemmed from events at the limit of being implantation-like or radioactivity-like. The energy loss of energetic light particles that are not detected with 100% efficiency in the scintillator or in the MUSIC detectors can indeed simulate a radioactivity event. An additional rejection criterion is obtained by the silicon detectors number 1 and 7 used as vetos when they fired in coincidence. The counting rates were low enough to ensure that the radioactive decay of a given 41Ti nucleus occurs with about 90% probability before the implantation of a second 41Ti nucleus. A small correction of branching ratios was performed to take this effect into account. Events characterized by too close lying implantation times were excluded in the half-life determination. We obtained a half-life of TI/2 = (81 4-4) ms for 41Ti which has to be compared to the literature value [ 16] of T1/2 = (80 4- 2) ms. Another important quantity for the determination of absolute branching ratios is the dead time of the data acquisition. The branching ratios were determined by using the time-correlation method which takes into account: (i) the total number of 41Ti implantations with subsequent decays (within a 3T~/2 time window) and (ii) the total number of implanted 41Ti atoms. The branching ratio is the ratio of the first quantity and the second one. This method is not only sensitive to the overall dead time but also to its fluctuations. One may easily see this effect by taking test cases like (i) 25% dead time over the entire counting period, and (ii) 50% of dead time for half of the counting period, with no dead time for the other half. The two cases result in the same average 25% dead time, but events of the type "implantation plus time-correlated decay" are observed with efficiencies of 56.25% in the first case and 62.5% in the second. In our experiment, the average dead time was measured to be 24% for the 41Ti tuning (it stayed close to that value for the other tunings). Making the assumption of a constant dead time was found to result in overestimating the branching ratios of 41Ti by about 10%, a difference of similar magnitude as in the test cases just mentioned. Only a full record of dead-time variations according to time and beam-pulse intensity fluctuations could allow for a proper "correlated dead-time" correction. We thus made a rough correction of 10% for the branching ratios of the other nuclei observed in our experiment and estimated that this procedure induced an additional 10% uncertainty in the measured values.

3.2. Results obtained for 44CF, 47Mn,

48'49Fe,5°Co

Apart from tuning on 41Ti for calibration purposes, three different tunings of the FRS were used to transmit the nuclei 44Cr, 48Fe and 5°C0 with maximum yield, respectively. The corresponding values of Bpl and Bp2 are given in Table 1. Some additional nuclei were transmitted simultaneously with the nominal ones. Thus, e.g., 47Mn and 49Fe could b e studied although their yields were far from optimum.

L. Faux et al./Nuclear Physics A 602 (1996) 167-180

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Table 1 Magnetic rigidities Bpl and Bp2 used to tune the spectrometer FRS for the nuclei listed in the first line. Also listed are the other nuclei observed simultaneously in the silicon telescope Selected nucleus

Bpj (Tm) Bp2 (Tm) Other nuclei implanted in the telescope

41Ti

44Cr

48Fe

5°C0

6.561 5.601 43V, 42Ti' ,~,41Sc 38,39,40Ca' 37,38K

6.398 5.323 46Mn, 45Cr 42,43V, 41,42Ti

6.404 5.215 5°C0, 49Fe 46,47,48Mn, 45,46Cr

6.403 5.154 52Ni, 5tCo 48'49,5°Fe, 47,48Mn

The counting rates were low enough to ensure that, even when several fl-delayed proton emitters were observed in the same FRS tuning, the radioactivity events could always be unambiguously time-correlated to the preceding implantation event. The measured fl-delayed charged-particle spectra recorded during time-correlation windows corresponding to three half-lives of the respective nuclei are shown in Figs. 4a-e for the nuclei 44Cr, 47Mn, 48'49Fe, and 5°C0, respectively. The spectra of 5°C0 and 49Fe show pronounced peaks. The determination of half-lives was performed with the decay of the most prominent proton peaks for 5°C0 and 48'49Fe, while a broader energy range (from 0.8 MeV to 6 MeV) was used for 44Cr, 47Mn, and 48Fe (see below). The proton energies, branching ratios and half-lives are given in Table 2. The number of events in the mCo peaks allows a half-life determination for restricted proton energy limits, i.e. the two most prominent peaks. The result is shown in Fig. 5. A value of TI/2 = (444-4) ms was obtained with a least-squares fit by using one exponential component after subtraction of the background, assuming that no daughter activity contributes to this peak. The properties of the daughter nucleus 5°Fe are unknown, but its fl-delayed proton branching ratio is probably very weak. A similar procedure was employed for 48Fe. The protons observed in the peak at Table 2 Measured proton energies, absolute branching-ratios and half-lives for all isotopes studied in the present experiment. For 44Cr and 49Fe, the values already reported in the literature [7,17] are also listed Nucleus

