- Email: [email protected]
The decay of 49Mn: First results from the chalk river on-line isotope separator
Volume 91B, number 2
PHYSICS LETTERS
7 April 1980
THE DECAY OF 49Mn: FIRST RESULTS FROM THE CHALK RIVER ON-LINE ISOTOPE SEPARATOR J.C. HARDY, H. SCHMEING, E. HAGBERG, W. PERRY, J. WILLS, E.T.H. CLIFFORD, V. KOSLOWSKY and I.S. TOWNER Atomic Energy Limited of Canada, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ 1JO J. CAMPLAN and B. ROSENBAUM Laboratoire Rend Bernas, CSNSM, 91406 Orsay, France R. KIRCHNER GSI Darmstadt, 6100 Darmstadt, Germany and
H. EVANS Queen's University, Kingston, Ontario, Canada K7L 3N6 Received 21 January 1980
The t3+-decayof 49Mn (j1r, T = 5/2-, 1/2; tl/2 = 384 -+ 17 ms) has been observed; it exhibits a superallowed branch to its mirror, the 49Cr ground state, and a (6.4 -+2.6)% allowed branch to the 7/2- state at 272 keV. The deduced Gamow-Teller matrix elements agree very well with shell model calculations that employ a modified Kuo-Brown interaction.
Manganese-49 is a T = 1/2 nucleus, which decays predominantly by a superallowed/3-decay transition to the ground state of its mirror, 49Cr. The structural simplifications inherent in such mirror decays make them particularly'attractive for comparison with model calculations [1]; but at the same time experimental accuracy is difficult to achieve since the lifetimes are short, there is a near absence of H-delayed 7-rays, and, above mass 40, mirror nuclei are well removed from /3-stability. Thus, although the mass of 49Mn is known [2,3], neither its lifetime nor its decay properties have so far been measured. We wish to report here the first characterization of the decay of 49Mn. The nuclide was produced by bombarding a 2 mg/cm 2 target of natural magnesium with 95 MeV 28Si ions from the Chalk River upgraded MP tandem. Reaction products recoiled from the target directly into a FEBIAD (forced electron beam induced
arc discharge) ion source [4], from which a 40 keV beam of ions was extracted into the new Chalk River isotope separator. This separator is based on a 135 ° inhomogeneous (n = 1/2) magnet of 1 m radius; its dispersion is such that adjacent beams at A ~ 50 are separated at the image plane by 45 mm (perpendicular displacement); and its measured resolution (M/6M, where 6M is the full width at half maximum in mass units of a beam of ions with mass M at the image plane) with the FEBIAD source is > 2000. Ions of a selected mass, in this case A = 49, pass through a narrow slit on the image plane and thence through an electrostatic deflector and two quadrupole doublets into a shielded collection chamber 5 m away. With this system, contamination from adjacent masses has been determined to be less than one part in 104 . The separated beam in the collection chamber passes entirely through a collimator 6 mm in diameter and 207
Volume 91B, number 2
PHYSICS LETTERS
7 April 1980
1
1
r
io*
10 5 49Cr
49Mn
1
r
511 keV
o __J W Z Z 10 4
I.u
.T~,z = 384-+17ms
T 0
a0- - - O
n,W 13-
Z
J-5 TIME
03
~1o
8_
49Cr 49Cr
3
I I
~
-I0 ~
15
(s)
49Cr
1
0 o
i0 z
20O
4OO ENERGY
6OO (keV)
Fig. 1. Partial spectrum of v-rays accumulated for ~ 1 h from the A = 49 isotope-separated source, with peaks attributed to the decays of 49Cr and 49Mn marked accordingly. The two small peaks at 581 keV and 727 keV are associated with the decay products of 232Th and are present in the room background Based on the measured decay properties of 49Mn, we deduce that the spectrum corresponds to a collection rate of ~ 300 atoms/s of 384 ms 49Mn. The inset shows the decay of the annihilation radiation. onto the tape o f a compact tape-transport system similar to that described in ref. [5]. In the study of 49Mn, several different modes o f operation were employed: (a) to optimize the detection of short-lived "),-rays, the collection point was viewed directly b y a Ge(Li) detector while the tape advanced slowly, carrying away long-lived activity (42 min 49Cr); (b) to obtain lifetime information, activity collected on the tape was moved periodically 7.6 cm to a counting position in front o f a Ge(Li) detector, data collection then being in 16 sequential spectra; and (c) to measure absolute branching ratios, the tape passed into a curved slot in an aluminum block at the counting position, the block being thick enough to annihilate all positrons from the decay o f 49Mn. A portion o f the ")'-ray spectrum observed at the collection point (mode (a)) is shown in fig. 1. The most prominent "/-rays are known lines associated with the decay [6] of 49Cr, but in addition there is a clear peak at 272 keV. Sequential measurements (mode (b)) 208
