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Evidence for proton radioactivity of 113Cs and 109I
Volume 137B, number 1,2
PHYSICS LETTERS
22 March 1984
EVIDENCE FOR PROTON RADIOACTIVITY OF 113 Cs AND 1°91 ¢? T. FAESTERMANN, A. GILLITZER, K. H A R T E L 1, p. KIENLE and E. NOLTE Physik Department, Technische Universitiit Miinchen, D-8046 Garching, West Germany Received 16 December 1983
By the bombardment of 58 Ni and 54 Fe targets with 250 Mev 58 Ni beams two new proton radioactive nuclides have been oroduced and identified as 113Cs and tentatively as 109 I. The observed half-lives and proton energies are T1/2 = 0.9 +~:~ us, Ep = 0.98 + 0.08 MeV for 113Cs and rl/2 > 25 us, Ep = 0.83 + 0.08 MeV for 1°91.
Recently the first two cases o f proton radioactive nuclei have been found, namely 151Lu [1] and 147Tm [2]. This new decay mode of nuclei, which has been searched for extensively during the last decade (see references in ref. [1 ] ), determines the borderline o f nuclear stability for neutron deficient nuclei and yields information on nuclear properties very far f r o m ' the valley of/3-stability. Not only can the various mass predictions be tested by the measured proton energy, but also the lifetime can be used for the determination of the anguar m o m e n t u m and spectroscopic factor involved in the emission o f the unbound proton. We report here on the observation o f two other proton radioactive nuclei. For one, with Z = 55, the main decay mode is presumably the emission of a 2d5/2 proton. Preliminary results of these experiments have already been presented elsewhere [3]. A new method was developed, which allows the measurement o f proton radioactivities with halflives down to the ns region and very low production cross sections. This may prove to be an important progress for further studies to pin down the borderline o f proton instability of nuclei. The Munich MP Tandem-linear accelerator combination [4] was used to accelerate 58Ni ions to 250 MeV. Pulsed Supported by the Bundesministerium ft~r Forschung und Technologie. I Visitor from 2. Physikalisches Institut, Universit:,it Heidelberg, West Germany. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
beams o f typically 109 particles/s with pulse periods ranging from 0.2/as to 50/~s traversed an annular gas detector system at the end o f which the target was mounted not visible by the detector (see fig. 1). Evaporation residues recoiling from the target were collected 5 cm downstream in a catcher foil consisting o f 0.4 mg/cm 2 aluminum or mylar with a central hole o f 5 mm diameter to let the primary beam pass through. Choosing the correct target thickness is crucial for this technique since only those evaporation residues produced in a 0.7 mg/cm 2 thick layer can be stopped in the catcher, and this layer may not be too deep in the target, because after a thickness o f about 2 mg/cm 2 the 58Ni ions are slowed down from 250 MeV to the energy of the Coulomb barrier, 210 MeV. Total thicknesses from 5.5 mg/cm 2 to 6.5 mg/cm 2 for the 58Ni targets and of 5.8 mg/cm 2 for the 54Fe target were used. Charged particles from the decay o f nuclei in the catcher foil could be observed in backward geometry by the detector system [5] with a large solid angle (7% o f 47r). It consisted o f a parallel plate avalanche counter (PPC), which yields a fast timing signal, and a Bragg curve spectroscopy (BCS) ionization chamber for particle identification [6]. The preamplifier signal o f the BCS chamber was branched and integrated with a long and a short shaping time constant thus generating an energy-proportional and a strongly Zdependent signal. In addition the drift time o f the electrons from the end of the Bragg curve to the Frisch grid was measured in order to deduce the range 23
Volume 137B, number 1,2
!
PHYSICS LETTERS
100ram
i
J-IONIZATION CHAMBER1
I
I I I I I I I I I.
