Doctoral Defense of Barak Hadina 18 th January 2008 In-Beam Study of Extremely Neutron Deficient Nuclei Using the Recoil Decay Tagging Technique Opponent: Dr. Paddy Regan, Department of Physics University of Surrey Guildford, UK

What are the ‘Big Questions’ in Nuclear Physics? 1.What are the limits of nuclear existence ? What combinations of protons and neutron can exist ? 2.How do nuclear properties change at extremes of proton and neutron separation energy; how is the single-particle structure altered under ‘extreme’ conditions including rotational stress ? 3.Are there any new collective effects or symmetries apparent in extreme nuclei, such as extra proton-neutron pairing at N ≈ Z ? 4.What effect do nuclear structural properties have on explosive nucleosynthesis (such as the rapid-proton capture process)?

What are the limits of nuclear existence ? Nuclear chart is ‘bounded’ by the proton and neutron drip-lines S n,S p <0. Particle ‘drip lines’ are defined by ground-state, particle radioactivity. Some cases,  -decay also puts a limit on what is ‘bound’.

Nuclear mass formulas (vital inputs for nuclear astrophysics calculations) are excellent at explaining ‘known’ masses…. BUT….. at the extremes of N : Z (e.g., 106 Te, 110 Xe) which one is correct ? Need data to constrain these models……  decay can determine masses to high accuracy (if the parent mass is known). Mass differences for Sn (Z=50) isotopes

Nuclear Excited States – Nuclear Microscopy… Nuclei can exist in either the ground state or an excited state Decays from excited states tell us about the internal structure!!! Each nucleus is different….but groups of structural patterns do appear….

Evidence for Nuclear Shell Structure? Nuclei with magic numbers of neutrons/protons have: N=50 N=82 N= state at higher energy Low values of B(E2: →0 + ) (i.e. small transition probability) N=126 N=82 N=50 E(2 + ) and B(E2) apparently correlated….small E(2 + ) usually has large B(E2) and vice versa.

Signatures of Nuclear Structure ? E(2 + ) value related to B(E2) by Raman’s empirical relation. E(4 + ) / E(2 + ) = R 4/2 is a measure of collective effects in nuclei. –Rotor = nuclear quadrupole deformation has E x (I) ~ I(I+1), R 4/2 = (4x5) / (2x3) = 20 / 6 = 3.33 –Perfect (harmonic, spherical) quadrupole vibrator has, E x (I) ~ I, R 4/2 = 4 / 2 = 2.00 –Axially asymmetric rotor, gamma-soft nucleus, R~2.5

Fusion-Evaporation Reactions Typical geometrical cross-section in fusion is given by ~  b 2, where b ~1.2(A 1 +A 2 ) 1/3 fm. i.e., expect  tot ~30 fm 2 = 0.3 barns (1b= m 2 ) KE beam > E Coulomb to form compound nucleus. E beam & Q-value lead to excitation energy, lost by particle evaporation. Reaction used in thesis to create 106 Te was 54 Fe+ 54 Fe → 108 Te*, Main evaporation channels from 108 Te* are 3p → 105 In, 2pn → 105 Sn, 2p → 106 Sn, a2p → 102 Cd, etc. since S p < S n. Most exotic final channel is 2n → 106 Te,  ~25nb. Problem ? How do you select events with 25nb out of 0.3b ? i.e., need channel selection to accuracy of ~1 part in 10 7 ? b

Spectroscopy ‘beyond’ the drip-line ? Coulomb barrier → unbound protons and/or  s are hindered in their decay, i.e., unbound nuclei have finite lifetimes. If gamma-ray decays of excited states are short (fs → ns) compared to ground state decay, these can decay before. The discrete ground state decay can be used as a ‘tag’ for the gamma-rays as coming from the specific nucleus of interest → Recoil Decay Tagging Technique

T 1/2 ( 108 Te) = 2s; E  =3.3 MeV (2p2n chan)

Nuclear Alpha Decay Conservation of energy m X c 2 = m X’ c 2 +T X’ +m  c 2 +T  Q-value for the decay Q = (m X - m X’ - m  ) × c 2 = T X’ + T  Conservation of momentum p  = p X’ so p  2 / 2m  >> p X’ 2 / 2m X’ T  >> T X’ typically 5.0 MeV : 0.1 MeV The Coulomb potential  1/r outside nucleus The nuclear potential << 0 inside nucleus Q-value typically 5 → 6 MeV b Is ~40fm (determined by decay Q-value). a is typically ~8 fm. For A~200, barrier height, B at a is typically 34 MeV The  -particle inside can tunnel to the outside, the wave function decays exponentially through barrier

~ × per MeV Geiger-Nuttall rule log t 1/2 = (const /  Q) - const

Height of the Coulomb barrier C = Radius where barrier is cleared b = k 2 = ( 2m/  2 ) × ( V (r) - Q )    a × e - k r through the barrier Probability to penetrate barrier P = e - 2 G where the Gamow factor G is G = ( 2m/  2 ) 1/2  a b ( V(r)- Q) 1/2 dr P is bigger (G smaller) for larger Q Geiger-Nuttall rule follows Nuclear Alpha Decay 1 z Z’ e 2 4  0 a 1 z Z’ e 2 4  0 Q Angular momentum adds an additional centrifugal potential V centrifugal = ( +1)  2 / 2m  r 2 adding an dependent additional barrier Decays favour large Q and small value % % 25.0 % 74.0 % Cm PuQ g.s. = MeV t 1/2 = 163 d    = ( -1 ) aside:

Physics at N≈Z, np Pairing Protons and neutrons in same orbitals, extra coupling might mean extra collectivity. Most likely at N≈Z (due to large spatial overlap of orbitals). Problem? Formation of heavy N~Z nuclei hard with fusion- evaporation reactions ( 106 Te, S n ~10.5 MeV, S p ~1.3 MeV). nnppnp T=1, S=0 T=0, S=1 TZ:TZ:+1 0

First data on 106 Te, E(2 + ) significantly lower in than expected from shell model calculations. What is missing, evidence for np pairing ? Note, experimental ‘tour de force’ at  =25nb !!! 106 Te

110 Xe (Z=54, N=56) has T z =+1. Most exotic Xe isotope studied to date. One expects E(2 + ) to increase and R 4/2 to decrease compared to 112 Xe... BUT the opposite happens! Is this evidence for new collective effects which only are apparent at N≈Z (i.e., np pairing ) ? (a) use 2-step decay to select 110 Xe events. (b) RDT gives 1 st 110 Xe decay scheme. 110 Xe Xe (Z=54) isotopes

RDT also applicable to heavier, alpha/proton drip-line systems. Fine structure (i.e., l -dependence in  decay) shows up dramatically in odd-odd systems (such as 170 Ir and 166 Re). More complicated  spectrum than A~110 region. Fine structure allows RDT to be used on specific single-particle coupling configurations.

Tagging gives level scheme. Structural assignment based on (a)Fine structure of alpha-decay (b)B(M1)/B(E2) branching ratios (c)Alignment properties

Summary of new physics present in thesis First experimental investigation of excited states in VERY neutron deficient nuclei 106,7 Te and 110 Xe. –Energy of (7/2 + ) state in 107 Te important for the rp-process. – 106 Te new in-beam ‘world record’ at 25 nb cross-section. Possible evidence for np-pairing ? – 110 Xe, increased collectivity compared to 112 Xe. Evidence for np-pairing ? Evidence for rotational bands and significant