1. -delayed fission of neutron-deficient Fr and At isotopes Lars Ghys1, Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven 2 Belgian Nuclear Research Centre, SCK•CEN, Mol, Belgium

2. Outline -delayed fission Experimental set-up Calibration method
Introduction
Fission at low excitation energies : systematics
Experimental set-up
ISOLDE
“Windmill” system
Calibration method
Experimental results
df of 202Fr
df of 200Fr
df of 194,196At
Summary
‘parasitic’ decay information
Conclusions & outlook
03/06/2013
INPC conference 2013

3. β-delayed fission : introductionQ Bf  
DF
QEC
βdf of 180Tl[1] :
Expected :
Symmetric 180Tl  180*Hg → 90Zr + 90Zr
Observed:
Asymmetric
180Tl  180*Hg → 100Ru + 80Kr
Fission properties of other nuclei in Pb-region??
Near or sub barrier fission → sensitivity to microscopic shell effects
Known ranges of spins & parities
‘classical’ U-region (N/Z ~ 1.5) and Pb-region (N/Z ~ 1.3) → study isospin dependence
Study partial half lifes as a function of Q and Bf
Deze slide nog veranderen!!
[1]: A.N. Andreyev et al./ PRL 105 (2010)
03/06/2013
INPC conference 2013

4. Fission at low excitation energies: systematics
df of n-deficient At & Fr?
[1]: A.N. Andreyev et al./ submitted to RMP
03/06/2013
INPC conference 2013

5. ISOLDE facility - CERN 2 µA of protons on UCx target
1.4 GeV protons
HRS mass
separator
GPS mass
GPS target
Windmill
position
HRS target
2 µA of protons on UCx target
isotopic (isomeric) pure beam
beam energy of keV
S.Rothe et al./ Nature Comm 4 (2013) 1835 .
03/06/2013
INPC conference 2013

6. Experimental set-up: “Windmill system”
Annular Si Si
Isotopically pure beam
from ISOLDE
C-foils
20 mg/cm2
Si detectors Ge
C-foils
Si detecors
241Am ff
/ 
ISOLDE beam
Ge detector Ge
annular Si Si
03/06/2013
INPC conference 2013

7. Calibration method – coincident fission events
Measurement : pulse height detector 1 (X1) and detector 2 (X2)
calibration
E1 = (a1 + a’1M1)X1 + b1 + b’1M1 energy calibration det 1 (Lohengrin – ILL)
E2 = (a2 + a’2M2)X2 + b2 + b’2M energy calibration det 2 (Lohengrin – ILL)
M1E1 = M2E Conservation of linear momentum
M1 + M2 = A Conservation of mass
Extraction of M1, M2, E1 & E2
Additional corrections
Energy loss in carbon foil (SRIM simulations)
ΔEfront = 0.3 MeV ; ΔEback = 1.2 MeV [S. Sels]
neutron emission
Sifront
Siback
~900 Ǻ
03/06/2013
INPC conference 2013

8. -delayed fission of 202Fr (IS466) - preliminary
Total Kinetic Energy (TKE) ≈ 147 MeV
202Fr
2 isomers !
202Fr
A.N. Andreyev et al./ PRL 105 (2010)
180Tl
180Tl
Binning?
TKE ≈ 135 MeV
03/06/2013
INPC conference 2013

9. -delayed fission of 200Fr (IS534)
After ~20 hrs of measurement:
6 fission fragments
2 events in coincidence
→ sufficient to extract TKE and partial βdf half life
200Fr
200Fr
PRELIMINARY
Preliminary calibration
Bdf = being patient (low statistics)
But, even with low statistics : TKE and branching (partial half lifes) can be extracted
03/06/2013
INPC conference 2013

10. -delayed fission of 194,196At (IS534) - preliminary
2 isomers!
TKE  152 MeV
196At
196At
TKE  153 MeV
Courtesy to Simon Sels
03/06/2013
INPC conference 2013

11. Mass distributions :Comparison
180Tl
194At
Counts/ 4 amu
196At
202Fr
Mass (amu)
03/06/2013
INPC conference 2013

12. Fission at low excitation energies: systematics
196Po
194Po
202Rn
[1]: A.N. Andreyev et al./ submitted to RMP
03/06/2013
INPC conference 2013

13. ‘Parasitic’ information on decay : 202Fr
Electrons (~106)
 Particles (~107)
202Fr : energy spectrum (Si)
-  summing
Fission fragments (116)
202Fr
~44 hrs of measuring
Pure  spectra
() -  coincidences
 Construct decay schemes
198At
202Rn
198Po
03/06/2013
INPC conference 2013

14. Conclusions & outlook Conclusions
df of 200,202Fr and 194,196At confirmed and studied at ISOLDE
Transition region from asymmetric to symmetric fission
Due to long measurement times  large statistics on decay spectroscopy
Lifetime determinations
Fine structure in  decay
Identification of isomeric states ….
Outlook
Determine df branching and partial half lifes  compare with other nuclei
Separation of isomers in 202Fr (CRIS) and 194At (RILIS)
Discussion with theoretical groups (P. Möller, S. Panebianco, M. Warda, Dubna)
03/06/2013
INPC conference 2013

