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Laser

Lecture 2

Bradley Cheal, University of Liverpool Properties from optical spectra

• Isotope shifts ➔ Charge radius r2 2 h i! h 2 i • Hyperfine splitting

➔ Nuclear (measurement) ➔ Magnetic dipole moment ➔ Electric quadrupole moment Q s !h 2i ➔ Sensitive probe of nuclear wave functions

➔ Single particle level migration

➔ Existence of a nuclear state at all Exploring the nuclear chart with lasers

Where are the magic numbers? Do they change away from stability?

What can single-particle phenomena tell us about the nuclear force?

What are the origins and features of collectivity and deformation?

What are the consequences of pairing?

Why do some nuclei have a halo character?

Do proton emitting nuclei have very large charge radii?

Can we learn about super heavy elements / island of stability? Obtaining reaction products from targets

Measure nuclear decays in the vicinity of the nuclear reaction

But sometimes we want to extract the products in the form of a beam Two beam production methods: In-flight and ISOL In-flight

Separator Primary beam (eg. FRS) Foil target High ion beam (MeV) ISOL ( On-Line)

Stopping volume Either: — the volume of a “thick” target itself Primary beam — or a separate “stopper” - eg. buffer gas Extraction

Low energy ion beam (~30keV) Mass separator Example ISOL facilities for high resolution laser spectroscopy

ISOLDE, CERN

IGISOL, JYFL, Finland

ISAC-I, TRIUMF, Canada ISOLDE, CERN ISOLDE: Isotope Separator On-Line (ISOL) DEvice Produce singly charged beams of radioactive isotopes (RIBs) with an energy of eg. 30-60keV

Ioniser Extraction as a beam (singly-charged + ions) Target

30kV ISOLDE: Isotope Separator On-Line (ISOL) DEvice CERN Linear Accelerators

Part a linear accelerator under construction at CERN, “LINAC 4”

LINAC4 is 80m long and located 12m underground Accelerates protons to 160 MeV Production of proton pulses/bunches Linear Accelerators

length of nth drift tube velocity penergy pn / / / CERN Synchrotrons

Proton Synchrotron Booster (PSB) Takes the beam from LINAC 2/4 Accelerates protons to 1.4 GeV ~2uA to ISOLDE (~half) Beam manipulation

High energy beams are steered and focussed with magnets

(large) bending magnet

Quadrupole Ion beams at ISOLDE ISOL beam manipulation Double x-y steerer

Quadrupole (triplet)

Einzel lens Beam diagnostics Faraday cup Currents down to ~1pA Can be segmented for position info.

Wire grid / beam scanner Micro-channel plates, Ion counting with rates < 105/s Collinear laser spectroscopy

1 E = mv2 = E = mvv 2 ) Collinear laser spectroscopy Laser (30kV) PMT Tuning potential

From ion source Doppler broadening Effects of energy spread and emittance

➔ Residual broadening of spectral peaks ➔ Reduction in resolution & sensitivity

➔ Needs higher laser power Wide ion beam ➔ Increases background (requires wide laser beam)

➔ Peak skewing ➔ Reduction in sensitivity Focussing

Problems solved using a cooler… Ion beam cooler for cooling

• Quadrupole rods with RF applied focus the ions to the axis • Weak axial field guides ions to end

He buffer gas Ions lose energy (and therefore energy spread) through collisions Need to reduce the background Problem: continuous non-resonant of into PMT

+ Ion beam Laser beam (continuous) -

Particle detectors Imaging

Segmented photomultiplier tube

Solution: detect photons only in coincidence with ions … but isobaric contaminants still reduce the effectiveness Ion beam cooler for bunching

Apply a trapping potential to the end electrode

PMT 200ms Cooler bunching technique

Ungated

Gated (64μs - 70μs)

Time of flight (50ms accumulation)

Background suppression 50ms / 6μs = ~104 TRIUMF (UBC, Vancouver) Accelerator types: Cyclotron

mv2 qvB = r v ! = r

qB ! = c m

⇒ frequency is constant ⇒ apply via RF to “dees” TRIUMF Accelerator: Cyclotron

p at 500MeV and up to 100uA Availability from conventional ISOL facilities

ISOLDE: Thick target, hot cavity High yields...... if and τ1/2 permit

1.4 GeV protons, 2μA JYFL, Finland A complementary technique: IGISOL (JYFL)

• Reaction products recoil from thin foil targets • Slowed or “stopped” in He buffer gas • Products carried out in supersonic jet • Ions captured by fields, gas pumped away A complementary technique: IGISOL (JYFL)

