The windmill

The windmill

The chamber

The windmill takes its name from its rotating wheel and its designer (Dutch Peter Dendooven). It is a chamber with 10 thin carbon foils (20 ug/cm²) meant as sample holders. The beam enters the chamber and is deposited on one foil. Direct decay observation can be made. The wheel is then rotated to bring a fresh foil while the remaining activity can still be observed at a decay station.

The carbon foils

10 thin carbon foils can be placed in the windmill at once. All foils are made from one large sheet of carbon to ensure uniformity of the foils. The thickness ranges from 19 ug/cm² to 22 ug/cm². Each foil is mounted on a cupper ring of diameter 10 mm and inner diameter 8 mm.

The thickness is such that alpha particles and fission fragment can fully exit the foils from any implantation depth in any direction. This allows for observation of the decay products on either side of the C-foil.

The C-foils are very fragile due to their thickness. Breathing on them is enough to tear them in pieces. Great precautions have to be taken when dealing with them. The vacuum system has also been design to minimise the stress on the C-foils while pumping or venting.

The number of foils being limited, the activity comes back rather quickly to the implantation region. This limits the half-life of the isotopes that can be studied.

The detectors

The windmill was originally designed to host two PIPS detector to detect alpha particles or plastic detectors to detect electrons. The setup has been since then upgraded and can host 4 PIPS detector, including an annular one for the beam to pass through, for a total solid angle of 27% at the implantation position.

A Ge detector can be placed in front of the setup, either in close geometry to the implantation position or to the decay position. Additionnal detectors can be placed at 90 degrees of the setup but with a lesser solid angle.

The diagnostics

A set of two plates with a different succession of holes are used to define 3 different beam sizes (2 mm, 4 mm, 6mm) each that can be sent through or to a plate that can be read by an ampmeter. The Faraday cup position (1 hole to the plate) can be used to tune with stable beam by minimising the amount of beam on the collimator while maximising that on the plate. The running mode (2 holes) is used during an experiment.

The first collimator is a device that can take some bit of activity and it should not be overlooked while proceeding with decontamination after a radioactive experiment.

The use of the windmill

The windmill was originally designed for the study of the alpha-decay of the exotic nuclei around Z=82.

It has recently found a new life as a mean of detection in the study of the changes in the mean-square charge radius of heavy isotopes around Z=82^[1,A] and in the EC-delayed fission of neutron-deficient isotopes in the same region. Both experimental campains are still current in ISOLDE (IS456 & IS466).

Most recent publications and theses with this device

[1] H. De Witte et al. Nuclear charge radii of neutron-deficient lead isotopes beyond N=104 midshell investigated by in-source laser spectroscopy. Physical Review Letters 98:112502 (2007)

[2] H. De Witte et al. Alpha-decay of neutron-deficient ²⁰⁰Fr and heavier neighbours. European Physical Journal A 23:243-247 (2005)

[A] H. De Witte. Probing the nuclear structure along the Z=82 closed shell: decay- and laser spectroscopic studies of exotic Pb, Bi and Po nuclei. Ph.D. thesis, KULeuven, 2004

[B] B.A. Marsh. In-source laser resonance ionization at ISOL facilities. Ph.D. thesis, University of Manchester, 2007.