1. On octupole nuclei and the status of SPEDE
George O’Neill
Thanks, good morning, as has been said I’m George O’Neill, I’m now in my second year at the university of Liverpool, and I’m going to go talk about the 221 radon data I’ve been analysing, as well as the 220 radon and 224 radium isotopes that Liam Gaffney analysed, and then will discuss a detector that’s currently in development, named SPEDE.

2. Octupoles Certain nucleon configurations have reflection asymmetry
(N, j, l) intruder orbitals interact with (N – 1, j – 3, l – 3) natural states
‘Octupole magic numbers’ ≈ 34, 56, 88 and 134
Can measure B(E3) transitions
Certain nuclear structures have a reflection asymmetery; essentially, they look like a pear. These octupole deformation occurs when N, j, l intruder orbitals interact with natural parity states that have j and l values 3 less, and the octupole-octupole part of the nucleon-nucleon force is enhanced. A series of octupole magic numbers results, at proton or neutron numbers 34, 56, 88 and In this diagram, we’ve got an even-even isotope, undergoing various degrees of octupole deformation. From left to right, we’ve got a quadrupole deformed nucleus with an octupole vibration averaging zero on the left, an intermediate case with a small potential barrier separating two degenerate deformed minima in the middle and an ideal rigid deformation on the right We can see what the corresponding levels look like in those potential wells; we have a staggering between odd and even states, and this is analogous to reflection asymmetric molecules such as Hydrogen chlorine, where we see an odd-even staggering in the rotational band. By looking at the E3 transitions and matrix elements we can directly determine the octupole collectivity unambiguously,

3. 220Rn results 220Rn, 3x105 ion/s 2.82 MeV/A
Okay, so here we have Liam Gaffney’s results from 220-Radon, taken in 2011, showing a good spectrum after doppler correction, with lots of the transitions identified and a good background subtraction as we expect at MINIBALL. The beam was incident on two different targets, with Nickel shown in blue and tin in red, each having a different cross-section when interacting via coulomb excitation with the beam. The facilities offered at MINIBALL mean both even- and odd-order electric multipole matrix elements could be extracted from the spectra with an accuracy of at least 10%.

4. 224Ra results 224Ra, 7x105 ion/s 2.83 MeV/A
And the same for 224-Radium. We can then take the information from these transitions, the intensities, centre of mass ranges, free matrix elements and normalisation factors, and use the GOSIA code to perform a minimisation to these matrix elements, and interpret the results.

5. 220Rn Ra
Unstretched E3 in 224Ra is a good indication of rotational octupole implying a rigid deformation rather than vibrational behaviour

6. 220Rn Ra interpretation
And when we do that, we see dynamic collectivity in 220-radon on the left here, and a static deformation in 224-radium.The results are consistent with the predictions of the mean-field approach, howhiever the predicted trend of the cluster model is ruled out.Ths, with further systematic data along these isotopic chains will allow us to constrain suitable candidates for further studies of the atomic electric dipole moment.

7. Studies of pear-shaped nuclei using accelerated radioactive beams Nature 497, 199–204
And this work was published in May in Nature. So that’s a set of really interesting results from the even-even isotopes, and I’m going to expand on that with a discussion on what I’ve been working on since I started my PhD last August, the odd-mass nuclei.

8. Odd octupoles Odd nucleon couples with core
(almost) Degenerate parity doublets
Enhanced Schiff moment (~3 orders of magnitude)
Improves limits on EDM
What we see in odd-mass octupoles is the motion of the unpaired nucleon coupling with the collective nuclear asymmetric structure, resulting in nearly-degenerate parity doublets being seen, as can be seen in the figure; compared to even-even nuclei there is much less of a defined stagger between states; the change in energy between states mainly occurs as the magnitude of parity increases. We can measure the energy splitting of the parity doublets, delta E, to determine the Schiff moment, rather than measuring the Q3 moment which was used for the even-even isotopes. It gives us a more precise measurement with increased sensitivity. This Schiff moment is related to the atomic electric dipole moment, which is a quantity predicted by various extensions to the standard model to lie within certain non-zero ranges; models such as various super-symmetry theories etc, and so offer an experimental method of excluding exotic theories at current probe-able energies.

