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Question 2D histograms for ROOT 

Forum: Biasing and Scoring
Date: 31 Jul, 2014
From: <Frank>

Hello,

I'm simulating photons into a scintillator that can be detected by a PMT. I'm recording hits positions in x, y and z separately and export the results to Root with commands like

 CreateTree::Instance()->PMTHitPosX[CreateTree::Instance()->PMTNumHits] = thePostPoint->GetPosition().x();
 CreateTree::Instance()->PMTHitPosY[CreateTree::Instance()->PMTNumHits] = thePostPoint->GetPosition().y();
 CreateTree::Instance()->PMTHitPosZ[CreateTree::Instance()->PMTNumHits] = thePostPoint->GetPosition().z();

 Float_t PMTHitPosX[MaxNum];
 Float_t PMTHitPosY[MaxNum];
 Float_t PMTHitPosZ[MaxNum];

 this->GetTree()->Branch("PMTHitPosX",&this->PMTHitPosX,"PMTHitPosX[PMTNumHits]/F");
 this->GetTree()->Branch("PMTHitPosY",&this->PMTHitPosY,"PMTHitPosY[PMTNumHits]/F");
 this->GetTree()->Branch("PMTHitPosZ",&this->PMTHitPosZ,"PMTHitPosZ[PMTNumHits]/F");

Therefore, after a simulation, I get a Root TTree with 1D histograms for hits in x, y and z. This part works well.

As the hits positions are recorded separately in x, y and z, we have marginal histograms and cannot retrieve 2D or 3D histograms (number of hits for a precise XY or XYZ point). Would it be possible to get something like PMTHitPosXY or PMTHitPosXYZ?

I tried declaring a PMTHitPosXYZ[ ][ ][ ] and using GetPosition() (without .x() or .y() or .z()) but this is a threevector and cannot associate it a 3D array [ ][ ][ ].

I know TH2 histograms exist in Root, but I just cannot make the link from G4 to Root.

Thank you!

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