A better beam for ISOLDE

ISCOOL, the RFQ Cooler and Buncher, recently installed at ISOLDE, heralds a new generation of beam quality.

Jérôme Sarret working on the alignment of ISCOOL, ISOLDE’s new RFQ Cooler and Buncher.

As any good chef knows, the secret to a good dish lies in the quality of its ingredients. And at ISOLDE, unlocking the juiciest secrets of the nucleus needs a high-quality beam. One recently installed device, the RFQ Cooler and Buncher (RFQCB), will enhance the emittance and bunching properties of the ion beam, giving ISOLDE’s experiments a better shot at teasing out the properties of exotic nuclei.

The device, originally conceived in a PhD thesis by Ivan Podadera, was installed and commissioned over the past few weeks by the AB-ATB-IF, AB-OP and PH-IS groups. At ISOLDE, radioactive nuclides are produced in thick high-temperature targets and the RFQ cooler will use a buffer gas, segmented cylinder and RF quadrupole to slow the ions, delivering a lower emittance continuous or bunched beam. This will enable the physicists in the various user groups to perform more detailed and higher resolution spectroscopic analysis on the ions than ever before.

Due to the design ISCOOL’s capacity is three orders of magnitude higher than previously built RFQCBs at about 100 million ions per bunch. Bunching also allows more precise experimentation by reducing background noise. The COLLAPS experiment used laser spectroscopy, overlapping the ion beam with a laser beam and observing the fluorescence of the excited photons, to demonstrate this. They took snapshot readings as bunches passed in 12 µs ‘gates’: "this drastically reduced the background noise to one ten-thousandth of that experienced in a continuous beam," said Kieran Flanagan of the COLLAPS team.

Within the RFQCB, the buffer gas dampens the ion motion but also causes some diffusion. As Project Coordinator Pierre Delahaye explains, "to keep the beam along the axis of the ion cooler, four rods create an RF quadrupolar field to radially confine the ions." A further electric field moves and bunches the beam: "This works along the longitudinal axis, dragging the ions through the device to the end where they are then trapped by a potential difference, until they are released in 10-20 µs bunches."

Despite the diffusion, transmission rates through the device are as high as 70-80% for nuclei with atomic masses above 39 (potassium) and 50% for 23 (sodium) and above.

"The RFQ Cooler and Buncher will provide beam for most of the ISOLDE experiments, including mass spectroscopy experiments at MISTRAL and ISOLTRAP," explains Project Coordinator, Hanna Frånberg. "The increase in beam quality means that the teams can test nuclei stressed further from stability". Using the cooler will allow ISOLDE users to push their experiments further and, it seems, to perform some really tasty physics.