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SUMMARY:Beta-decay as a probe of the  isospin doublet in 8Be
DTSTART;VALUE=DATE-TIME:20240528T135000Z
DTEND;VALUE=DATE-TIME:20240528T141000Z
DTSTAMP;VALUE=DATE-TIME:20260514T231301Z
UID:indico-contribution-23075@indico.ific.uv.es
DESCRIPTION:Speakers: Maria Jose Garcia Borge (ISOLDE-CERN)\nSince the mid
 -60s\, the presence of a 2$^+$ doublet in $^8$Be\, constituted by the 16.6
  and 16.9 MeV excited states\, has been observed [1-3]. An intriguing aspe
 ct of this doublet is its status as the best-known instance featuring a co
 mplete isospin mixing\, where the 16.6 MeV ($^7$Li+p) and 16.9 MeV ($^7$Be
 +n) levels can be decomposed in an equal mixture of two pure isospin (T=0 
 and T=1) levels [4]. While indications of this behaviour have hinted throu
 gh R-Matrix fits in reaction experiments [5]\, direct confirmation is stil
 l pending.\n\nThe 2$^+$ isospin doublet in $^8$Be\, comprising the 16.6 an
 d 16.9 MeV excited states\, has been consistently observed [1-3] since the
  mid-1960s\, through reaction experiments. Notably\, this doublet stands o
 ut as the most well-known instance of a total isospin mixing\, where the 1
 6.6 MeV ($^7$Li+p) and 16.9 MeV ($^7$Be+n) levels exhibit an equal mixture
  of two pure isospin (T$=0$) and (T$=1$) states [4]. While R-Matrix fits i
 n reaction experiments have hinted at this behaviour [5]\, direct confirma
 tion remains pending.\n\nThe beta decay of the 1-proton halo nucleus $^8$B
  into $^8$Be offers a valuable avenue for probing the isospin composition 
 of the doublet through selective Fermi and Gamow-Teller components. Howeve
 r\, resolving the feeding to the 2$^+$ doublet poses challenges. Within th
 e Q$_{\\text{EC}}$ window (Q$_{\\text{EC}}$ = 17.9798(1)\\) MeV)\, the pre
 dominant ($\\geq$ 88 %) decay mode leads to a broad \\(2$^+$) state at 3 M
 eV [6]\, extensively studied due to its significance as a primary source o
 f high-energy solar neutrinos [7]. Additionally\, beta decay can occur via
  electron capture (EC) at 17\,640 MeV. Assuming the EC decay occurs in the
  core with the halo proton as a spectator\, the strength of this unobserve
 d branch is estimated from the decay of $^7$Li to be a branching ratio of 
 \\(2.3 $*$ 10$^{-8}$)[8].\n\n\nThe IS633 experiment\, conducted by the MAG
 ISOL collaboration at the CERN/ISOLDE facility's decay station (IDS)\, is 
 focused on investigating the 2$^+$ doublet of $^8$Be through the beta deca
 y of $^8$B [9\,10]. A mass-separated 50 keV $^8$BF$_2$ beam was implanted 
 in a (30\, $\\text{mg/cm}^2$) carbon foil catcher. Through EC/($\\beta^+$)
  decay\, $^8$B feeds the excited states of $^8$Be\, which subsequently bre
 ak up into two $\\alpha$ particles or a proton and a $^7$Li\, depending on
  the level fed. Detection of the breakup fragments is done through a syste
 m of particle telescopes\, each comprising a Double-Sided Silicon Strip De
 tector (DSSD) stacked with a thick Si-PAD detector. These telescopes\, arr
 anged in pairs of opposite-facing detectors\, enabled precise data collect
 ion.\n\nIS633 represents a significant advancement\, achieving a two-order
 -of-magnitude improvement in statistics over the preceding benchmark exper
 iment JYFL08 [9]. High-statistics data from IS633 enabled the resolution o
 f the continuum spectrum of $^8$Be from 1 MeV up to 17 MeV. Notably\, the 
 16.6 MeV and 16.9 MeV doublet were resolved for the first time in a beta d
 ecay study.\n\nThis contribution provides a comprehensive description of e
 xperiment IS633\, including the analysis of excitation spectra using R-mat
 rix methods and an alternative approach based on beta recoil. These comple
 mentary analyses have facilitated the determination of isospin mixing in t
 he doublet and the identification of the Fermi and Gamow-Teller components
 .\n\n**Refrences**\n\n[1] F.C. Gilbert\, Phys. Rev. 93 (1954) 499.\n\n[2] 
 B.J. Farmer et al.\, Nucl. Phys 15 (1960) 626.\n\n[3] E. Matt et al.\, Phy
 s. Lett. 9 (1964) 174.\n\n[4] T. Nilsson et al.\, Hyperfine Int 129 (2000)
  67.\n\n[5] C.P. Browne et al.\, Phys. Lett. 23 (1966) 371.\n\n[6] P. von 
 Brentano\, Physics Reports 264 (1996) 57.\n\n[7] J.N. Bahcall and C. Peña
 -Garay\, New Journal of Physics 6 (2004) 63.\n\n[8] M.J.G. Borge et al.\, 
 J. Phys. G 40 (2013) 035109.\n\n[9] O. Kirsebom et al.\, Phys. Rev. C 83 (
 2011) 065802.\n\n[10] S. Viñals\, PhD thesis. Complutense University of M
 adrid (2020).\n\nhttps://indico.ific.uv.es/event/7332/contributions/23075/
LOCATION:ADEIT - Valencia
URL:https://indico.ific.uv.es/event/7332/contributions/23075/
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