[Gate-users] (again) Energy Distribution and positron annihilation

[Alexandre Dupas]

Dear Emmanuel,

> I am indeed using a water filled phantom and this might be why I get
> annihilation inside the detectors elements, especially because this PET
> system is dedicated to small-animal imaging and there the detector and very
> close to each other.

Well, if you are using a water filled phantom, you shouldn't have
annihilation inside your detector (or very very very unlikely).
Something else must be wrong! Maybe if you show us your faulty Gate
macro?

> You said your simulations showed a maximum annihilation distance of 2 cm
> which seems pretty huge to me for a positron. Were your simulation done with
> Gate, Geant4 or any other Monte Carlo package ?

The simulations were run using Gate v5. We also looked at a
publication (for which I have no reference right now) which gave us
similar maximal distance for 512 keV positron. I agree with you and
your findings, the mean distance is lower than 1cm for 18-FDG
positron. The 2.5cm is the *maximal* distance (and very few
annihilation reach this mark). I roughtly estimate the mean
annihilation distance just below 0.5cm (but I should take my data back
to give a more accurate value).

> I have no granted access to
> any database regarding positron interaction, yet for an electron at about 1
> Mev (which is higher than the max 0.6 Mev of a 18-F disintegration) and
> according to NIST ESTAR database the mean range is less than 1 cm so I would
> say it is less for a positron since the cross section should be higher for
> anti-particle. This is a bit of an awkward comparison I admit though.

I must admit I cannot really comment on that.

Best regards,
Alexandre

>
>
>
> Best Regards,
>
>
>
> --
>
> BRARD Emmanuel
>
> emmanuel.brard at alumni.u-psud.fr
>
> Medical Physics - PhD Student
>
> IPHC IMABIO Bat 25 - 23 rue du Loess
>
> 67037 Strasbourg Cedex 2
>
> FRANCE
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