II
RANGE
CATCHER FOIL --
TARGET
ILo f
R
"~
~
/
~:
a-PARTICLE
• 1
'
~: -i =
::
,,,,,,
,,,,,,,,,,.l
If I l l l l
MAGNET
ANODE
FRISCH GRID : r /
I
~.. /
22 March 1984
loll
II
I PULSED BEAM
III
4 / II I
EQUIPOTENTIAL
RINGS
AVALANCHE COUNTER
IIIIIIII Fig. 1. Schematic drawing of the experimental setup. Evaporation residues from the target are stopped in the catcher foil. Decay protons and c~-particlesare measured with an avalanche counter and an ionization chamber and identified by determination of their Bragg peak and range.
of the particles in the detector gas. By this redundant identification of the detected particles background was reduced to a level which allows the measurement of cross sections below 1 #b. With isobutane as counter gas and a pressure of 60 mbar the 16 cm deep ionization chamber could stop protons with energies up to 1.6 MeV and a-particles with energies up to 5 MeV. Protons could be distinguished from a-particles for energies larger than 0.5 MeV. The PPC, operated at 10 mbar pressure, was optimized for high detection efficiency (between 10% and 60%) of protons. Grids were used as electrodes to avoid energy losses in additional foils. The time signal of the PPC was also used to measure the time delay of events with respect to the beam pulses. The accessible range of half-lives was limited to more than about 10 ns by the time of flight of the evaporation residues from the target to the catcher. In a 20 h bombardment of 58Ni with a 250 MeV 58Ni beam a line of protons was observed (see fig. 2) in the intervals between beam pulses, which were 1.6 #s long and separated by 3.2/as. This line cannot be due to a background from prompt protons which show a continuous energy spectrum. After corrections for energy losses in the catcher foil, the entrance window of the detector and the PPC gas volume the 24
58Ni+SSNii]t
~',.>,.~
LO 03 l--
Z
.0
W > W
TIME
o
0.0
1°0
ENERGY [MEV]
2.0
0°8 [LIS]
3.0
Fig. 2. Spectrum of protons observed between beam pulses for the reaction of 250 MeV 58Ni with 58Ni. Due to the conditions on the particle identification and range spectra, protons can be detected with energies between 0.5 MeV and 1.6 MeV. The inset shows the time dependence of the events in the line.
decay energy of the protons amounts to 0.98 + 0.08 MeV. The energy loss correction was determined using measured energy losses of a-particles and the assumption that the evaporation residues are equally
Volume 137B, number 1,2
PHYSICS LETTERS
distributed across the total thickness of the catcher foil. The uncertainty of the correction contributes the main part to the quoted error for the decay energy. The width of the line of about 200 keV (FWHM) is consistent with the broadening of a monoenergetic proton line by the implantation range in the catcher foil, by energy loss straggling in the entrance window and by the angular dependence of the energy loss. The intensity of the proton line as a function of the time between the beam pulse and detection is plotted in the inset of fig. 2. It shows a decay which is fitted by a half-life of T1/2 0.9_+1.3 0.4/as (T1/2 < 6/as at the 95% confidence level). The efficiency for catching the recoils is quite uncertain and therefore the calculated production cross section for the emitter nucleus of 30/ab is estimated to be correct only within a factor of two. In a similar experiment 54Fe was bombarded for 12 h with a 58Ni beam of 250 MeV. The spectrum of protons measured during the time between beam pulses, which were 2/as long and separated by 6.4 #s, is shown in fig. 3. Once more a proton line was observed with a considerably lower energy of 0.83 + 0.08 MeV after corrections for energy losses. The proton line did not show an observable decay within the 4.4 /as beam-off period yielding a lower limit for the halflife of T1/2 > 25/as. The production cross section for this proton emitter is estimated to be 40/ab with an uncertainty of a factor of two.
58N i + S~Fe o I-z IJJ
o 0.0
1.0 ENERGY
200
3o0
[MEV]
Fig. 3. Out-of-beam proton spectrum for the reaction of 250 MeV 58 Ni with 54 Fe using the same identification procedure as for the spectrum of fig. 2.