15. Collaboration Comenius University, Bratislava A.N. Andreyev D. Jenkins
V. Truesdale
H. de Witte
J. Elseviers
L. Ghys
M. Huyse
D. Radulov
M. Rajabali
C. Van Beveren
P. Van den Bergh
P. Van Duppen
B. Andel
S. Antalic
Z. Kalaninova
X. Derkx
J. Lane
V. Liberati
T. E. Cocolios
E. Rapisarda
ISOLDE & RILIS
K. Nishio, Y. Nagame, JAEA, Tokai, Japan
F.P. Hessberger, SHIP (GSI), Darmstadt, Germany
U. Koster, ILL, Grenoble, France
R. Page, OLL, University of Liverpool
P. Möller, Los Alamos National Laboratory, US
L. Ghys
L. Popescu
D. Pauwels
03/06/2013
INPC conference 2013

16. Thank you!
03/06/2013
INPC conference 2013

17. Back-up
03/06/2013
INPC conference 2013

18. Total kinetic energy (TKE) and prompt neutron emission
TKE when no prompt neutrons are emitted = 147(1) MeV Fragment excitation energy ≈ [Q(fission)-TKE+QEC]/2
N/Z (202Rn) = 1.35
1.30 < N/Z < 1.40
Rode punten een beetje groter maken. (gedaan -> cluster)
Up to 2 prompt neutrons can be emitted in a fission event.
TKE when 2 prompt neutrons are emitted : 148(1) MeV
03/06/2013
INPC conference 2013

19. Pulse height defect (PHD)
Why use Si detectors?
intrinsic efficiency ≈100 %
good energy resolution : FWHM ~ 1.5 MeV
compact size
Disadvantage : Pulse height defect
Energy loss in detector dead layer
Electron – hole recombination
Nuclear collisions
Schmitt calibration [1] : E = aX + b → E = (a+a’M)X+ b + b’M
[1] : H. W. Schmitt et al. / Phys. Rev. 137 (1965) B837 – B847
Lowering of recorded pulse height
03/06/2013
INPC conference 2013

20. Calibration measurements – Lohengrin, ILL
Fission framents from 235U(n,f)
Energy/q and mass/q selection
Energy accuracy of ~100 keV
5 detectors tested : 2x PIPS; 3x surface barrier (1x annular)
Mass range : 48 – 149 amu Energy range : 12 – 100 MeV
03/06/2013
INPC conference 2013

21. Calibration Measurements – Lohengrin ILLSB
PIPS
Pulse height defect = Eion – aXion - b
Schmitt calibration valid for typical fission masses (80 – 136 amu) and energies (30 – 100 MeV)
180Hg – fission products:
EL  61 MeV ; MH  100 amu
EH  74 MeV ; ML  80 amu
Perpendiculair vs. Grazing (45°) incidence  effect of dead layer
03/06/2013
INPC conference 2013

22. Calibration method
E1 = (a1 + a’1M1)X1 + b1 + b’1M1 Schmitt calibration det 1
E2 = (a2 + a’2M2)X2 + b2 + b’2M2 Schmitt calibration det 2
M1E1 = M2E Conservation of linear momentum
M1 + M2 = A Conservation of mass
Extraction of M1, M2, E1 & E2
Additional corrections
Energy loss in carbon foil (SRIM simulations)
ΔEfront = 0.3 MeV ; ΔEback = 1.2 MeV [Simon]
neutron emission
Close geometry  wide angular range of detected particles
ILL calibration measurements
03/06/2013
INPC conference 2013

23. Total kinetic energy 202Rn: TKE ≈ 147 MeV 180Hg σ = 8(1) MeV
Preliminary: up to 2 prompt neutrons / fission event possible
180Hg
TKE = 134.6(7) MeV (no neutrons)
σ = 5.6(8) MeV
0 or 1 prompt neutron emitted
03/06/2013
INPC conference 2013

24. βdf in the neutron-deficient Pb region
nucleus
facility
reference
180Tl
ISOLDE, CERN
A.N. Andreyev et al./ PRL 105 (2010)
192,194At
SHIP, GSI
A.N. Andreyev et al./ PRC 87 (2013) 14317
178Tl
V. Liberati et al., submitted
186,188Bi
J. Lane et al./ PRC 87 (2013) 14318
200,202Fr
L. Ghys et al. - preliminary
194,196At
V. Truesdale, S. Sels et al - preliminary
200,202Fr
192,194,196At
186,188Bi
178,180,182Tl
03/06/2013
INPC conference 2013

25. ‘Parasitic’ information on decay : 202Fr
- plot : 202Fr
Single ’s
202Fr
198At
Kleurencode gebruiken!
03/06/2013
INPC conference 2013
M. Huyse et al./ PRC 46 (1992) 1209