The Helium source

The Helium atoms needed for the experiments must be in the $2^3$S state instead of the ground state, since the energy gap between the ground state $1^1$S and the first excited state $2^1$S is too big to use it for laser cooling, and because this transition does not satisfy the condition $\Delta m \neq 0$ for absorption of a photon.

To prepare the atoms to the He($2^3$S) state, the Helium gas is led on the outside of a glass tube with a needle in it, and then enters the glass tube through a small hole in the glass at the end of the needle. The needle is put on a voltage of -580 V, which causes a discharge and a current of 3 mA. Due to this discharge, a very small percentage of the ground state Helium atoms will be excited to the $2^3$S state by electron impact. Since the ground state Helium atoms are not detected by the channeltron, they do not disturb the measurements. In our experiments the percentage of He($2^3$S) atoms in the signal detected by the channeltron varied between 30% and 60%. The rest of the detected particles are photons, electrons and He($2^1$S) atoms, which are all unaffected by the used laser light. The ground state Helium atoms are not detected by the channeltron.

To reduce the mean velocity of the beam, the Helium source is cooled with liquid nitrogen of a temperature of 77 K. The mean velocity of Helium at a given temperature is given by [5]:

$$\bar{v} = \sqrt{\frac{8k_B T} {\pi M}}$$ (2.1)

with $k_B$ the Boltzmann's constant and $M$ the mass of the Helium atom. At room temperature this gives a mean velocity of 1250 m/s for Helium atoms. In the experiments done with the cooled He source, a mean velocity of 900 to 1000 m/s is found, which corresponds to a temperature of 150 to 190 K.

Directly behind the glass tube of the Helium source is the nozzle. This is a pinhole with a diameter of 0.5 mm separating the He source from the next vacuum chamber with a much better vacuum. Because of this difference in pressure between both sides of the nozzle, the gas coming through the nozzle is expanded supersonically.

Behind the nozzle there is a second pinhole of 0.5 mm, the skimmer, which reduces the gas pressure in the collimator chamber. These two diaphragms roughly define the beam of atoms that enter the rest of the setup.