22 March 1984
In the reaction of 250 MeV 58Ni with 58Ni the compound nucleus 116Ba is formed with an excitation energy of 58 MeV. Even-Z evaporation residues have not to be considered as proton radioactive because the protons are bound due to the additional pairing energy. The nuclei 114Cs and 1101 reached by (pn) and (apn) evaporation have been shown to have bound protons [7]. The 58Ni(58Ni, p3n)ll2Cs reaction is expected to have a much smaller cross section than the observed one, since only 18 MeV is available for evaporation of the four nucleons and excitation of the residual nucleus. This leaves the (58Ni, p2n) and the (58Ni, ap2n) channel leading to ll3Cs and 109I as the only reactions populating possible proton radioactive nuclei. Based on evaporation calculations and cross-section systematics one would expect a cross-section ratio of the order of 10 for the (p2n) with respect to the (ap2n) channel. The nucleus 109I however can be excluded as the source of the 0.98 MeV protons, because it should have also been reached by the 54Fe(58Ni, p2n) reaction, but the proton line observed there has smaller energy and longer half-life. With this in mind we can conclusively assign the 0.98 MeV proton radioactivity to ll3Cs populated with the 58Ni(58Ni, p2n) reaction. Following similar arguments as above, the proton line of 0.83 MeV observed with the 58Ni + 54Fe reaction can only be due to the decay of 109I or 105Sb (in 106Sb the protons are also bound [7] ). We consider the former as more probable because the production cross section is similar to that for ll3Cs, thus favouring also the (p2n) evaporation channel. In conclusion we assign the observed 0.98 MeV proton line definitely to the decay of ll3Cs and the 0.83 MeV line tentatively to 109I. Of the 1975 mass predictions [8] the shell model approach of Liran and Zeldes agrees best (Qp = 0.93 MeV for ll3Cs, 0.94 MeV for 109I) with the measured decay energies; only the absolute values but not the relative change are reproduced by J~inecke and Eynon (1.07 MeV and 0.47 MeV). Of the more recent predictions the calculations of M611er and Nix [9] agree well (0.93 MeV and 1.05 MeV), while those of Feix and Hilf [10] (0.55 MeV and 0.37 MeV) underestimate the proton decay energies by about 0.4 MeV. For a discussion of the lifetimes of the observed proton radioactivities we extrapolate the calculations of Feix and Hilf [10], which reproduced the measur25
Volume 137B, number 1,2
PHYSICS LETTERS
ed half-life of 151Lu, to the measured decay energies using the energy dependence of a simple Gamow factor. Within the framework of the shell model the protons outside the Z = 50 core fill first the 2d5/2 and 2g7/2 orbitals. For ll3Cs with a proton energy of 0.98 MeV we expect a half-life of 0.4 tas for an L = 2 and 120/as for an L = 4 decay. Thus the observed lifetime favours a d5/2 over a g7/2 configuration for the proton emitting state, although the large error in the measured decay energy corresponds to an order of magnitude uncertainty in the lifetime. The observed lifetime shows this to be the first nucleus which only decays by proton emission, since the expected lifetimes for the competing c~- or/3-decay are about six orders of magnitude longer. This is also consistent with the negative result in searches for 113Cs at online isotope separators [11,12]. For a 0.83 MeV proton decay of 1°91 we calculate a half-life of 7/2s for L = 2 and of 2 ms for L = 4. The lower L-value cannot be ruled out considering the large uncertainty of
26
22 March 1984
the decay energy. Therefore both configurations are possible for the state emitting the 0.83 MeV protons.
References [1] S. Hofmann et al., Z. Phys. A305 (1982) 111. [2] O. Klepper et al., Z. Phys. A305 (1982) 125. [3] T. Faestermann et al., Proc. Intern. Conf. on Nuclear physics (Florence, 1983) p. 311. [4] E. Nolte et al., Nucl. Instrum. Methods 158 (1979) 311. [5 ] K. Hartel et al., to be published. [6] Ch. Schiessl et al., Nucl. Instrum. Methods 192 (1982) 291. [7] A. P/'ochocki et al., Proc. 4th Intern. Conf. on Nuclei far from stability (Helsing~r, 1981) CERN 81-09 (1981) p. 163. [8] S. Maripuu (ed.), At. Data Nucl. Data Tables 17 (1976). [9] P. MiSllerand J.R. Nix, At. Data Nucl. Data Tables 26 (1981) 165. [10] W.F. Feix and E.R. Hilf, Phys. Lett. 120B (1983) 14. [11] J.M. D'Auria et al., Nucl. Phys. A301 (1978) 397. [12] P.O. Larsson et al., preprint GSI-15-83, to be published in Z. Phys. A.