Proton energy (keV)

Branching ratio (%)

44Cr 47Mn 48Fe 49Fe

900-1100 6514-20 9594-33 19784-29 15384-24 10834-16 27904-41 20344-30 9504-50 21814-50 31404-50 1960±50

74-3 3.44-0.9 3.64-1.1 434-10 54-1 44-1 434-12 11 4-3

5°Co 44Cr

49Fe

Half-life (ms)

1004-50 444-7 704-3

444-4 534- 3

754-10

Ref.

this work this work this work this work this work this work this work this work [ 71 [71 [7 ] [ 17]

L. Faux et aL/Nuclear Physics A 602 (1996) 167-180

==

173

I/3 300

E

d) '#-e

p,

°o ,s

2OO lO

11111 5

100

"~ 60

b) 47Mn

I

e) °Co

150

100 40 50 20

2

tO

K

Jl_, •

t,/) 100

80

4

Energy(MeV)

c) 'SFe

60

40

20

1

2

Energy(MeV)

Fig. 4. fl-delayed charged-particle spectra obtained for the nuclei (a) 44Cr, (b) 47Mn, (c) 48Fe, (d) 49Fe and (e) 5°C0.

960 keV exhibit an exponential decay with a half-life of T1/2 -- (51 + 17) ms. As shown in Fig. 6, a decay study of all protons with energies higher than 800 keV yields a half-life slightly shorter but compatible with the first one of T1/2 = (45 + 11) ms. This value is very close to the value independently obtained from fitting the decay events due to fl particles (T1/2 = (44 + 11) ms). As the half-life determination from fl particles and from signals above 800 keV yield independent results, we adopt an error-weighted mean value of the two last values of TI/2 = (44 4- 7) ms. In view of the limited statistics, the half-life determination of 47Mn was performed on the whole spectrum including fl particles and protons. A fit with an exponential plus a constant yields a half-life of TI/2 = (100 4-50) ms. The constant corresponds to uncorrelated fl decays due to other nuclei. In the case of 44Cr~ the implantation of 637 nuclei does not allow to clearly observe

174

L. Faux et al./Nuclear Physics A 602 (1996) 167-180 u) 500 400 0 ( j 300

5°Co

200

100 90 80

70 60

~

so

~

40 '

I

I 210

'

'

' 41O

'

•

' 610

'

'

' 80

I

'

'I;0'

'120' ' '110' ' '160

'

Time(ms) Fig. 5. Decay curve of the main 5°Co proton peaks at 2.03 MeV and 2.79 MeV.

the peaks already reported in Ref. [7]. These previous results, however, did not include any branching ratio for proton emission. The activity that we see around 950 keV, the proton energy corresponding to the decay of the T = 2 IAS in 44V, allows to extract an upper limit of the branching ratio of (7 4- 3)% for this channel. The reason why we state here only an upper limit is that either the high-energy tail o f / 3 particles or /3-delayed protons stemming from levels fed by Gamov-Teller decay seem to partially mask the IAS proton decay. The upper limit thus determined is nevertheless interesting when considered together with the similar T = 2 IAS proton decay branches observed in the decay of 4SFe and 52Ni [4]. The observation of 49Fe in this experiment is a by-product of the tuning on 5°Co. The main/3-delayed proton peak observed in our experiment confirms the earlier results 1!! O ~)

4SFe

70 60 50

T , / z = 4 5 + 11 m s

4O

2O

8

7 6

5 .

0

.

.

.

.

/.

.

50

.

I

100

.

.

.

.

I

150

i

L

.

.

i

200

.

.

.

.

I

250

.

.

.

.

I

,

300

Time(ms) Fig. 6. Decay curve of 48Fe determined with charged particles yielding signals above 800 keV (see text).

L. Fauxet al./Nuclear PhysicsA 602 (1996) 167-180

175

obtained by Cerny and coworkers [ 17]. The improved statistics allow us to redetermine the proton energy with a slightly better accuracy. In addition, two other peaks are observed as reported in Table 2. A half-life measurement made on the main 1.97 MeV proton peak yields a value of T1/2 = (70 + 3) ms. Together with the previous value of Ti/2 = (75 4- 15) ms [ 17], we deduce a new half-life value of Tt/2 = (71 4- 3) ms for 49Fe.

4. Interpretation of the results The nuclei studied in this work have isospin projections of Tz = - 2 and - 3 / 2 . For proton-rich nuclei with such isospin values, the/3 decay is expected to feed dominantly the IAS. When the IAS is proton-unbound, a small isospin impurity of a few percent allows proton emission to dominate over y de-excitation.