yielded a half-life of 350 -+ 80 ms for that peak and revealed a major short-lived component o f the annihilation radiation - see the inset to fig. 1 - with a fitted half-life of 384 -+ 17 ms. Evidently, both are features o f the same nuclear decay. The observed half-life alone indicates 49Mn as the source o f activity, since all other A = 49 nuclides that can be produced with our b e a m target combination are known to have very different lifetimes. The argument is clinched b y a precise measurement o f the observed "}'-ray energy based on the known energies [6] of the 49Cr lines. The result, 272.3 + 0.4 keV, agrees closely with the energy of the first excited state (271.4 + 0.4 keV [7]) in 49Cr, the/3 +decay daughter of 49Mn. No other short-lived ")'-rays were observed below 3 MeV. With the source o f the observed activity unambiguously identified, the branching ratios for the 3-transitions from 49Mn were determined b y measuring the intensity o f the 272 keV 7-ray relative to the short-lived component of the 511 keV radiation as observed with
7 April 1980
PHYSICS LETTERS
Volume 91B, number 2
experimental mode (c). Since all positrons annihilated in a controlled geometry, the intensity o f the 511 keV peak could, after correction for annihilation in flight (8.1 -+ 2.0%; see ref. [8]), be related directly to the total positron decay intensity. We established the detection efficiency b y placing standard sources ,1 of 133Ba and 22Na at the same position in the a l u m i n i u m block as that taken b y the separated 49Mn samples; this calibration was sufficient since these sources provided a 7-ray line at 276 keV (133Ba) as well as radiation from the annihilation of positrons (22Na). Our result gives the intensity of the 272 keV 3,-ray as (6.4 -+ 2.6)% of the total n u m b e r of 49Mn disintegrations. It is corrected for real coincidences b e t w e e n 272 keV 7-rays and annihilation radiation, a 10% effect with our geometry; the electron capture probability is negligible (EC//3 + < 0.1%). The results obtained for the decay o f 49Mn are summarized in fig. 2 and table 1. The low log ft value measured for the ground-state transition shows it to be superallowed, and this establishes the s p i n - p a r i t y of 49Mn to be the same as that o f 49Cr, which is k n o w n [7,9] to be 5 / 2 - . This assignment is also consistent with $1 The 133Baand 22Na sources were prepared and their activities measured (to +-1% and -+2%, respectively) by the Chalk River radioisotope standardization group: J.S. Merritt, A.R. Rutledge and L.V. Smith.
J l
0
49Cr
5/ 2-//
7716 5/~49Mn 384ms
/~1 6.4 /3~ 93.6
4.76 3.67
Fig. 2. Proposed decay scheme for 49Mn. the observed allowed/3 + branch to the 7 / 2 - excited state at 272 keV. In general an allowed/3-transition is described [ i ] by
K/G~ (1 + 6 r ) t = f v ( 1 ) 2 ( I - 6c) + fAR2(o'~) 2 ' where K = 1.230618 X 10 - 9 4 erg2cm6s, t is the experimental partial half-life, G~r is the effective vector coupling constant, fir and 6 c are radiative and chargedependent mixing corrections, f v and fA are statistical rate functions calculated for vector and axial-vector decay, (1) and (a~> are the corresponding nuclear matrix elements, and R is the axial vector coupling con-
Table 1 Summary of results for the observed decay branches of 49 Mn. Transition to ground state branching ratio (%) partial half-life a) t (ms) end-point energy Ema x (/3) (keV) statistical rate functions : fV log ft corrections : 8r (%) 8 e (%)
fA
93.6 -+ 2.6 410 -+ 22 6692 +- 24 11390 -+ 190 11790 -+ 190 3.67 +- 0.02 2.04 0.57
272 keV state 6.4 -+ 2.6 6000 +- 2500 6420 -+ 24 9700 -+ 160 4.76 -+ 0.20 2.06
experimental
0.434 +- 0.052
0.260 -+ 0.053
calculated <(I~) : KB f9 KB f9 + f8 r KB* f9 + f8 r
0.623 0.302 0.375
0.078 0.248 0.227
a) Based on a total 49Mn half-life of 384 -+ 17 ms. 209
Volume 91B, number 2
PHYSICS LETTERS F
stant expressed in units of G V. For our present purposes we shall ignore renormalization effects in R, and use the value 1.237 +- 0.008 derived from the decay o f the neutron [1 ]. The data for the observed 49Mn transitions have been analyzed with the same techniques as used in ref. [ 1 ] to extract experimental values for the G a m o w - T e l l e r matrix elements. The essential components of the calculations, together with the results, appear in table 1. Also shown in table 1 are the results of three shell model calculations for the G a m o w - T e l l e r matrix elements. For A = 49 nuclei, large-scale shell model calculations in the full fp-shell model space are impractical. Instead, we considered first only t9 configurations (f = f7/2), the residual interaction and single-particle energies being taken from the G-matrix calculations o f Kuo and Brown [10] (KB) corrected for core polarization: viz. Gbare + G3p - lh" The results are given in the table as the first line o f " c a l c u l a t e d
210
7 April 1980
pole centroids of the interaction [ 11 ]. By shifting the energy centroids o f f9 configurations from those with f8r, the effect of this modification is to weaken the corrections to ( o , ) from perturbation theory. The calculated values o f (G~> shown in the last line of the table are in excellent agreement with experiment, and indicate that the KB* interaction not only improves agreement with experimental binding energies [ 11 ] but may also be efficacious in calculating transition probabilities. Further tests in the same mass region would be valuable.