9. Internal conversion Internal conversion coefficient, α = N(ɣ)/N(e-)
ICC increases as Z, λ →∞, Etrans→0
Internal conversion dominates some transitions vs. gamma decay
221Rn, 1x105 ion/s 48 hour beam time
120Sn target, 2 mg/cm2
The octupole deformation is predicted to be strongest specifically around the proton number 88, that of radium, and neutron number Using the ISOLDE facility good, clean beams of various radon and radium isotopes have been shown to be achievable. The internal conversion coefficient, alpha, increases, that is to say decays are more likely to go via internal conversion, as Z increases and transition energy decreases, and also as multipolarity increases, and we can see that here in this plot that I have adapted from a HIE-ISOLDE proposal. We can see that for 221-Rn incident on a 120 tin target after 48 hours that especially at low energies, for example 70 keV, the number of counts in the electron transitions is much clearer than any gamma ray peak, exemplified by the fact this is a log scale. These numbers were calculated using GOSIA assuming E1-E3 moments are constant and the same as those in 220-Rn, M1 matrix elements extrapolated using the rotational model, and energies scaled according to the relative behavior of the even-even radon isotopes.

10. 221Rn results
Kα 324% ↓
221Rn, 6x103 ion/s 30 hour, 2.85 MeV/u 120Sn target, 2 mg/cm2
224 71% ↓
Kβ 56% ↓ % ↓
287 23% ↓
This time last year, we managed around 24 hours of 221 radon beam of intensity around 6000 particles per second, incident on a 120-Tin target of density 2mg/cm^2 at MINIBALL. After Doppler correction, we can see 4 peaks fairly clearly as I’ve marked on. It, and the other odd radon nuclei, are relatively unprobed as far as the nuclear structure is concerned; all we know is the ground state has a spin of 7/2, tentatively with positive parity. We can tell this nucleus is highly converted due to the high number of X-rays seen. Hopefully in a couple of years we might be able to build on this, something which is already a very interesting spectra; this is the first time anyone has ever excited this nucleus, and I should say, it’s really exciting working on these results. What I’m trying to do for now is to establish what configuration these gamma-rays are representing by simulating various different models in GOSIA.
273 15% ↓

11. 221Rn results
To really exemplify the point about not being able to do much with the spectra, here’s the entire gamma-gamma matrix projection. What we have proven is that it is possible to Coulomb excite odd-mass radon nuclei and observe decays using the MINIBALL apparatus, the first time this has been done anywhere. We’ve also shown that an electron spectrometer will yield definite results, given that there are so many X-rays relative to gamma-rays observed.

12. 221Rn results
To really exemplify the point about not being able to do much with the spectra, here’s the entire gamma-gamma matrix projection. What we have proven is that it is possible to Coulomb excite odd-mass radon nuclei and observe decays using the MINIBALL apparatus, the first time this has been done anywhere. We’ve also shown that an electron spectrometer will yield definite results, given that there are so many X-rays relative to gamma-rays observed.

13. Electron spectrometers
Semi-conductors; silicon
Segmented
Need to control δ–e- flux
Now used in-beam with γ–ray spectrometers
So moving on now to the work on electron detection. I’ve mentioned already that nuclei increasingly undergo internal conversion as Z increases, and that a lot of the interesting nuclei lie in these high-mass regions, so it is important to detect electrons from the internal conversion process. The detectors used for electron spectroscopy are typically a semi-conductor material, normally segmented silicon. Often detectors use some method to ensure the detection of delta electrons is kept to a minimum, for example a thin foil or high voltage barrier. In order to gain as much information about the nucleus as possible, in-beam electron detectors are starting to be used in conjunction with in-beam gamma-ray detectors in order to help understand details such as the multipolarity of the transitions, as well as detecting transitions that would not necessarily be visible in a gamma-ray spectrum due to X-rays. If we go through these pictures, we have a schematic of SACRED, which stands for Silicon Array for ConveRsion Electron Detection, the precursor to that in the lower right, SAGE in Jyvaskyla, coupled with the JUROGAM germanium gamma ray detectors, and in the top right, the detector which is the focus of my work, SPEDE, which will be going into MINIBALL.

