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Optimizing magnets for RIBs using a Channeltron

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An email that was sent by Mike Trinczek regarding magnet optimization for low-intensity RIBs.

Hi Everyone,

I noticed this eLog entry last night:

Summary: "38K counts too low to measure on Faraday Cups.
Detail: After changing to 39K the beam is too weak to be seen on the
Faraday cups. No optimizing of the magnets has been done, TITAN
experimenters were asked to call ISAC Ops if the beam is deficient for
their experiment."

and thought that I should re-comment about optimizing the magnets for RIBs.

In general, we use Faraday cups whenever possible.  The cups should be set
on the most sensitive scale possible so that we have the best resolution
possible when making adjustments.  If you don't understand what this
means, ask Beam Delivery.

Cups (normally with Reverse Bias) work for strong RIBs, such as 8Li.  For
weaker RIBs we can use the channeltrons.  The best one to choose for
optimizing the magnets is IMS:CEM20.  Channeltrons die due to too much
integrated charge.  So they should be used sparingly.  Do not let the beam
sit on them for a long time.  Five minutes or so is fine.  They can easily
take beam intensities of 10^5 and even 2x10^6.

How can you predict how high the intensity is?  Look in the eLog for the
Yield measurement entry:

Summary: Yields for Li and K at 70 uA
Detail: beam current 70 uA p+
ITE:TBHT 230 A
ITE:TGHT 100 A
ITE:BIAS 20 kV

Li-8 : 1.1e8/s
Li-9 : 1.9e7/s
Li-11 : 2600/s
K-37 : 1100/s
K-38m : 1.0e4/s
K-38g : 2.2e5/s
K-46 : 6.6e6/s
K-47 : 8.4e5/s
K-49 : 1.1e4/s
K-50 : 560/s
K-51 : 29/s


From this, we can see that 38K (m plus g) is low enough to use on a
Channeltron, but 46K is too high.  If there isn't an eLog entry, contact
Colin.

To use IMS:CEM20 do the following:
1.  Insert upstream CI Faraday cup such as FC14 or FC19.
2.  Put FC on most sensitive scale and make sure there is no measurable
current.  If there is current, abort!
3.  Insert CEM20 and wait (1 minute or so) for its Bias to ramp up.
4.  Remove FC, but be ready to put it back in!
5.  If counts on CEM20 rise above 2e10^6, put FC in and abort.
6.  If counts are below 2e10^6, optimize the magnets until satisfied.  If
counts go above 2e10^6, abort.
7.  When done, put in upstream FC to block beam, then remove CEM20.
8.  Continue to deliver beam.

This should help us with Beam Delivery.  In the future we will have a
Pepper Pot downstairs to uniformly attenuate the beam by a factor of 10
and 100.  This will allow us to use the channeltrons for beams of higher
intensities (and bridge the gap to the intensities where we can use
Faraday cups).

All operators should read and understand this.  If you do not understand,
ask someone from Beam Delivery.

Thank you,

Mike.

************************

Edit: September 15, 2009 (JA):
Hi All,

some more remarks for optimising the K isotopes. For the isotopes with a
mass greater than 47 the background from the corresponding Ca isotopes is
much  higher than the K. Therefore optimising on the cup current or on one
of the channeltrons will set the magnets to the Ca values. After doing so
they should be scaled back to the K isotopes. For example after optimizing
on mass 48 the magnets have to be scaled from 48Ca to 48K.

If there are any doubts on the mass setting don't hesitate to call.

Friedhelm

Friedhelm
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