I'm sure my problem is a simple issue, but let me explain it

I would like to use a gradient to control what is scattered at the beginning and at the end of my point array. Please find in attachment a small scene of what I am talking about (created using Clarisse 3.6). As you can see, the result is not exactly what I expected.
Can you give me some help, or an other way to do that? I would like to avoid container here (where I can paint my ids), because I will loose the procedural nature of a point array linked to a scatterer.
Thank you for your time!
Cheers,
Clément