Decay of 49Fe and mCo In the cases of 49Fe and 5°Co, the most intense proton peak is attributed to the deacy of the IAS. Assuming first that these states are unmixed and using k = 6127 4- 9 as determined by Wilkinson [18], one may calculate l o g f t = k / B ( F ) . For the T = 3/2 nucleus 49Fe with B ( F ) = 3, one obtains l o g f t = 3.31. For 5°Co with T = 2 and B ( F ) = 4, one gets l o g f t = 3.19. If one combines these values with the experimentally observed half-lives and the Q~ values estimated by Audi and Wapstra [ 19], it is possible to predict branching ratios of (54 ± 6)% and (55-4-5)% for the feeding of the T = 3/2 and T = 2 IAS in 49Mn and 5°Fe, respectively. Due to isospin impurities, one has to expect experimental feedings smaller than these values. The most intense proton peaks observed in our experiment have absolute branching ratios in reasonable agreement with the theoretical estimates. Similar to the conclusion already drawn by Cerny et al. [17] for the 1.96 MeV proton peak of 49Fe, we attribute the 2.79 MeV peak observed in the 5°Co decay to the proton decay of the T = 2 IAS in 5°Fe (see Fig. 7). The energy location of this state is greatly eased by that of the mirror state in 5°Cr known to lie at an excitation energy of (8425 ± 5) keV [20]. The observed energy of 2.79 MeV fits very well to a transition from the T = 2 IAS at (8480 4-48) keV excitation energy in 5°Fe to the 1 1 / 2 - state at 1542 keV in 49Mn [21]. The only other possible neighbouring state in 49Mn would be the 9 / 2 - state at 1059 keV, but this would place the IAS in 5°Fe at (7997 i 48) keV excitation energy, a value which is much lower than that observed in the mirror nucleus. A further indication that the 2.73 MeV transition is connected to the 1 1 / 2 - state is given by the quadratic coefficient c in the IMME. This term is always [2] within 10% of the unifbrmly charged sphere estimate <,. The c value obtained is 1.002 x cs if one assumes the 1 1 / 2 - state as final state compared to a much larger value (2.0 x Cs) if the 9 / 2 - level is taken instead. We thus interpret the 2.79 MeV proton emission as shown in the partial decay scheme of 5°Co (Fig. 7). The 2.03 MeV transition cannot be placed between known states in this decay scheme.

176

L. Faux et al./Nuclear Physics A 602 (1996) 167-180

T=2 44(4)ms 6+ 5O

T=2

6+

IAS

2.790 M e V / 43(12)°/,

8.48

13/2

/ /

7+ 6~, 5,11/2 -

I

5.69 5.21

9/2

4.41

7/2 5/2

4.15

49 Mn+p 0

50

Fe

Fig. 7. Partial decay scheme of 5°Co. Energy levels and branching ratios in bold characters are from the present experiment. Energy levels are given in MeV relative to the 5°Fe ground state [ 19]. The 2.79 MeV line is attributed to the proton decay of the T = 2 IAS in 5°Fe. The 2.03 MeV transition could not be placed between known states in this decay scheme.

In the case of 49Fe, we may place the 1.54 MeV transition with some confidence as originating from the feeding through Gamow-Teller transition of the 7 / 2 - , 5 / 2 - state at 4.38 MeV excitation energy in 49Mn decaying by proton emission to the 2 + state in 48Cr. The new partial decay scheme of 49Fe is shown in Fig. 8. The 1.08 MeV line cannot be attributed to a transition between known states in the decay scheme.

Decay of 48Feand 44CF In contrast to the cases of 49Fe and 5°C0, the decays of the nuclei 48Fe and 44Cr do not show large branching ratios for proton emission. At first sight, this observation is surprising as the feeding of the IAS is calculated to be rather large: (43 + 11 )% for the T = 2 IAS in 4SMn and (42 -t- 5)% for the T = 2 IAS in 44V. In 48Fe, the observed peak at 959 ± 33 keV fits well the expected energy for the proton decay of the 48Mn IAS to the ground state in 47Cr as calculated with the IMME [(961±54) keV] but the observed intensity is quite small: (3.6 ± 1.1)%. For 44Cr, we could not clearly see the peak at 950 keV reported in Ref. [7] but only extract an upper limit of 7% for the proton decay of the 44V IAS, again significantly lower than the expected feeding.

L. Fauxet al./Nuclear Physics A 602 (1996) 167-180 _ T=3/2

177

70(3) ms

7/2

49Fe

7/2-5/2-

5.27

719"-RI9-

1.978MeV •4 ~ 7 / 2 j

7/2- I _ _

4.85 4.82 I A S

5/24.38

1.s38 aey"

/

4

2+ y

3.51

5122.84

0+

2.09

48 Cr + p 0

49

Mn

Fig. 8. Partial decay scheme of 49Fe. Energy levels and branching ratios in bold characters are from the present experiment. The other levels in 49Mn are taken from the mirror nucleus 49Cr. Energy levels are given in MeV relative to the 49Mn ground state [ 191. The 1.08 MeV transition could not be placed between states in this decay scheme.

