[Halld-physics] cerenkov hits in hdgeant

[Richard Jones]

Nathan

    * First, I was wondering why a hit requires at least two photoelectrons (in hitCerenkov.c), why not just one?

I considered that the randoms from single-photoelectron counting would be too high to be usable.  These are large phototubes and they will probably have high single-photoelectron counting rates.

    *   Next, what is the origin of the 25% efficiency factor which is applied when storing Cerenkov photons to the stack (in gustep.F)? I understand why an efficiency factor is needed, but why this particular value?

This is the photon detection efficiency of the phototube, typically between 20% and 30% in the visible.  Cerenkov light peaks in the blue, where the numbers are closer to 20% for typical bialkali phototubes, but I chose 25% to be as generous as I thought realistically possible.  The other important number is the material of the phototube window that determines how much of the Cerenkov spectrum gets through.  I assumed quartz windows (fused silica) which is a reasonable compromise between efficiency and cost.

    * In your description of the Cerenkov Counter in the CD2 Baseline document (November 16, 2006), you mention that the mirror optics are not optimized, making the simulation unrealistic. Do you recall how large of an effect this had on the number of detected photons?

The mirror geometry was worked out by Eugene Chudakov, and implemented in Geant by me.  I think that comment just means that this is a first-pass attempt to make focusing optics for this geometry, and that it can probably be improved by studying light paths generated by the particular particles of interest for Cerenkov PID.  The thing about optimization is that one does not know how much improvement is possible until the improvement can be demonstrated.

-Richard J.

On 3/20/2011 9:09 PM, Nathan Sparks wrote:
> Hi Richard,
>
> I have some questions about the Cerenkov simulation. First, I was wondering why a hit requires at least two photoelectrons (in hitCerenkov.c), why not just one? Next, what is the origin of the 25% efficiency factor which is applied when storing Cerenkov photons to the stack (in gustep.F)? I understand why an efficiency factor is needed, but why this particular value?
>
> In your description of the Cerenkov Counter in the CD2 Baseline document (November 16, 2006), you mention that the mirror optics are not optimized, making the simulation unrealistic. Do you recall how large of an effect this had on the number of detected photons? I am now thinking that this is why I am seeing so few hits. Finally, do you have any suggestions for a method to use when optimizing the geometry?
>
> Regards,
> Nathan
>

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