References [1 ] S. Raman, C.A. Houser, T.A. Walkiewicz and I.S. Towner, At. Data Nucl. Data Tables 21 (1978) 567. [2] D. Mueller, E. Kashy, W. Benenson and H. Nann, Phys. Rev. C12 (1975) 51. [3] A.H. Wapstra and K. Bos, At. Data Nucl. Data Tables 19 (1977) 177. [4] R. Kirchner and E. Roeckl, Nucl. Instrum. Methods 133 (1976) 187. [5 ] P. Dam, E. Hagberg and B. Jonson, Nucl. Instrum. Methods 161 (1979) 427. [6] S.V. Jackson, E.A. Henry and R.A. Meyer, Phys. Rev. C12 (1975) 2094. [7] M.L. Halbert, NucL Data Sheets 24 (1978) 175. [8] J.C. Hardy, H. Schmeing, J.S. Geiger and R.L. Graham, Nucl. Phys. A223 (1974) 157. [9] J.O. JBnsson, L. Sanner and B. Wannberg, Phys. Scr. 2 (1970) 16. [10] T.T.S. Kuo and G.E. Brown, Nucl. Phys. Al14 (1968) 241. [11] A. Pores, E. Pasquini and A.P. Zuker, Phys. Lett. 82B (1979) 319; E. Pasquini and A.P. Zuker, Proc. Topical Conf. on Physics of medium light nuclei (Florence, 1977), eds. P. Blasi and R.A. Ricci (Editrice Compositori, Bologna, 1978) p. 62.
PHYSICS LETTERS
7 April 1980
THE DECAY OF 49Mn: FIRST RESULTS FROM THE CHALK RIVER ON-LINE ISOTOPE SEPARATOR J.C. HARDY, H. SCHMEING, E. HAGBERG, W. PERRY, J. WILLS, E.T.H. CLIFFORD, V. KOSLOWSKY and I.S. TOWNER Atomic Energy Limited of Canada, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ 1JO J. CAMPLAN and B. ROSENBAUM Laboratoire Rend Bernas, CSNSM, 91406 Orsay, France R. KIRCHNER GSI Darmstadt, 6100 Darmstadt, Germany and
H. EVANS Queen's University, Kingston, Ontario, Canada K7L 3N6 Received 21 January 1980
The t3+-decayof 49Mn (j1r, T = 5/2-, 1/2; tl/2 = 384 -+ 17 ms) has been observed; it exhibits a superallowed branch to its mirror, the 49Cr ground state, and a (6.4 -+2.6)% allowed branch to the 7/2- state at 272 keV. The deduced Gamow-Teller matrix elements agree very well with shell model calculations that employ a modified Kuo-Brown interaction.
