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HY

Target system

Three types of target system are available for the production target placed at the "F0" chamber. Two are for gas targets, and the other one is for solid targets.

Water-cooled gas target system

Target gas is enclosed in a 80- (or 40-) mm-long cell. The cell have 20-mm-diameter holes on the beam direction, and sealed with Havar foils, usually 2.5 micron-thick. The target pressure can be increased to 1 atm or more.

For intense beams depositting much more than 2 W in the target gas, the gas may heat up and break the window foils. We experienced such breaking for an 40Ar beam of 100-200pnA.

Cryogenic gas target system

A cryogenic target system was built in 2006. By using this, the target can be 3–4 times thicker with the same pressure (usually operated at 1 atm or less) compared to the room-temperature target. Most of the recent CRIB experiment uses this system.

• See published paper: H. Yamaguchi et al., Nucl. Instr. and Meth. A 589 (2008) 150-156.

The cryogenic target system has more stability against the heat deposit by the beam. A long time operation using a thin (2.5 microns) Havar foil, at which heat deposit is 2 W was proven to be possible. However, we experienced that a small hole was made on the window foil in such cases:

• The heat at the entrance Havar was 2W, but the exit foil was thicker (10 microns), with the heat deposition of 8W.
• The heat was 1–2 W at exit Havar, but using 3He gas at 150 Torr made a hole on the foil. (The pressure was too low?)
• The heat was 1–2 W at exit Havar, but beam was focused too good (~1–2 mm).

The target is cooled by liquid nitrogen, which is supplied from a portable bottle. Currently we have two large bottle (250 L) and two small (120 L) bottles. The large one can supply liquid nitrogen for nearly 2 days, when the target gas circulation rate is small. Therefore, we have to exchange the bottle at least once in 1-2 days. The exchange takes about 20 minutes, and we can refill the liquid-nitrogen bottle at the supplier close to the CRIB room. The refilling takes 2-3 hours for 200 L. The refilling and crane operations must be done by licenced persons (some of the CNS members are licenced).

Water-cooled solid target

A water-cooled holder is used for solid targets. The holder has two holes with diameters of 20 mm.

How to optimize the F0 target pressure?

For an intense beam production the target gas pressure should be as high as possible, but the possibility of window foil breaking increases much if you go beyound 1 atm (760 Torr). On the other hand, the secondary beam energy can be lower and broader, if the target is too thick.

Here is the recommended way to optimize the thickness:

-Define the secondary energy width you want for the experiment (W1). In the experiment the energy width can be changed with the F1 slit opening width. The width of 16 mm corresponds to 1% in momentum and 2% in energy. Typical setting is +/- 5-10 mm. An opening more than +/- 15mm results in a much worse transmission in the WF, and not recommended. Note that the width can be broadened more by the energy straggling at the F3 PPACs, if you will use them.

-Calculate the secondary beam energies, when the production reaction took place 1) at the entrance of the target, and
2) at the exit of the target.

-The secondary beam energy can be calculated as follows:

1. Calculate the primary beam energy loss in the entrance Havar foil (2.5um), and the target gas up to the reaction point.
2. Calculate the secondary beam energy at production by the kinematics.
3. Calculate the secondary beam energy loss in the remaining gas after the reaction point, and the exit Havar foil (2.5 um).
4. After all these energy loss, the remaining energy is the secondary beam energy.

-Compare 1) and 2), and obtain the energy width of the produced secondary beam (W2).

-Check if W1 and W2 are comparable. If W2 > 2 x W1, you may reduce the pressure and recalculate.

-Also check if the central energy (reaction at the center of the target gas, the average of 1) and 2) ) is acceptable.

-Practically, the final pressure should be 760 Torr > p> 200 Torr. Otherwise it will be difficult to have a stable target.

Notices

In any of the 3 systems, a target ladder for other targets (like ZnS for the beam tuning) is installed, and it can be remotely controlled. The production target is attached to the ladder for the water-cooled gas/solid targets, while the cryogenic gas cell is separated and must be move in/out manually. (Do not make a collision between the gas cell and ladder!)

The secondary beam is produced by making reaction with the target, and the ions only going to forward angle is accpepted. The acceptance of CRIB is 5.6 msr by the specification (but effective acceptance is known to be nearly twice).

The target and other things in the F0 chamber are often irradiated by intense beams and easily get contaminated by radioactive materials having long lifetimes. You have to be aware of this fact, and if you need to open the chamber, please observe "safety rules at F0", which is also found in this documentation.

F0 wheels and slit

You can put degrader, collimator and charge stripper at F0 slit/rotating wheel

-F0 slit (Q1 monitor) A slider placed after F0, before Q1 magnet. It has 3 slots at 50, 100, 150 mm. 50 mm is usually used for a 1-mm-diameter collimator. The other two are used for degraders or charge stripper (please make sure what are installed).

-F0 wheel Can be rotated nearly 360 degrees around F0 target ladder/gas cell. We can use it for degraders in front of or behind the target.

Questions:

• What will you use as the production target?
• How much is the target thickness?
• How much is the production reaction cross section?
• How much is the expected rate of the secondary beam that go into D1?