Root code:

///////////////////////////////////////////////
// second 0-volt data with pi filters replaced
// lower QET biases

TChain *e = new TChain("rqDir/eventTree")
TChain *z = new TChain("rqDir/zip1")

int nsparse = 1

char* datapath ="/data/chocula/fritts/data/k100proc/midasrq/byseries/"

e->Add(Form("%s/%s/umn*root",datapath,"07200727_1922"));
z->Add(Form("%s/%s/umn*root",datapath,"07200727_1922"));

z->AddFriend(e)

TCut crand = "EventCategory" 

z->SetAlias("ffrac","PFOFamps/PTOFamps")
z->Draw("PFOFamps/PTOFamps:PTOFamps",!crand+"ffrac>0&&ffrac<1&&PTOFamps>0&&PTOFamps<0.5e-6")

TCut cf = "ffrac>0.5"

z->Draw("PTOFamps>>h(200,0.0e-6,0.5e-6)",!crand+cf)
z->SetAlias("PTOFkeV","PTOFamps*59.54/0.1675e-6")
z->SetAlias("PTOF0keV","PTOFamps0*59.54/0.1675e-6")
z->Draw("PTOF0keV>>h(100,-10,10)",crand)
h->Fit("gaus") // Sigma        3.68842e-01   1.06691e-02
// noise 369 eVee, similar to Run72