14. SPEDE SPectrometer for Electron DEtection Will be situated at MINIBALL
Si detector in backwards geometry
Can be used for many areas of nuclear structure
SPEDE stands for the spectrometer for electron detection and will be positioned in the backwards geometry; this picture shows what the set up will be inside the chamber surrounded by MINIBALL itself. The beam comes in from the left hand side, passing through the center of SPEDE. It interacts with the target, represented by this red rectangle, and subsequent particles are detected in the CD detector on the right. The conversion electrons are emitted with some angular dependence, and many will then be detected by SPEDE. Most delta electron flux will be removed with a thin foil due to space constraints. We will also bias the target to stop delta electrons.

15. SPEDE Changes will result in new chamber Similar to T-REX
Easy access to both detectors
Adding in the new detector, as well as the new target mounting system and the accompanying electronics and cooling requires a new, larger chamber. This will be a barrel chamber, similar to the one used on T-REX currently, and which will have the shell mounted on runners in order to provide access to each detector and target holder. It can be seen here, and although this isn’t final yet, we are looking to confirm it by the end of next week.

16. SPEDE – numbers PCB ø – 12 cm, Detector ø – 5 cm (0.5 mm thick)
24 segments
Depletion voltage ~50V
Cooled to -30∘C
Simulated FWHM: keV
The diameter of the PCB is 12cm, and the detector itself has a diameter of 5cm, a depth of 500 micrometers, and 24 segments.
The detector depletes at about 50V, and the maximum leakage current is 200nA at 150V.
It will be cooled with ethanol down to minus 30 degrees celsius, and it has a simulated resolution of 9.2 keV at 678 keV.

17. SPEDE – where we are
Detector & PCB ready – mechanical design finalised in coming days
Preamplifiers tested
In-beam testing and commissioning at JYFL – spring 2014
On track for installation when HIE-ISOLDE comes online
SPEDE does exist, as a piece of bonded silicon, that can be seen here. The result is shown in this picture here on the lower right. The mechanical design will be finalised next Wednesday.
Pre-amplifiers have been tested, with the setup shown on the left here, with reasonable results in terms of peak-noise ratio and so on.
As things stand, we have testing and commissioning of the detector in-beam agreed at the jyvaskyla lab in Finland, which will take place early 2014.
The beam used will be a pulsed cocktail beam for testing, and it is proposed that a 197Au beam at 4.8 MeV/u that is incident upon an 112Cd target will be used for commissioning. The K130 cyclotron used at JYFL can be manipulated to match conditions at HIE-ISOLDE; the testing and commissioning does not require radioactive ion beams.
At the moment I believe everything is on track for completion in late 2014, and so will be ready for when the ISOLDE upgrades are scheduled to be completed.

18. Summary Simulation of 221Rn configuration on-going
Combined e--γ ray spectroscopy unlocking secrets of nuclei
SPEDE will enhance experiments for all users of MINIBALL
Should have first results from it 2015
So….in summary, the work on 221-radon is continuing, by modelling various different transitions. The work on it, and the heavy octupole nuclei in general will be greatly benefitted by the inclusion of an electron spectrometer, SPEDE, which will give more data for all users of MINIBALL to help in the study of nuclei in the future. We should have initial results from experiments at MINIBALL using radioactive beams in 2015.

19. Collaboration A quick thank you to the rest of the collaboration, and
CEA
Marie-Delphine Salsac, Magda Zielinska
Technischen Universität Darmstadt
Thorsten Kröll, Marcus Scheck, Sabine Bönig, Michael Thürauf
University of Florida
C.Y. Wu
University of Jyväskylä
Tuomas Grahn, Janne Pakarinen, Philippos Papadakis, Joonas Konki, Sanna Stolze
University of Cologne
Nigel Warr, Lars Lewandowski, Burkhard Siebeck, Tim Steinbach, Andreas Vogt
KVI Groningen
Lorenz Willman
KU Leuven
Liam Gaffney, Kasia Wrzosek-Lipska, Tim De Coster, Nele Kesteloot
University of Liverpool
Peter Butler, Rolf-Dietmar Herzberg, George O'Neill, Jim Thornhill
University of Michigan
Tim Chupp
Yale University
Christian Bernards
A quick thank you to the rest of the collaboration, and

20. Summary Simulation of 221Rn configuration on-going
Combined e--γ ray spectroscopy unlocking secrets of nuclei
SPEDE will enhance experiments for all users of MINIBALL
Should have first results from it 2015
I’d like to welcome any questions