The use of 3He, dilution or TU inserts becomes quite dangerous when a leak allows air to enter the sample well of a cryostat, cryofurnace or cryomagnet. A plug of ice or air grows around the insert and blocks it inside the sample well of the cryostat.
As the temperature rises, the air below the plug will expand, raising the pressure inside the sample well. In some circumstances, the insert can be ejected from the cryostat like a rocket
We explain below the safe working practice for the insertion and removal of a sample holder insert that must be followed on our instruments. Please, contact our cryogenic experts when installing and removing an insert on an instrument.
- Make sure that there is no leakage from the sample well of the cryostat (test for leak-tightness).
- Fit the insert into the sample well, and purge the well twice by pumping and back-filling with helium gas to atmospheric pressure.
- Pump the exchange gas from the sample well. Partially fill the space available in the sample well with helium gas.
- Inserts not motor-driven should be attached to the cryostat with a chain, to prevent them being ejected by excessive pressure in the sample well.
BE CAREFUL! The phase during which the sample warms up is a critical one!
The instructions given below explain the procedures to be applied for checking whether a plug has been formed, and, if so, the action to be taken to ensure that the insert is removed in the safest possible way.
- Attach the insert to the cryostat with the chain.
- Bring the sample well to atmospheric pressure of helium.
- Slacken the locking screw; try to rotate the insert from right to left and to move it up and down. The aim is to ensure that no plug of solid air or ice has formed between the well and the insert.
- Re-tighten the locking screw.
- There are two possibilities:
EITHER rotation / translation was possible OR rotation / translation was impossible:
Rotation / translation possible:
- Set the temperature of the sample well to that required for the removal of the sample.
- Once this temperature has been reached, ensure that the insert is still free to move, unfasten it, remove it.
Rotation / translation impossible:
- Take the following immediate precautions:
- Limit the number of people around the cryostat
- In no circumstances should those working on the insert lean over the insert.
- Warn a member of the cryogenics staff and ask them for advice on the action to be taken (if different from above); establish a safety zone around the cryostat (about 5m.)
- Thoroughly heat the cryostat, purging it of helium and nitrogen.
- Break the cryostat vacuum.
- Rinse with helium the sample well.
- Check that rotation and translation are now possible (that the ice has melted).
- Loosen the insert and remove it from the sample well.
This working procedure has been approved by the chairman of our Internal Safety Commission (CIS), the safety engineer and the head of the Advanced Neutron Environment team.