AERE Cyclotron

01-02-2026

The AERE cyclotron was made operational in 1949, after a series of meetings in 1945 and 1946 that recognised the need for a particle accelerator at Harwell, in the moulds of American cyclotrons such as the ones found at Berkeley and Harvard. Construction started in 1946.

Constructed on the east half of Hangar 7 (given Harwell is a former air base), the facility was to be furnished with enough laboratory space for 50 industrial scientists, and an additional 20 female and 30 male scientists (for some reason, the gender was specified in the original design documents).

The accelerator was built at a cost of £139k (£5.1M in 2026) for the pre-accelerator (Cockcroft-Walton design), and £130k (£4.8M) for the frequency modulated accelerator. The total cost was estimated to be 9.9M in 2026 pounds.

Shielding

3 different shielding configurations were considered for the cyclotron:

Excavation

The accelerator was to sit partially (or fully) underground, with the soil acting as a natural radiation screen. This was originally discarded, but reconsidered once the need for a 304mm concrete foundation was made clear. However, further consideration was dropped once it was deemed the "dearest" of the three methods.

Water tanks

Water tanks, filled with borated water (due to boron's high neutron absorption cross section, and an estimated daily neutron production of 3.8*10^8 neutrons/cm² for 100 MeV energies) were a possibility, and given that Berkeley had already used water tanks, it seemed enticing.

This solution would provide easy shielding reconfiguration (all that had to be done was draining the tanks), but borated water also leads to the production of tritium, which in itself "activates" the shielding.

Water would be deployed in tandem with concrete shielding, and would sit atop the accelerator, as that would be the section which would face the highest neutron intensity (according to contemporary calcuations). Tanks would be 89mm thick.

Concrete

Concrete, which is the go-to solution in modern synchrotrons, was considered the easiest and cheapest option. Many different thicknesses were considered, with 152mm thick concrete being installed, in addition to the aforementioned 89mm tanks on the roof, filled with sodium metaborate solution if necessary.

For comparision, modern synchrotrons employ up to 1550mm (at Diamond Light Source) of Barytes concrete on the storage ring, or up to 2050mm on the booster roof.

Harvard's design characteristics were a big influence, as their cyclotron used between 77mm and 2500mm (yes, that is a wide range) of earth in combination with 127mm of water.

Energy

The original power design for the accelerator was 150 MeV, however, that was later bumped to 170 MeV with upgrades.

A study was carried out in 1948 to increase energy above 170 MeV, but that would require a significant amount of work, which would see the magnet power increase from 250kW to 400kW (or new, larger poles and more powerful coils). These modifications would take several months (up to 6) and were ultimately not applied.

Energy specifications were similar to Harvard's cyclotron, and remained so until the late 1970s.

Power delivery

The cyclotron was outfitted with a 15kV, 700kW power pack, with an additional 500 kVA available for auxilliary equipment.

Among those, one of the most power hungry would be the deuteron gas generator, which used electrolysis to generate deuteron gas for the Van de Graaf generator. This process would consume up to 5ccs of heavy water a week, which provided more than the 2 litres required by the generator.

Upgrades and successors

In 1955, a 3 GeV Cockcroft-Walton cyclotron was considered, which would utilise betatron acceleration instead of the more complex and expensive RF systems of the time. Technical, bureaucratic and organisational challenges prevented the project from succeeding, resulting in an unsuccesful model that was ultimately scrapped.

In 1958, a series of new accelerators were proposed, such as an ion accelerator with an [energy of 12 MeV], a proton accelerator with a beam energy of up to 100 MeV (compared to the protons of 12 MeV of the previous design), and much tighter resonance/fluctuations specifications.

These proposals would ultimately lead to an additional 11 accelerators, with the 50 MeV Proton Linear Accelerator (PLA) and the max. 50 MeV (protons) Variable Energy Cyclotron (VEC). Of these 11, the AERE cyclotron would "outlive" 5 of them.

These accelerators were employed in many areas, varying from physics research, applied research and isotope production.

Alternative use proposals

Most notably, the cyclotron was to have a similar fate to Harvard's cyclotron, which was turned into a proton therapy centre. This idea was studied in 1973, with interest continuing all the way to 1978, when a funding decision had to be made.

The accelerator ended up not becoming a proton therapy centre, with now two centres being available in the UK and treating up to 1500 patients a year each.

The cyclotron was disused in the late 1970s, and was not repurposed.

Decommissioning

Decommissioning began in the 1990s and was not completed until 2006.

Present

Harwell is currently home to a large synchrotron and a number of other facilities of similar calibre, such as neutron/muon sources, laser facilities (soon to be home to the world's most powerful laser) and a number of other facilities tending to other scientific areas.

Not much remains from Hangar 7, and certainly not much from the original 110 inch cyclotron, but great effort has been made to preserve communications and pictures of the cyclotron in its heyday.