Last Update : 2015/01/27

1 Questions related to parameter setting
Q1.1 How can we precisely simulate motion of low-energy neutrons? How can we use nuclear data library in PHITS?
A1.1 In general, nuclear reaction models such as Intra-Nuclear Cascade (INC) or Quantum molecular dynamics (QMD) cannot simulate reaction processes attributed to induced neutrons below 20 MeV because of complex resonance structures. Therefore, nuclear data libraries are very useful for the simulation. To utilize the data library, emin(2) and dmax(2) in the [parameters] section have to be set appropriately, e.g. emin(2)=1.0e-10 and dmax(2)=20.0.

Q1.2 How can we simulate motion of photons, electrons and positrons?
A1.2 As the default setting of PHITS , photons, electrons and positrons are immediately cutoff when they are created. Thus, you have to set emin(12-14) and dmax(12-14) in the [Parameters] section. The typical minimum and maximum values of these parameters are 1.0e-3 and 1.0e2 for emin(12-14) and dmax(12-14), respectively, although dmax(14) can be extended up to 1.0e5. However, if you set emin(12) and emin(13) to 1.0e-3, computational time becomes very long. Thus, the recommended emin(12) and emin(13) in most cases are 1.0e-1. Please see in more detail. Note that dmax should be set to greater than (i.e. should not be equal to) the incident energy.

Q1.3 Does the default setting of nuclear reaction models give the most accurate result?
A1.3 In general, the default models give the best results in most cases from version 2.52. It should be noted that you have to use nuclear data library to simulate neutrons below 20 MeV (see A1.1).

Q1.4 What kind of simulation does event-generator mode suit for?
A1.4 Event generator mode suits simulations by which the event-by-event information is necessary to be obtained, e.g. neutron detector response calculations and design of semi-conductor devices. It is also useful for the simulation that must determine energy and type of charged particles produced by low-energy neutron interactions. In concrete, event generator mode generally suits does the simulations using [T-Deposit], [T-LET], [T-SED], [T-Yield] and/or [T-Product] tallies. On the other hand, it is not suit for the simulations only using [T-Track] and/or [T-Cross] tallies, such as shielding calculation. See Manual "4.2.19 Event Generator Mode" section in more detail.

Q1.5 When should I change the mode for the statistical uncertainty (the setting of istdev)?
A1.5 We generally recommend the history variance mode (istdev=abs(2)), where the statistical uncertainty depends on the total history number (maxcas*maxbch), except for the case of shared-memory parallel computing, where only the batch the batch variance mode (istdev=abs(2)) can be selected. However, the computational time occasionally becomes extremely long in the history variance mode, especially in the case of tallies using a lot of memories, e.g. xyz mesh tally with very fine structure. When you perform the PHITS calculation with such conditions, please change to the batch variance mode and set maxbch to be more than 10.

Q1.6 Is it possible to use nuclear data libraries other than JENDL-4.0?
A1.6 Yes. PHITS can use nuclear data libraries that are written in the ACE format, i.e. MCNP format. The followings are the instruction how to use a new nuclear data library in PHITS.
  1. Open "xsdir" contained in the package of the new nuclear data library with a text editor, and copy all addresses of nuclear data (e.g. 1001.80c 0.999167 xdata/endf71x/H/1001.710nc 0 1 4 17969 0 0 2.5301E-08)
  2. Open "xsdir.jnd" included in the "data" folder of PHITS with a text editor, and paste the copied addresses at the end of the file.
  3. Creating a new folder with appropriate name written in the address file in the XS folder of PHITS, and copy data files of the new nuclear data library to the created folder.
  4. Explicitly specify the library ID in the [material] section of your PHITS input file (e.g. 1H.80c or 1001.80c for the above example). If you do not specify the library ID, PHITS automatically find the data library of the nucleus earlier written in " xsdir.jnd". Thus, JENDL-4.0 is selected when the data for the nucleus is available.
Q1.7 How can I obtain neutron fluxes emitted from photonuclear reactions with a good statistical uncertainty?
A1.7 You can bias the photonuclear cross section using the "pnimul" parameter. For example, the probability to cause photo-nuclear reaction becomes double when you set pnimul=2.0, and the weights of secondary particles emitted from the photonuclear reaction are 0.5. If you set very high value for pnimul, e.g. above 100.0, the photon fluxes might be changed. Thus, please check whether the photon fluxes does not change very much when you use this parameter.

Q1.8 I cannot restart PHITS simulation due to error related to the inconsistency of tally. Why?
A1.8 It is due to the loss of significant digit in the PHITS input file. In that case, you have to set "ireschk = 1", then PHITS skips the consistency check.