PHYSICS LETTERS
22 March 1984
EVIDENCE FOR PROTON RADIOACTIVITY OF 113 Cs AND 1°91 ¢? T. FAESTERMANN, A. GILLITZER, K. H A R T E L 1, p. KIENLE and E. NOLTE Physik Department, Technische Universitiit Miinchen, D-8046 Garching, West Germany Received 16 December 1983
By the bombardment of 58 Ni and 54 Fe targets with 250 Mev 58 Ni beams two new proton radioactive nuclides have been oroduced and identified as 113Cs and tentatively as 109 I. The observed half-lives and proton energies are T1/2 = 0.9 +~:~ us, Ep = 0.98 + 0.08 MeV for 113Cs and rl/2 > 25 us, Ep = 0.83 + 0.08 MeV for 1°91.
Recently the first two cases o f proton radioactive nuclei have been found, namely 151Lu [1] and 147Tm [2]. This new decay mode of nuclei, which has been searched for extensively during the last decade (see references in ref. [1 ] ), determines the borderline o f nuclear stability for neutron deficient nuclei and yields information on nuclear properties very far f r o m ' the valley of/3-stability. Not only can the various mass predictions be tested by the measured proton energy, but also the lifetime can be used for the determination of the anguar m o m e n t u m and spectroscopic factor involved in the emission o f the unbound proton. We report here on the observation o f two other proton radioactive nuclei. For one, with Z = 55, the main decay mode is presumably the emission of a 2d5/2 proton. Preliminary results of these experiments have already been presented elsewhere [3]. A new method was developed, which allows the measurement o f proton radioactivities with halflives down to the ns region and very low production cross sections. This may prove to be an important progress for further studies to pin down the borderline o f proton instability of nuclei. The Munich MP Tandem-linear accelerator combination [4] was used to accelerate 58Ni ions to 250 MeV. Pulsed Supported by the Bundesministerium ft~r Forschung und Technologie. I Visitor from 2. Physikalisches Institut, Universit:,it Heidelberg, West Germany. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
beams o f typically 109 particles/s with pulse periods ranging from 0.2/as to 50/~s traversed an annular gas detector system at the end o f which the target was mounted not visible by the detector (see fig. 1). Evaporation residues recoiling from the target were collected 5 cm downstream in a catcher foil consisting o f 0.4 mg/cm 2 aluminum or mylar with a central hole o f 5 mm diameter to let the primary beam pass through. Choosing the correct target thickness is crucial for this technique since only those evaporation residues produced in a 0.7 mg/cm 2 thick layer can be stopped in the catcher, and this layer may not be too deep in the target, because after a thickness o f about 2 mg/cm 2 the 58Ni ions are slowed down from 250 MeV to the energy of the Coulomb barrier, 210 MeV. Total thicknesses from 5.5 mg/cm 2 to 6.5 mg/cm 2 for the 58Ni targets and of 5.8 mg/cm 2 for the 54Fe target were used. Charged particles from the decay o f nuclei in the catcher foil could be observed in backward geometry by the detector system [5] with a large solid angle (7% o f 47r). It consisted o f a parallel plate avalanche counter (PPC), which yields a fast timing signal, and a Bragg curve spectroscopy (BCS) ionization chamber for particle identification [6]. The preamplifier signal o f the BCS chamber was branched and integrated with a long and a short shaping time constant thus generating an energy-proportional and a strongly Zdependent signal. In addition the drift time o f the electrons from the end of the Bragg curve to the Frisch grid was measured in order to deduce the range 23
Volume 137B, number 1,2
!
PHYSICS LETTERS
100ram
i
J-IONIZATION CHAMBER1
I
I I I I I I I I I.