Gas volume

Thin foil targets Cyclotron beam He JYFL Accelerator: Cyclotrons

K130 p, d, α, 32S…

MCC30/15 p, d Beam time! JYFL, Finland Beam from K130 cyclotron (inc. heavy ions)

100μA p @ 30MeV 50μA d @ 15 MeV Magnet … and n converter

Ions Laser PMT

Thin foil targets, He buffer gas, Supersonic gas jet extraction. • Fast (sub-ms) extraction • Chemically unselective Commissioning of the IGISOL 4 Commissioning of the IGISOL 4 laboratory Tuneable (dye) laser

Complicated molecules with many rotational and vibrational states

Leads to a continuous range of fluorescence from the band head Tuneable (dye) laser Tuneable (dye) laser CW and pulsed lasers Commissioning of the IGISOL 4 laboratory Optical manipulation in the ion cooler-buncher J=1

J=1

Weak? Collinear Pulsed Short λ? J=2 laser In-cooler J=0

• Focus of slow / trapped ions ➜ always efficient • Can use broadband/pulsed lasers ➜ large λ range

Cheal et al. Phys. Rev. Lett. 102, 222501 at IGISOL 4

Electrostatic switchyard

Penning trap mass spectrometer RF cooler-buncher

Collinear laser line Quadrupole moments of manganese

{ 6D,8P, 4D... CEC entry

Atomic ground state

No sensitivity to quadrupole moments

Can’t compare shell model interactions Optical pumping in ISCOOL

A~106/s 80% branch A~2×108/s

Optical pumping

C. Babcock, PhD Thesis, University of Liverpool (2016) Quadrupole moments of manganese

GXPF1A uses full pf space LNPS adds the νg9/2 and νd5/2 orbitals

C. Babcock, H. Heylen et al. PLB 760 387 (2016) ISOL target and ion source (eg. ISOLDE)

Ionisation takes place using e.g. surface ionisation

In beam is then mass filtered downstream Spectroscopy in the ion source Can’t detect photons, so use many lasers to resonantly ionise

Ionisation potential

Ion detection or counts decay spectroscopy Ion

Tune/scan first step Atomic gs. frequency Advantages… and disadvantages 59Cu 1/2-1/2 58Cu 1/2-1/2 (In-Source) (In-Source)

59Cu 1/2-3/2 • Sensitive particle detection (HR technique) (rather than photon detection) • Doppler broadening • High power lasers - broadband ⇒ Low resolution (more tolerable if heavy element) Other Approaches Collinear Ionisation

cf. TJ Procter JPCS 381 012070 (2012) Multiple photon detection

S. Malbrunot-Ettenauer CERN-INTC-I197 (2017) Extract beams for high resolution spectroscopy

Transport to experiments, including a set-up for high resolution laser spectroscopy (see in a moment)

Problems of isobaric components (for any experiment) - swamp the - misidentification / interpretation Neutron converter…? Release curve…? Apply laser ionisation: Laser Ion Source

Deliver to experiments inc. HR laser spec.

• Try to suppress surface ionisation • Selectively enhance yield

• Purified beam for single Z as well as A

• Higher yield (A,Z) • Lower background Apply laser ionisation: Laser Ion Source

Post accelerated Zn beam (but isobars still present)

Laser identifies the peaks caused by the Summary: Laser spectroscopy at RIB facilities Collinear spectroscopy (high resolution) Photon ΔE=mvΔv detector Mass analyzing Cooler-buncher V=Vdc+Vrf cos(t) magnet

(a) or Ion(b) beam Laser beam V=Vdc-Vrf cos(t) Doppler tuning electrodes Particle or decay counting eg. ISOLDE Decay Station ion

Radioactive isotopes IP extracted as an ion beam

In-source method (higher sensitivity, lower resolution) photon Laser beams step-wise resonantly ionize the reaction products leaving the target Ion source (laser on/off comparisons and scanning) gs

Cheal, Cocolios and Fritzsche, Phys. Rev. A 86, 042501