Manganese-49 is a T = 1/2 nucleus, which decays predominantly by a superallowed/3-decay transition to the ground state of its mirror, 49Cr. The structural simplifications inherent in such mirror decays make them particularly'attractive for comparison with model calculations [1]; but at the same time experimental accuracy is difficult to achieve since the lifetimes are short, there is a near absence of H-delayed 7-rays, and, above mass 40, mirror nuclei are well removed from /3-stability. Thus, although the mass of 49Mn is known [2,3], neither its lifetime nor its decay properties have so far been measured. We wish to report here the first characterization of the decay of 49Mn. The nuclide was produced by bombarding a 2 mg/cm 2 target of natural magnesium with 95 MeV 28Si ions from the Chalk River upgraded MP tandem. Reaction products recoiled from the target directly into a FEBIAD (forced electron beam induced
arc discharge) ion source [4], from which a 40 keV beam of ions was extracted into the new Chalk River isotope separator. This separator is based on a 135 ° inhomogeneous (n = 1/2) magnet of 1 m radius; its dispersion is such that adjacent beams at A ~ 50 are separated at the image plane by 45 mm (perpendicular displacement); and its measured resolution (M/6M, where 6M is the full width at half maximum in mass units of a beam of ions with mass M at the image plane) with the FEBIAD source is > 2000. Ions of a selected mass, in this case A = 49, pass through a narrow slit on the image plane and thence through an electrostatic deflector and two quadrupole doublets into a shielded collection chamber 5 m away. With this system, contamination from adjacent masses has been determined to be less than one part in 104 . The separated beam in the collection chamber passes entirely through a collimator 6 mm in diameter and 207
Volume 91B, number 2
PHYSICS LETTERS
7 April 1980
1
1
r
io*
10 5 49Cr
49Mn
1
r
511 keV
o __J W Z Z 10 4
I.u
.T~,z = 384-+17ms
T 0
a0- - - O
n,W 13-
Z
J-5 TIME
03
~1o
8_
49Cr 49Cr
3
I I
~
-I0 ~
15
(s)
49Cr
1
0 o
i0 z
20O
4OO ENERGY
6OO (keV)
Fig. 1. Partial spectrum of v-rays accumulated for ~ 1 h from the A = 49 isotope-separated source, with peaks attributed to the decays of 49Cr and 49Mn marked accordingly. The two small peaks at 581 keV and 727 keV are associated with the decay products of 232Th and are present in the room background Based on the measured decay properties of 49Mn, we deduce that the spectrum corresponds to a collection rate of ~ 300 atoms/s of 384 ms 49Mn. The inset shows the decay of the annihilation radiation. onto the tape o f a compact tape-transport system similar to that described in ref. [5]. In the study of 49Mn, several different modes o f operation were employed: (a) to optimize the detection of short-lived "),-rays, the collection point was viewed directly b y a Ge(Li) detector while the tape advanced slowly, carrying away long-lived activity (42 min 49Cr); (b) to obtain lifetime information, activity collected on the tape was moved periodically 7.6 cm to a counting position in front o f a Ge(Li) detector, data collection then being in 16 sequential spectra; and (c) to measure absolute branching ratios, the tape passed into a curved slot in an aluminum block at the counting position, the block being thick enough to annihilate all positrons from the decay o f 49Mn. A portion o f the ")'-ray spectrum observed at the collection point (mode (a)) is shown in fig. 1. The most prominent "/-rays are known lines associated with the decay [6] of 49Cr, but in addition there is a clear peak at 272 keV. Sequential measurements (mode (b)) 208
yielded a half-life of 350 -+ 80 ms for that peak and revealed a major short-lived component o f the annihilation radiation - see the inset to fig. 1 - with a fitted half-life of 384 -+ 17 ms. Evidently, both are features o f the same nuclear decay. The observed half-life alone indicates 49Mn as the source o f activity, since all other A = 49 nuclides that can be produced with our b e a m target combination are known to have very different lifetimes. The argument is clinched b y a precise measurement o f the observed "}'-ray energy based on the known energies [6] of the 49Cr lines. The result, 272.3 + 0.4 keV, agrees closely with the energy of the first excited state (271.4 + 0.4 keV [7]) in 49Cr, the/3 +decay daughter of 49Mn. No other short-lived ")'-rays were observed below 3 MeV. With the source o f the observed activity unambiguously identified, the branching ratios for the 3-transitions from 49Mn were determined b y measuring the intensity o f the 272 keV 7-ray relative to the short-lived component of the 511 keV radiation as observed with
7 April 1980
PHYSICS LETTERS
Volume 91B, number 2
experimental mode (c). Since all positrons annihilated in a controlled geometry, the intensity o f the 511 keV peak could, after correction for annihilation in flight (8.1 -+ 2.0%; see ref. [8]), be related directly to the total positron decay intensity. We established the detection efficiency b y placing standard sources ,1 of 133Ba and 22Na at the same position in the a l u m i n i u m block as that taken b y the separated 49Mn samples; this calibration was sufficient since these sources provided a 7-ray line at 276 keV (133Ba) as well as radiation from the annihilation of positrons (22Na). Our result gives the intensity of the 272 keV 3,-ray as (6.4 -+ 2.6)% of the total n u m b e r of 49Mn disintegrations. It is corrected for real coincidences b e t w e e n 272 keV 7-rays and annihilation radiation, a 10% effect with our geometry; the electron capture probability is negligible (EC//3 + < 0.1%). The results obtained for the decay o f 49Mn are summarized in fig. 2 and table 1. The low log ft value measured for the ground-state transition shows it to be superallowed, and this establishes the s p i n - p a r i t y of 49Mn to be the same as that o f 49Cr, which is k n o w n [7,9] to be 5 / 2 - . This assignment is also consistent with $1 The 133Baand 22Na sources were prepared and their activities measured (to +-1% and -+2%, respectively) by the Chalk River radioisotope standardization group: J.S. Merritt, A.R. Rutledge and L.V. Smith.
