Superheavy Element Research at the Berkeley Gas-filled Separator. Jacklyn M. Gates Lawrence Berkeley National Laboratory

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1 Superheavy Element Research at the Berkeley Gas-filled Separator Jacklyn M. Gates Lawrence Berkeley National Laboratory

2 Outline BGS Gas mixtures

3 LBNL Heavy Element Group Heino Nitsche Ken Gregorich Jacklyn Gates Gregory Pang Paul Ellison Oliver Gothe Nick Esker Joe McLaughlin Group Leader, UCB Prof. of Chem. Senior Staff Scientist Project Scientist Postdoctoral Fellow Graduate Student/Postdoctoral Fellow Graduate Student Graduate Student Undergraduate Student With help from The Nuclear Structure Group Walter Loveland (Oregon State U.) and others

Senior Staff Scientist Project Scientist Postdoctoral Fellow Graduate Student/Postdoctoral Fellow

4 Berkeley Gas-filled Separator (BGS) Beam Rutherford Detectors Beam Trajectory Gas-Filled Chamber Punchthrough Detector Target Quadrupole Magnet Focal Plane Detector Gradient- Field Magnet Flat-Field Dipole Magnet MWAC EVR Trajectory Compound nucleus recoils are ejected from the target with the momentum of the beam In 0.5-Torr He, they experience many charge-changing collisions, giving 100% charge acceptance Average charge is (nearly) proportional to velocity, giving LARGE velocity acceptance Compound nucleus evaporation residues (EVRs) are focused on the detector array everything else has a smaller magnetic rigidity, and takes a left turn

In 0.5-Torr He, they experience many charge-changing collisions, giving 100% charge acceptance Average charge is (nearly) proportional to velocity, giving LARGE

5 Berkeley Gas-filled Separator (BGS)

6 Understanding Magnetic Rigidity in He Gas Back to basics... experimental average charge LBL(SASSY), Dubna, Julich BGS Z > 98 Betz, Whitkower Q = 0.641(v/v 0 )Z 1/ v/v 0 Z 1/3 Back in 1948, Neils Bohr suggested a q = vz 1/3 dependence Two problems: 1) There is lots of scatter. Deviations are +/- 10%. Can this be understood in terms of the electronic shell structure of the stripped ions? 2) Strong deviations at low velocities due to the high ionization potential of He.

7 Ghiorso and Armbruster suggest that deviations are due to electronic shell structure of stripped ions

8 q exp - sunusiodal correction Understanding Magnetic Rigidity in He Gas Sinusoidal correction based on electronic structure of stripped ion LBL(SASSY), Dubna, Julich BGS Z > 98 Betz, Whitkower Q = 0.641(v/v 0 )Z 1/ x = (v/v 0 )Z 1/3 m = b = c = d = q ave = mx + b + csin[(2π/32)(z-mx-b-d)] v/v 0 Z 1/3 Semi-empirical understanding of why this works: If the stripped ion is in an f- orbital, the most loosely bound electrons are inner electrons, and are less available for stripping by the gas, giving a lower q. If the stripped ion is in a p- orbital, the most loosely bound electrons are outer electrons, and are readily available for stripping by the gas, giving a higher q. But problems arise at low velocities!

647 q ave = mx + b + csin[(2π/32)(z-mx-b-d)] 1 2 4 6 8 10 12 14 16 v/v 0 Z 1/3 Semi-empirical understanding of why this works: If the stripped ion is in an f- orbital, the most loosely bound

9 Understanding Magnetic Rigidity in He Gas Iodine and uranium data show a break below v = 1.6v 0 v 0 2v 0 v 0 2v 0 The red lines trend toward q = 2.5 at v = 0 because the first of ionization potential of He is 25 ev. This is usually between the second and third ionization potentials of heavy elements.

5 at v = 0 because the first of ionization potential of He is 25 ev.

10 Understanding Magnetic Rigidity in He Gas After applying a slow velocity correction... q exp - (sinunsoidal+slow correction) LBL(SASSY), Dubna, Julich BGS Z > 98 Betz, Whitkower Q = 0.641(v/v 0 )Z 1/ v/v 0 Z 1/3 x = (v/v 0 )Z 1/3 m=0.641 b= c=0.517 d= q ave =mx+b+csin[(2π/32)(z-mx-b-d)] q slow (v/v 0 ) linear from (1.6,q ave ) to (0,2.5) Cause of low-velocity problem: average charge tends toward positive value at low velocities because of the large He ionization potential. Apply correction based on simplest assumption: Charge changes linearly between v/v 0 = 1.6 and q = 2.5 at v=0

235 1 2 4 6 8 10 12 14 16 v/v 0 Z 1/3 x = (v/v 0 )Z 1/3 m=0.641 b=-0.235 c=0.517 d=74.647 q ave =mx+b+csin[(2π/32)(z-mx-b-d)] q slow (v/v 0 ) linear from (1.

