REX-ISOLDE Post Accelerator

The Radioactive beam EXperiment (REX) [HKS+_{98, KSE}+_{03, CAB}+_{04] at} ISOLDE is used for the post acceleration of radioactive isotopes. Beams of singly-charged atoms are delivered to the Penning trap, REX-TRAP where they are cooled and bunched. The bunches are then sent to REX-EBIS, an electron beam ion source, where they are converted to highly charged ions. The highly charged ions that satisfy the specified mass-to-charge ratio,A/q, are extracted as ion bunches, mass separated and injected into the linear accelerator REX-LINAC where they accelerated to an energy around 3.0 MeV/u.

Charge Breeding

Figure 3.14: REX-TRAP (left) and REX-EBIS (right) in the ISOLDE hall CERN.

The charge breeding system at REX-ISOLDE comprises of REX-TRAP [SAB+_02], REX-EBIS [Wen02, WCF+_{03] and an achromaticA/q separator [RKS}+_{99], which} guides the mass-separated beam towards the REX-LINAC. 60 keV 1+ _{ions deliv-} ered by ISOLDE enter the gas filled REX-TRAP and are retarded due to collisions with the dipole-polarised buffer gas. This allows for the continuous injection of ions in the trap. The cylindrical structure of REX-TRAP is located on a 60 keV high voltage platform with a magnetic field of 3 Tesla provided by a supercon- ducting solenoid bore. Such a strong magnetic field provides radial confinement of ions within REX-TRAP, whilst a quadrupole electric potential provides lon- gitudinal confinement of the ions. Further selectivity of the species of interest is provided through sideband cooling [ABD+_{05], which couples the magnetron} and reduced cyclotron motions of the ions. After a typical accumulation and cooling time of 20 ms the ions are extracted from the trap in short bunches and re-accelerated to 60 keV and injected into REX-EBIS for charge breeding. Trans- mission through REX-TRAP is dependent upon the number of ions accumulated. For an intensity of less than 105 _{ions per bunch an efficiency of up to 45% can be} reached. Efficiency decreases with increasing number of ions and can be as low as 10% for intensities of 107 _{ions per bunch.}

REX-EBIS produces highly charged ions by bombarding ions sent from REX- TRAP with mono-energetic 5 keV electrons. Current densities of around 150 A/cm are obtained by compressing a 0.2 A electron beam using a 2 T magnetic field created by a superconducting solenoid. For a certain operational frequency, the breeding time can be varied. Varying the breeding time changes the charge state distribution of the produced ions, allowing for certain A/q values to be opti- mised. It is therefore possible to tune REX-EBIS to the optimal charge state of the ions required for post-acceleration by the REX-LINAC. The beam extracted from REX-EBIS has a typical bunch width of∼100µs. As only one charge state from the total charge state distribution is chosen by theA/q selector, the maxi- mum breeding efficiency of REX-EBIS is about 30%. As the number of positively charged ions that can be stored in REX-EBIS is around 2x1010 _{per breeding cy-} cle, REX-EBIS does not limit the intensity of the post-accelerator. The number of desired nuclei produced is often several orders of magnitude smaller that the residual stable gas ions bred in REX-EBIS. Mass separation of the beam extracted from REX-EBIS is performed by a S-shaped separator with a mass resolution of

A/q

∆A/q ≈ 100 -150. The separator has a transmission efficiency of about 75-90%. The chosen, highly charged radioactive ions are then sent to the REX-LINAC for post-acceleration.

REX Linac

Figure 3.15: REX-LINAC in situe at the ISOLDE hall CERN.

The REX-LINAC, shown in situ in Figure 3.15 and schematically in Figure 3.16, consists of four resonant structures, each adjustable of a certain energy range. The first stage of acceleration occurs when 5 keV ions from REX-EBIS are accelerated to an intermediate energy of 300 keV/u by the RFQ structure. The IH structure then further accelerates the ions to an adjustable energy of 1.1 - 1.2 MeV/u. The final acceleration stages consist of three 7-gap resonators delivering energies in the range of 0.8 - 2.25 MeV/u, and a 9-gap resonator delivering the final, maxi- mum energy of 3.0 MeV/u. The resonance frequency of the 7-gap resonators is 101.28 MHz, whist the 9-gap resonator has a frequency of 202.56 MHz. The ratio of the time over which the LINAC is on over the total time between EBIS pulses is known as the duty cycle and is 10%.

The total transmission through REX-LINAC is in the order of 80%. When considering the efficiencies of REX-TRAP, REX-EBIS and the A/q separator, the overall efficiency of REX-ISOLDE is≈5%. The overall efficiency is defined as the ratio of the number of ions reaching the target to the number of ions delivered by ISOLDE to the front of REX-TRAP.

Figure 3.16: Schematic layout of the REX-LINAC [Rex]. The figure shows the acceler- ating units of REX-LINAC, a bending magnet which guides the ions to one of the two beam lines and the 65◦ beam line leading to the target position of MINIBALL.