The calibration with 41Ti and the observation of "normal" branching ratios for 49Fe and 5°C0 lead us to conclude that the low branching ratios observed for 48Fe and 44Cr in the same set of data cannot arise from an experimental deficiency. Similar to what has already been concluded for the case of 52Ni [4], the only possible explanation for such reduced proton intensities is the competition with 7 de-excitation of the IAS. This hypothesis is qualitatively supported by barrier-penetration calculations yielding tunneling half-lives of the same order of magnitude as for typical y decays. We thus conclude that despite its low branching ratio the proton peak at 959 keV observed in the decay of 48Fe has to be assigned to a previously unobserved T = 2 IAS in 48Mn with a mass excess of 6m = - ( 2 6 3 0 4 4- 36) keV. The proposed partial decay scheme of 48Fe is shown in Fig. 9. Since already three members of the quintet were known [22], one has the possibility to test the quadratic form of the IMME. The agreement with the simple quadratic form is very good, as already found for 52Ni [4]. Although the uncertainty on the mass excess of the IAS in 48Mn from this work is rather large, it is still much less than the systematic effect seen by Comay, Kelson and Zidon [ 3 ] when comparing experimental masses of proton-unbound nuclei to predictions using the Garvey-Kelson relations. In the case of the (T = 2, A = 48) quintet, only the

178

L. Faux et al./Nuclear Physics A 602 (1996) 167-180 T=2

51(17) m s

0+ 48

.~2 0.959 MeV

IAS

/

3 . 6 ( 1 . 1 ) /% /

2.99

/

2.4

1- --

2.3 512 -

(2.03)

312

r

47 Cr+p

1-

t:

1+

T

4

+

0.52 0.42

0 48 Mn

Fig. 9. Partial decay scheme of 48Fe. Energy levels and branching ratios in bold characters are from this experiment. The energy levels are given in MeV relative to the 48Mn ground state [ 19]. = - l state observed in this work is proton-unbound. An energy stabilization specific to this state, due to a radial expansion o f the proton wave function (Thomas-Ehrman shift), would thus be seen in a deviation o f the quadratic form of the IMME. Such an effect is not observed. As mentioned earlier [2], this does not mean that there is no T h o m a s - E h r m a n shift but rather that it is not specific to unbound states. Instead, a gradual increase of stabilization with decreasing binding energy seems to be possible. This effect, however, would be incorporated in the b and c terms o f the IMME. Decay o f 47Mn

For this nucleus, the observed delayed proton activity indicates feeding of excited states in 47Cr above the 4.8 MeV proton-emission threshold and thus above the T = 3 / 2 IAS at 4.3 MeV. The observed peak at 651 keV is characterized by a log f t value o f 4.35 and is assigned to a Gamow-Teller transition.

5. C o n c l u s i o n s

The relativistic heavy-ion beams at GSI have been shown to be competitive with that of other facilities for the production o f proton-rich nuclei in the fp shell. The isotopic

L. Faux et al./Nuclear Physics A 602 (1996) 167-180

179

separation and identification o f the nuclei o f interest with the spectrometer F R S has yielded first spectroscopic studies o f several nuclei. There is accumulating e v i d e n c e for 3/ de-excitation o f the p r o t o n - u n b o u n d T = 2 IAS in Tz = - 1

nuclei. The /3-delayed

charged-particle spectra allowed to measure the masses o f three analog states. For A = 48, the quadratic f o r m o f the I M M E is able to fit the new mass o f the p r o t o n - u n b o u n d ( T = 2, T z = - 1 )

analog states as well as the three known masses o f the other proton-

b o u n d analog states. This gives s o m e confidence in the predictive p o w e r o f the I M M E to calculate the masses o f very proton-rich nuclei, even if they are particle unbound. This is especially interesting when considering 45Fe and 48Ni, the experimentally best accessible candidates for two-proton radioactivity. The results reported here show that a better understanding o f the radioactive decays could be obtained for these nuclei if 3' detection were added. However, the efficiency o f such detectors should be high, even if the beam-intensity improvements at G S I will be realized.

Acknowledgements It is a pleasure for us to thank the GSI accelerator staff for p r o v i d i n g an intense and reliable 58Ni beam. We w o u l d like to express our gratitude to K.-H. Behr, A. Brtinle and K. B u r k a r d for the technical support during the experiment. We a c k n o w l e d g e the financial support from the I N 2 P 3 - G S I collaboration programme.

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