J l
0
49Cr
5/ 2-//
7716 5/~49Mn 384ms
/~1 6.4 /3~ 93.6
4.76 3.67
Fig. 2. Proposed decay scheme for 49Mn. the observed allowed/3 + branch to the 7 / 2 - excited state at 272 keV. In general an allowed/3-transition is described [ i ] by
K/G~ (1 + 6 r ) t = f v ( 1 ) 2 ( I - 6c) + fAR2(o'~) 2 ' where K = 1.230618 X 10 - 9 4 erg2cm6s, t is the experimental partial half-life, G~r is the effective vector coupling constant, fir and 6 c are radiative and chargedependent mixing corrections, f v and fA are statistical rate functions calculated for vector and axial-vector decay, (1) and (a~> are the corresponding nuclear matrix elements, and R is the axial vector coupling con-
Table 1 Summary of results for the observed decay branches of 49 Mn. Transition to ground state branching ratio (%) partial half-life a) t (ms) end-point energy Ema x (/3) (keV) statistical rate functions : fV log ft corrections : 8r (%) 8 e (%)
fA
93.6 -+ 2.6 410 -+ 22 6692 +- 24 11390 -+ 190 11790 -+ 190 3.67 +- 0.02 2.04 0.57
272 keV state 6.4 -+ 2.6 6000 +- 2500 6420 -+ 24 9700 -+ 160 4.76 -+ 0.20 2.06
experimental
0.434 +- 0.052
0.260 -+ 0.053
calculated <(I~) : KB f9 KB f9 + f8 r KB* f9 + f8 r
0.623 0.302 0.375
0.078 0.248 0.227
a) Based on a total 49Mn half-life of 384 -+ 17 ms. 209
Volume 91B, number 2
PHYSICS LETTERS F
stant expressed in units of G V. For our present purposes we shall ignore renormalization effects in R, and use the value 1.237 +- 0.008 derived from the decay o f the neutron [1 ]. The data for the observed 49Mn transitions have been analyzed with the same techniques as used in ref. [ 1 ] to extract experimental values for the G a m o w - T e l l e r matrix elements
210
7 April 1980
pole centroids of the interaction [ 11 ]. By shifting the energy centroids o f f9 configurations from those with f8r, the effect of this modification is to weaken the corrections to ( o , ) from perturbation theory. The calculated values o f (G~> shown in the last line of the table are in excellent agreement with experiment, and indicate that the KB* interaction not only improves agreement with experimental binding energies [ 11 ] but may also be efficacious in calculating transition probabilities. Further tests in the same mass region would be valuable.
References [1 ] S. Raman, C.A. Houser, T.A. Walkiewicz and I.S. Towner, At. Data Nucl. Data Tables 21 (1978) 567. [2] D. Mueller, E. Kashy, W. Benenson and H. Nann, Phys. Rev. C12 (1975) 51. [3] A.H. Wapstra and K. Bos, At. Data Nucl. Data Tables 19 (1977) 177. [4] R. Kirchner and E. Roeckl, Nucl. Instrum. Methods 133 (1976) 187. [5 ] P. Dam, E. Hagberg and B. Jonson, Nucl. Instrum. Methods 161 (1979) 427. [6] S.V. Jackson, E.A. Henry and R.A. Meyer, Phys. Rev. C12 (1975) 2094. [7] M.L. Halbert, NucL Data Sheets 24 (1978) 175. [8] J.C. Hardy, H. Schmeing, J.S. Geiger and R.L. Graham, Nucl. Phys. A223 (1974) 157. [9] J.O. JBnsson, L. Sanner and B. Wannberg, Phys. Scr. 2 (1970) 16. [10] T.T.S. Kuo and G.E. Brown, Nucl. Phys. Al14 (1968) 241. [11] A. Pores, E. Pasquini and A.P. Zuker, Phys. Lett. 82B (1979) 319; E. Pasquini and A.P. Zuker, Proc. Topical Conf. on Physics of medium light nuclei (Florence, 1977), eds. P. Blasi and R.A. Ricci (Editrice Compositori, Bologna, 1978) p. 62.