Low energy gamma can not be absorbed

Hi, all:
I am simulating a pi+ particle with 50GeV energy in the water medium, using FTFP_BERT physics module.
I found there some some chances that geant4 will produce a ~1e-5 MeV energy gamma particle (via pi- hBertiniCaptureAtRest process). This extrem low energy gamma will continue to be rayleigh scattered and will never be absorbed. Thus the simulation is caught in endless loop.

Here are some excerpts of the simulation output:

G4WT0 > *********************************************************************************************************
G4WT0 > * G4Track Information:   Particle = pi-,   Track ID = 5435,   Parent ID = 5225
G4WT0 > *********************************************************************************************************
G4WT0 > 
G4WT0 > Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
G4WT0 >     0 2.66e+03     74.3      712       141        0        0         0       World initStep
G4WT0 >     1 2.66e+03       78      707       139     1.19     6.35      6.35       World hIoni
G4WT0 >     2 2.66e+03     91.1      688       134     4.77     23.2      29.5       World hIoni
G4WT0 >     3 2.66e+03       96      681       131     1.79     8.67      38.2       World hIoni
G4WT0 >     4 2.66e+03      111      659       125     5.27     26.2      64.4       World hIoni
G4WT0 >     5 2.66e+03      121      645       121     3.77     17.4      81.8       World hIoni
G4WT0 >     6 2.67e+03      153      600       108     12.5     55.6       137       World hIoni
G4WT0 >     7 2.67e+03      163      588       104     3.24     15.3       153       World hIoni
G4WT0 >     8 2.67e+03      182      561      96.3     6.85     33.3       186       World hIoni
G4WT0 >     9 2.67e+03      199      537      89.1     6.64     28.5       215       World hIoni
G4WT0 >    10 2.67e+03      199      536      88.6    0.256     1.45       216       World hIoni
G4WT0 >    11 2.67e+03      214      517      81.5     6.24     24.2       240       World hIoni
G4WT0 >    12 2.67e+03      216      513      79.6      1.5     4.87       245       World hIoni
G4WT0 >    13 2.67e+03      227      498        74      4.5       18       263       World hIoni
G4WT0 >    14 2.66e+03      236      485      69.3     3.99     15.5       279       World hIoni
G4WT0 >    15 2.66e+03      242      476      65.1     3.04     11.2       290       World hIoni
G4WT0 >    16 2.66e+03      255      457      58.1     6.56     23.4       313       World hIoni
G4WT0 >    17 2.66e+03      269      437      51.2     6.92     24.6       338       World hIoni
G4WT0 >    18 2.66e+03      280      420      44.4     6.48     19.7       357       World hIoni
G4WT0 >    19 2.66e+03      288      407      39.3     5.15     15.8       373       World hIoni
G4WT0 >    20 2.66e+03      293      401      36.2     2.78     7.62       381       World hIoni
G4WT0 >    21 2.66e+03      299      392      31.7     4.44     11.1       392       World hIoni
G4WT0 >    22 2.66e+03      300      391        31     0.36     1.09       393       World hIoni
G4WT0 >    23 2.66e+03      304      384      27.5     3.48     8.58       402       World hIoni
G4WT0 >    24 2.66e+03      308      378      24.1     3.47        7       409       World hIoni
G4WT0 >    25 2.65e+03      310      374      21.4     2.34     4.02       413       World hIoni
G4WT0 >    26 2.65e+03      312      370      18.9     2.46     4.56       417       World hIoni
G4WT0 >    27 2.65e+03      314      367      16.7     2.22     3.71       421       World hIoni
G4WT0 >    28 2.65e+03      316      365      14.6     2.14        3       424       World hIoni
G4WT0 >    29 2.65e+03      317      363      12.7     1.84     2.39       426       World hIoni
G4WT0 >    30 2.65e+03      318      362      10.9     1.82     1.91       428       World hIoni