11 Symmetric Reactions magnetic rigidity (Tm) CN CN Ca Pb Pu ES ES Ca Pu Pb velocity (V 0 )

2 CN CN 48 48 Ca+ 206 242 Pb Pu ES ES 48 48 Ca+

12 Symmetric Reactions: 48 Ca+ 206 Pb beam xfers 254 No EVRs

13 Symmetric Reactions: 48 Ca+ 206 Pb beam xfers 254 No EVRs

14 Symmetric Reactions: 48 Ca+ 206 Pb beam xfers 254 No EVRs

15 All Low Energy Events: 48 Ca+ 208 Pb

16 Highly Asymmetric Reactions 2.4 magnetic rigidity (Tm) CN 22 Ne+ 242 Pu ES 22 Ne+ 242 Pu velocity (V 0 )

4 1.2 CN 22 Ne+ 242 Pu ES 22 Ne+ 242 Pu 1.

17 Asymmetric Reactions: 22 Ne+ 242 Pu beam xfers 104 EVRs

18 Asymmetric Reactions: 22 Ne+ 242 Pu beam xfers 104 EVRs

19 Asymmetric Reactions: 22 Ne+ 242 Pu beam xfers 104 EVRs

20 Problem areas for the BGS: low velocity recoils from highly asymmetric reactions

113 174 112 111 172 110 109 108 170 107 168 106 166

21 Problem areas for the BGS: low velocity recoils from highly asymmetric reactions

22 Old Average Charge Data from Betz and Whitkower v 0 2v 0

23 22 Ne+ 242 Pu: Br in He, H EVR (He) EVR (H 2 ) ES (He) ES (H 2 ) 2.5 BρMix( Acn, v, Zcn, 1.0) BρMix( Acn, v, Zcn, 0.0) BρMix( 235, v, 90, 1.0) BρMix( 235, v, 90, 0.0) v V/V 0

24 Predicted Combinations of H 2 :He

25 22 Ne+ 242 Pu: Br in 60:40 He:H 2 mixture EVR ES 2.5 BρMix( Acn, v, Zcn, 0.4) BρMix( 235, v, 90, 0.4) v V/V 0

27 22 Ne+ 242 Pu in 30% H 2 :70% He beam xfers 104 EVRs

28 22 Ne+ 242 Pu in 30% H 2 :70% He beam xfers 104 EVRs

29 22 Ne+ 242 Pu in 30% H 2 :70% He beam xfers 104 EVRs

31 13% H 2 : 87% He at 0.5 torr 21% H 2 : 79% He at 0.5 torr 30% H 2 : 70% He at 0.5 torr 30% H 2 : 70% He at 0.25 torr

32 Project Gamma-ray energy tracking array Covering ¼ of 4p solid angle with 28 segmented Ge crystals Mechanical support structure Acquisition system

33 Expeiments, Fall 2011 seven 4-crystal modules efficiency similar to GS g-ray track reconstruction 6-month campaign In-beam spectroscopy exp: 208 Pb( 48 Ca,2n) 254 No, 10x statistics improvement 208 Pb( 50 Ti,1-2n) 257,256 Rf, highest-z for in beam spect.

34 Preparation

35 Preparation

36 Installation

37 Installation

38 Installation

39 Installation

40 Installation

41 Target

42 Status Experiments with full GRETINA began 20 Sept with 48 Ca+ 208 Pb The BGS and the new focal plane detectors are in great shape. We cleanly identify recoil implants, alpha decays, and electron bursts at the expected rates for the beam intensities we have been running

43 Status

44 Status Experiments with full GRETINA began 20 Sept with 48 Ca+ 208 Pb The BGS and the new focal plane detectors are in great shape. We cleanly identify recoil implants, alpha decays, and electron bursts at the expected rates for the beam intensities we have been running GRETINA was clearly able to see ground state band in 254 No although some improvements can be made to increase resolution New 20-day beamtime beginning 18 Oct Start with 254 No to test updated software Then 256 Rf???

GRETA. I-Yang Lee. For the GRETA Steering Committee. Workshop on the Experimental Equipment for RIA March 18-22, 2003, Oak Ridge, Tennessee

GRETA. I-Yang Lee. For the GRETA Steering Committee. Workshop on the Experimental Equipment for RIA March 18-22, 2003, Oak Ridge, Tennessee GRETA I-Yang Lee For the GRETA Steering Committee Workshop on the Experimental Equipment for RIA March 18-22, 2003, Oak Ridge, Tennessee Gamma-ray Tracking Concepts Compton Suppressed Ge Veto Ν det = 100