II
RANGE
CATCHER FOIL --
TARGET
ILo f
R
"~
~
/
~:
a-PARTICLE
• 1
'
~: -i =
::
,,,,,,
,,,,,,,,,,.l
If I l l l l
MAGNET
ANODE
FRISCH GRID : r /
I
~.. /
22 March 1984
loll
II
I PULSED BEAM
III
4 / II I
EQUIPOTENTIAL
RINGS
AVALANCHE COUNTER
IIIIIIII Fig. 1. Schematic drawing of the experimental setup. Evaporation residues from the target are stopped in the catcher foil. Decay protons and c~-particlesare measured with an avalanche counter and an ionization chamber and identified by determination of their Bragg peak and range.
of the particles in the detector gas. By this redundant identification of the detected particles background was reduced to a level which allows the measurement of cross sections below 1 #b. With isobutane as counter gas and a pressure of 60 mbar the 16 cm deep ionization chamber could stop protons with energies up to 1.6 MeV and a-particles with energies up to 5 MeV. Protons could be distinguished from a-particles for energies larger than 0.5 MeV. The PPC, operated at 10 mbar pressure, was optimized for high detection efficiency (between 10% and 60%) of protons. Grids were used as electrodes to avoid energy losses in additional foils. The time signal of the PPC was also used to measure the time delay of events with respect to the beam pulses. The accessible range of half-lives was limited to more than about 10 ns by the time of flight of the evaporation residues from the target to the catcher. In a 20 h bombardment of 58Ni with a 250 MeV 58Ni beam a line of protons was observed (see fig. 2) in the intervals between beam pulses, which were 1.6 #s long and separated by 3.2/as. This line cannot be due to a background from prompt protons which show a continuous energy spectrum. After corrections for energy losses in the catcher foil, the entrance window of the detector and the PPC gas volume the 24
58Ni+SSNii]t
~',.>,.~
LO 03 l--
Z
.0
W > W
TIME
o
0.0
1°0
ENERGY [MEV]
2.0
0°8 [LIS]
3.0
Fig. 2. Spectrum of protons observed between beam pulses for the reaction of 250 MeV 58Ni with 58Ni. Due to the conditions on the particle identification and range spectra, protons can be detected with energies between 0.5 MeV and 1.6 MeV. The inset shows the time dependence of the events in the line.
decay energy of the protons amounts to 0.98 + 0.08 MeV. The energy loss correction was determined using measured energy losses of a-particles and the assumption that the evaporation residues are equally
Volume 137B, number 1,2
PHYSICS LETTERS
distributed across the total thickness of the catcher foil. The uncertainty of the correction contributes the main part to the quoted error for the decay energy. The width of the line of about 200 keV (FWHM) is consistent with the broadening of a monoenergetic proton line by the implantation range in the catcher foil, by energy loss straggling in the entrance window and by the angular dependence of the energy loss. The intensity of the proton line as a function of the time between the beam pulse and detection is plotted in the inset of fig. 2. It shows a decay which is fitted by a half-life of T1/2 0.9_+1.3 0.4/as (T1/2 < 6/as at the 95% confidence level). The efficiency for catching the recoils is quite uncertain and therefore the calculated production cross section for the emitter nucleus of 30/ab is estimated to be correct only within a factor of two. In a similar experiment 54Fe was bombarded for 12 h with a 58Ni beam of 250 MeV. The spectrum of protons measured during the time between beam pulses, which were 2/as long and separated by 6.4 #s, is shown in fig. 3. Once more a proton line was observed with a considerably lower energy of 0.83 + 0.08 MeV after corrections for energy losses. The proton line did not show an observable decay within the 4.4 /as beam-off period yielding a lower limit for the halflife of T1/2 > 25/as. The production cross section for this proton emitter is estimated to be 40/ab with an uncertainty of a factor of two.
58N i + S~Fe o I-z IJJ
o 0.0
1.0 ENERGY
200
3o0
[MEV]
Fig. 3. Out-of-beam proton spectrum for the reaction of 250 MeV 58 Ni with 54 Fe using the same identification procedure as for the spectrum of fig. 2.
