[time-nuts] vs Hg ion? Re: GPS clock stabilitiy, Rb vs Cs

[Magnus Danielson]

On 05/05/2013 09:28 PM, Poul-Henning Kamp wrote:
> In message<20130505205257.8497f166abb1e4918695353d at kinali.ch>, Attila Kinali w
> rites:
>
>> I also have never seen a H gas cell standard, probably for the same reason
>> of needing UV light.
>
> Hydrogen is very hard to contain.
>
> The way you *filter* hydrogen is to press it through a palladium film,
> and that doesn't take a particular high pressure.

Another aspect is that D1 and D2 lines are essentially the same 
(121.5674 nm and 121.5668 nm) while for rubidium its 794.760 nm and 
780.027 nm which is about 25000 times wider. This makes pumping 
state-selection hard for hydrogen but relatively easy with rubidium.

So, for hydrogen you have to do state-selection through magnets, which 
is what is being used in hydrogen masers.

There is so many practicalities that goes in to technical decisions.

Cheers,
Magnus