update CHARGE_FRACTION

This commit updates the CHARGE_FRACTION value to now represent approximately
the fraction of light reflected from each PMT. It also updates the value to be
closer to the true value based on a couple of fits.

2 files changed, 10 insertions, 2 deletions

diff --git a/src/likelihood.c b/src/likelihood.c
index 6cc990c..2b686f8 100644
--- a/src/likelihood.c
+++ b/src/likelihood.c
@@ -403,7 +403,7 @@ double nll_muon(event *ev, double T0, double *pos, double *dir, double t0, doubl
     gsl_integration_cquad_workspace_free(w);
 
     mu_noise = DARK_RATE*GTVALID*1e-9;
-    mu_indirect = total_charge/CHARGE_FRACTION;
+    mu_indirect = total_charge*CHARGE_FRACTION/10000.0;
 
     for (i = 0; i < MAX_PMTS; i++) {
         if (ev->pmt_hits[i].flags || (pmts[i].pmt_type != PMT_NORMAL && pmts[i].pmt_type != PMT_OWL)) continue;
diff --git a/src/likelihood.h b/src/likelihood.h
index 0da7db9..5efc8ef 100644
--- a/src/likelihood.h
+++ b/src/likelihood.h
@@ -18,7 +18,15 @@
  * value for n. */
 #define MIN_RATIO -10
 
-#define CHARGE_FRACTION 60000.0
+/* Roughly equal to the fraction of light which is reflected at each PMT.
+ *
+ * Note: The proper thing to do here would be to separately integrate over the
+ * track to calculate the amount of reflected light using the data from the
+ * PMTR bank. However, this would make the likelihood function almost twice as
+ * long, so we just assume a constant fraction of the light is reflected now. */
+#define CHARGE_FRACTION 0.3
+
+/* Dark rate of the PMTs (Hz) */
 #define DARK_RATE 500.0
 /* Single PE transit time spread (ns). */
 #define PMT_TTS 1.5