G4WT0 >    31 2.65e+03      319      360      9.52     1.41     1.49       430       World hIoni
G4WT0 >    32 2.65e+03      320      359      8.12      1.4      1.2       431       World hIoni
G4WT0 >    33 2.65e+03      320      359      6.99     1.13    0.939       432       World hIoni
G4WT0 >    34 2.65e+03      321      358      5.89     1.09    0.753       433       World hIoni
G4WT0 >    35 2.65e+03      321      358      4.96    0.934    0.595       433       World hIoni
G4WT0 >    36 2.65e+03      321      357      4.23    0.726    0.476       434       World hIoni
G4WT0 >    37 2.65e+03      321      357      3.48    0.759    0.395       434       World hIoni
G4WT0 >    38 2.65e+03      322      357      2.57    0.908    0.319       434       World hIoni
G4WT0 >    39 2.65e+03      322      356      1.83    0.735    0.242       435       World hIoni
G4WT0 >    40 2.65e+03      322      356     0.994     0.84    0.184       435       World hIoni
G4WT0 >    41 2.65e+03      322      356     0.066    0.928   0.0909       435       World hIoni
G4WT0 >    42 2.65e+03      322      356         0    0.066  0.00164       435       World hIoni
G4WT0 >    43 2.65e+03      322      356         0        0        0       435       World hBertiniCaptureAtRest
G4WT0 > Track (trackID 5435, parentID 5225) is processed with stopping code 2
...
...
SOME OTHER SECONDERIES PROCESSING
...
...
G4WT0 > *********************************************************************************************************
G4WT0 > * G4Track Information:   Particle = gamma,   Track ID = 5589,   Parent ID = 5435
G4WT0 > *********************************************************************************************************
G4WT0 > 
G4WT0 > Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
G4WT0 >     0 2.65e+03      322      356  1.21e-05        0        0         0       World initStep
G4WT0 >     1 2.65e+03      321      357  1.21e-05        0     1.18      1.18       World Rayl
G4WT0 >     2 2.65e+03      321      356  1.21e-05        0    0.652      1.83       World Rayl
G4WT0 >     3 2.65e+03      317      353  1.21e-05        0     6.84      8.68       World Rayl
G4WT0 >     4 2.67e+03      307      327  1.21e-05        0     34.6      43.3       World Rayl
G4WT0 >     5 2.65e+03      294      327  1.21e-05        0     21.4      64.7       World Rayl
G4WT0 >     6 2.64e+03      281      336  1.21e-05        0     18.4      83.1       World Rayl
G4WT0 >     7 2.64e+03      279      337  1.21e-05        0     2.88      85.9       World Rayl
G4WT0 >     8 2.64e+03      277      341  1.21e-05        0     5.92      91.9       World Rayl
G4WT0 >     9 2.64e+03      278      341  1.21e-05        0     3.25      95.1       World Rayl
G4WT0 >    10 2.64e+03      278      343  1.21e-05        0     3.31      98.4       World Rayl
G4WT0 >    11 2.66e+03      268      348  1.21e-05        0     21.9       120       World Rayl
G4WT0 >    12 2.66e+03      269      347  1.21e-05        0     1.76       122       World Rayl
G4WT0 >    13 2.66e+03      268      346  1.21e-05        0     1.61       124       World Rayl
G4WT0 >    14 2.67e+03      268      341  1.21e-05        0     7.35       131       World Rayl
G4WT0 >    15 2.67e+03      265      340  1.21e-05        0     3.01       134       World Rayl
G4WT0 >    16 2.66e+03      264      337  1.21e-05        0     5.25       139       World Rayl
G4WT0 >    17 2.66e+03      263      338  1.21e-05        0     1.05       140       World Rayl
...
...
CONTINUE SCATTERING
...
...

I am wondering why pi- capture at rest will produce such a low energy gamma particle and why this particle is not absorbed.

Thanks

After I set the optical property of the medium and regist optical physics process, this bug no longer appears. :sweat_smile: