Utilities

The accelerator will require supplies of electricity, water and air for routine operation. The specifications for each of these parameters will come from the manufacturer. The power requirements given are for a typical PET cyclotron with proton energy of about 17 MeV.

Power. Electrical services supplied to the vault will depend on the cyclotron chosen and its exact configuration. Example values for a typical PET cyclotron are given in Tables 3.1 and 3.2.

Table 3.2 describes the total installed capacity in kVA for the required circuits and breakers. Note that the services outlined above do not include HVAC, wall outlets or safety system power.

Chilled water. The closed circuit, de-ionized water cooling system which serves the cyclotron, beam lines and related equipment usually requires chilled water cooling. This can come from a facility chilled water supply or from a separate chiller, which is dedicated to the accelerator. Some typical operating parameters are given in Table 3.3.

Lighting. Normal lighting services include standard fluorescent fixtures and mercury vapour lights controlled locally. Emergency lighting is provided by battery pack lighting units which are activated upon loss of building power. The intensity of the light should be sufficient to clearly see all parts of the vault. This is important for safety and maintenance procedures, which often require close-in work.

TABLE 3.1. TYPICAL ELECTRICAL SERVICES FOR A PET CYCLOTRON

Installed capacity 100 kVA (115 kVA with a beam line) Normal operating mode 45–80 kW

Standby mode 5–20 kW

Voltages 480/3 F, 208/3 F, 208/1 F, 120/1 F

Voltage stability ±10%

Frequency 50–60 Hz

RF ground Type and location to be determined in consultation with the vendor

O/L protection To meet local electrical code

TABLE 3.2. TYPICAL POWER REQUIREMENTS FOR PET CYCLOTRON SUBSYSTEMS

Subsystem Description Volts Phase kVA

Cyclotron Main magnet P/S

Vacuum Cryopump comp.

Vacuum forepump Beam line* Combination magnet P/S^a

Quadrupole (2) P/S^a

Water system Pumps, etc. 208/120 3/1 10

Control system Computer system 120 1 1.0

Safety PLC c/w UPS system 120 1 1.0

Recommended installed capacity Total 115

a If included in supply.

TABLE 3.3. CHILLED WATER REQUIREMENTS FOR A TYPICAL PET CYCLOTRON

Supply temperature 15°C ± 1.5°C at heat exchanger Heat removal rate 80 kW (excluding beam lines) during operation

20 kW (excluding beam lines) on standby

Flow rate 200 L/min

Maximum pressure at inlet 1.0 MPa Pressure drop for heat exchanger 35 kPa

Compressed air. A compressed air unit is required for several components on the cyclotron. Some typical supply requirements are a source of instrument quality air with a dew point at least 10ºC below ambient temperature, a particle size below 5 mm, and an oil content not to exceed 1 part per million.

The supply pressure is 550 kPa with a flow rate of 150 L/min and a reserve volume of 100 L. There should be several compressed air outlets with valves located in the cyclotron vault. If there is equipment located outside the vault, one compressed air outlet should be provided in each room.

Gas supplies. Gases are required for the operation of the cyclotron and the production of radioisotopes. There are the ion source gases which must be supplied to the ion source, the target gases which are used for the production of radioisotopes, and helium gas which is used to cool the target windows and to transfer liquids in to and out of the targets. These gas cylinders should be in close proximity to the cyclotron, but not inside the vault. If they are inside the vault, they may become activated through neutron capture reactions or may be inconvenient to change when they are empty.

Hydrogen generator. The basic ‘feed-stock’ used to produce H^– ions in the cyclotron’s ion source is hydrogen gas. In many installations, this is obtained from a tank of ‘pure’ (so-called ‘five-nines’ grade of purity) H₂ gas.

Unfortunately, hydrogen gas is highly flammable, and many local fire codes — and common sense — prohibit storage of H₂ cylinders inside a building without special (and expensive) safety provisions. Some facilities have tried using small

‘lecture’ sized H₂ gas cylinders to stay compliant with local safety regulations, but these can be quickly depleted depending on machine utilization and require replacement, always — or so it seems — at an inconvenient time during a busy schedule of operation.

Some facilities elect to store their regular sized H₂ gas cylinders out of doors, with long gas line connections to the ion source. However, this requires long pump-out times and may increase the possibility of vandalism, or small leaks, or contamination during and after service interventions.

The best solution is the use of a ‘hydrogen generator’ which employs the electrolytic decomposition of pure water, in conjunction with diffusion through a palladium membrane, to obtain very high purity H₂ gas (up to ‘seven-nines’) without the problems associated with handling and storage of H₂ gas cylinders.

In addition, the purity of H₂ gas in cylinders — even from a reliable and reputable vendor — is sometimes poorer than stated on the certificate of analysis, causing difficult to diagnose problems with ion source operation. This problem is entirely eliminated (assuming the system is free of leaks) by using the H₂ generator.

Air flow. The air flow in the cyclotron vault should be set up so that the air exhaust is in the part of the vault which is most likely to have loose contamination, and the supply should be in the cleanest part of the vault. This will help in maintaining any contamination to the smallest possible area.