2 Questions related to error occurred in compiling or executing PHITS
Q2.1 I cannot compile PHITS.
A2.1 The status of most variables used in PHITS was changed from "static" to "dynamic". Consequently, PHITS 2.52 or later cannot be compiled by old Fortran compilers such as f77 and g77. Therefore, Fortran compilers recommended by PHITS office are Intel Fortran Compiler 11.1 (or later) and gfortran 4.71 (or later). See Manual "2.4 Compilation using makefile for Linux&Mac" section in detail.

Q2.2 Segmentation fault occurred during the execution of PHITS.
A2.2 It might be due to the overflow of the memory used in PHITS. In that case, you have to increase the maximum size of memory acceptable to PHITS. The maximum size is defined as "mdas" parameter in "" in the "src" directory. Thus, you have to increase this number, and re-compile PHITS. You may also have to increase "latmax" parameter if you would like to use a huge lattice structure such as voxel phantom. See Manual "2.9 Array sizes" or Advanced course (I) p28-29 in more detail.

Q2.3 An error occurred when I try to use "infl:" in my PHITS input file.
A2.3 When "infl:" command is used in your PHITS input file (let name input.dat), you have to type "file = input.dat" at the first line of "input.dat" file. Or you have to make another input file (let name whose first line is "file = input.dat", and use as the input file of the PHITS. For example
phits_c.exe < > output.dat
See Manual "2.7 Executable file" section in more detail

Q2.4 An error occurred when I try to execute PHITS on Linux or Unix console, but I can execute it on Windows using the same input file.
A2.4 Many reasons are considered to cause the error, but the most probable one is the difference of "return code" used in Linux (or Unix) and Windows. If you prepare your input file in your Windows computer, and transfer to your Linux (or Unix) system using FTP software, you have to check the status of transfer mode; i.e. you have to select "ASCII mode" in your FTP software.

Q2.5 Can PHITS be executed on Cygwin?
A2.5 Yes, you can. You can find the Cygwin option in "makefile" of PHITS .

3 Questions related to Tally
Q3.1 What is the difference between [t-heat] and [t-deposit] tallies?
A3.1 The two tallies give basically the same values of deposition energies as each other. However, the [t-heat] tally can also give the deposition energy estimated by the Kerma approximation for neutrons and photons. When you use this approximation, set e-mode=0 in [parameters] section and electron=0 in [t-heat] section for neutrons and photons, respectively. The [t-deposit] tally is useful for calculating deposition energy weighted by user defined function, such as Q(L) relationship for calculating dose equivalent. See Manual "6.4 [t-deposit]" section in more detail.

Q3.2 The track length or fluence of heavy ions calculated by [T-Track] or [T-Cross] is strange.
A3.2 It might be due to the miss-define of the energy mesh in the tally section. The energy of heavy ions should be defined in MeV in the tally section, although it should be written in MeV/nucleon in the [Parameter] section.

Q3.3 Results obtained by [T-LET] and/or [T-SED] tally are strange.
A3.3 You have to check the density of material selected by "letmat". If you select the material that is not used in your geometry, you have to define its absolute atomic densities (1H 6.893407e-2 16O 3.446704e-2) in [Material] section. See ParticleTherapy.inp in as example.

Q3.4 How can we estimate the statistical uncertainty from the tally output?
A3.4 Version 2.52 or later, the standard deviations or standard errors are correctly outputted in the tally results. See Manual "4.2.2 Number of history and bank" section in detail.

Q3.5 Can I use "dump" function when I execute PHITS in parallel machine?
A3.5 From version 2.30, it works. Please ask phits office how to use the function.

Q3.6 Why do some events deposit energies greater than the incidentenergy using [t-deposit] with "output=deposit" option.
A3.6 When exothermic nuclear reaction occurs, the total energy of secondary particles becomes greater than the incident energy. Such events are also observed when mesh = xyz or r-z and nedisp <> 0. This is due to the algorithm for considering the energy straggling of charged particle in PHITS. In that case, you have to set mesh = reg in [t-deposit], and define cells that correspond to each xyz or r-z mesh in [cell] section.

4 Questions related to source generation
Q4.1 How can we normalize PHITS outputs when I use the isotropic source (s-type=9 or 10, dir = -all) ?
A4.1 If there is nothing inside the sphere of the isotropic source, the fluence inside the sphere is normalized to 1/(pi*r12) (/source), where r1 is the radius of the sphere. Thus, if you would like to convert the tally output (/source) to the unit fluence, you have to multiply the result with pi*r12. It should be noted that the weight control method is employed in generating the isotropic source, and thus, the event-by-event information cannot be derived from the simulation using the isotropic source. If you would like to obtain event-by-event information for isotropic irradiation, you have to set "dir = iso" in the [source] section.

Q4.2 Source particle does not created in the cell where it should be.
A4.2 If you set the source generating surface (or point) exactly on the surface of a certain cell, PHITS sometimes miss-identify the cell where it should be. In this case, please move the source surface a little bit different from the cell surface.