22 March 1984
In the reaction of 250 MeV 58Ni with 58Ni the compound nucleus 116Ba is formed with an excitation energy of 58 MeV. Even-Z evaporation residues have not to be considered as proton radioactive because the protons are bound due to the additional pairing energy. The nuclei 114Cs and 1101 reached by (pn) and (apn) evaporation have been shown to have bound protons [7]. The 58Ni(58Ni, p3n)ll2Cs reaction is expected to have a much smaller cross section than the observed one, since only 18 MeV is available for evaporation of the four nucleons and excitation of the residual nucleus. This leaves the (58Ni, p2n) and the (58Ni, ap2n) channel leading to ll3Cs and 109I as the only reactions populating possible proton radioactive nuclei. Based on evaporation calculations and cross-section systematics one would expect a cross-section ratio of the order of 10 for the (p2n) with respect to the (ap2n) channel. The nucleus 109I however can be excluded as the source of the 0.98 MeV protons, because it should have also been reached by the 54Fe(58Ni, p2n) reaction, but the proton line observed there has smaller energy and longer half-life. With this in mind we can conclusively assign the 0.98 MeV proton radioactivity to ll3Cs populated with the 58Ni(58Ni, p2n) reaction. Following similar arguments as above, the proton line of 0.83 MeV observed with the 58Ni + 54Fe reaction can only be due to the decay of 109I or 105Sb (in 106Sb the protons are also bound [7] ). We consider the former as more probable because the production cross section is similar to that for ll3Cs, thus favouring also the (p2n) evaporation channel. In conclusion we assign the observed 0.98 MeV proton line definitely to the decay of ll3Cs and the 0.83 MeV line tentatively to 109I. Of the 1975 mass predictions [8] the shell model approach of Liran and Zeldes agrees best (Qp = 0.93 MeV for ll3Cs, 0.94 MeV for 109I) with the measured decay energies; only the absolute values but not the relative change are reproduced by J~inecke and Eynon (1.07 MeV and 0.47 MeV). Of the more recent predictions the calculations of M611er and Nix [9] agree well (0.93 MeV and 1.05 MeV), while those of Feix and Hilf [10] (0.55 MeV and 0.37 MeV) underestimate the proton decay energies by about 0.4 MeV. For a discussion of the lifetimes of the observed proton radioactivities we extrapolate the calculations of Feix and Hilf [10], which reproduced the measur25
Volume 137B, number 1,2
PHYSICS LETTERS
ed half-life of 151Lu, to the measured decay energies using the energy dependence of a simple Gamow factor. Within the framework of the shell model the protons outside the Z = 50 core fill first the 2d5/2 and 2g7/2 orbitals. For ll3Cs with a proton energy of 0.98 MeV we expect a half-life of 0.4 tas for an L = 2 and 120/as for an L = 4 decay. Thus the observed lifetime favours a d5/2 over a g7/2 configuration for the proton emitting state, although the large error in the measured decay energy corresponds to an order of magnitude uncertainty in the lifetime. The observed lifetime shows this to be the first nucleus which only decays by proton emission, since the expected lifetimes for the competing c~- or/3-decay are about six orders of magnitude longer. This is also consistent with the negative result in searches for 113Cs at online isotope separators [11,12]. For a 0.83 MeV proton decay of 1°91 we calculate a half-life of 7/2s for L = 2 and of 2 ms for L = 4. The lower L-value cannot be ruled out considering the large uncertainty of
26
22 March 1984
the decay energy. Therefore both configurations are possible for the state emitting the 0.83 MeV protons.
References [1] S. Hofmann et al., Z. Phys. A305 (1982) 111. [2] O. Klepper et al., Z. Phys. A305 (1982) 125. [3] T. Faestermann et al., Proc. Intern. Conf. on Nuclear physics (Florence, 1983) p. 311. [4] E. Nolte et al., Nucl. Instrum. Methods 158 (1979) 311. [5 ] K. Hartel et al., to be published. [6] Ch. Schiessl et al., Nucl. Instrum. Methods 192 (1982) 291. [7] A. P/'ochocki et al., Proc. 4th Intern. Conf. on Nuclei far from stability (Helsing~r, 1981) CERN 81-09 (1981) p. 163. [8] S. Maripuu (ed.), At. Data Nucl. Data Tables 17 (1976). [9] P. MiSllerand J.R. Nix, At. Data Nucl. Data Tables 26 (1981) 165. [10] W.F. Feix and E.R. Hilf, Phys. Lett. 120B (1983) 14. [11] J.M. D'Auria et al., Nucl. Phys. A301 (1978) 397. [12] P.O. Larsson et al., preprint GSI-15-83, to be published in Z. Phys. A.