Abstarct: In many situations to describe the effect of radiation in biology, the absorbed dose is quite inappropriate, because the mechanisms and effects, especially in the cellular and subcellular dimensions, are governed by non-uniform microscopic properties. It is clear that the microscopic pattern of interactions and absorbed energy of radiation is crucial to understanding the details of the mechanism of radiation effects. The microdosimetry domain includes the physical descxription of all microscopic patterns of dose distribution. The main purpose and major applications of microdosimetry are related to biological problems and these problems are strongly emphasized in physical studies, however microdosimetry can be used in other fields such as chemistry, physical detectors, microelectronics, protection against radiation and ... to give a more accurate descxription of macroscopic descxriptions of radiation. The aim of this study is to design and fabricate a neutron microdosimeter for medical applications and therefore its wall is soft tissue equivalent material. Given that the number of interactions in the microdosimeter is small, we need a device that increases the effect of this number, as well as a linear relationship between the initial number of ionization and excitation and the final number, to be informed of the amount of deposited energy. Therefore, in this study, a proportional counter was designed in which an axial anode wire surrounded by a cylindrical helix provided more uniformly multiplication electrons in a smaller volume than the detector. The presence of a helix increases the resolution and larger multiplication without applying more voltage to the anode. In the obtained working voltage, when the anode and helix voltages are 750 and 150 volts respectively, the presence of the helix increases the electric field intensity by more than 60%. In order to make a microdosimeter, we first examined the various parameters using a virtual laboratory. To design and optimize the microdosimeter response for particle transport and to investigate the parameters affecting the microdosimeter measurements such as the material and wall thickness of the microdosimeter, dimensions, gas, type, and energy of the source, etc., use the Monte Carlo simulation code GEANT4. We used COMSOL code to design the electric field, the effect of the helix, the voltages of the helix and the anode, the optimal distance between the helix and the anode, the thickness of the anode and the helix, and the effect of the transmittance coefficients on the field intensity. We have also written the program in Fortran programming language to calculate the stopping power and range. We also used the MCNPX code for verification. In order to study the different modes of optimization, microdosimetric quantities and their uncertainties and microdosimetric spectra were calculated and plotted. In this regard, several simulations were performed and according to their results, we made a laboratory sample of a straight cylindrical microdosimeter with a diameter of 25 and a wall thickness of 5 mm of polyethylene. In order to not conduct polyethylene, a copper laxyer with a thickness of less than 0.3 μm was used which does not change the microdosimeter spectra while conducting. We filled the inside of the microdosimeter with propane tissue equivalent gas at a pressure of 17.34 torr that equivalent to one micron site. It is also possible to use methane and propane gas by applying a density correction coefficient, for example up to 10 MeV energy this correction coefficient for methane and propane is 0.781 and 0.864, respectively. In order to eliminate the input noise to the measurement system, we used an aluminum housing and in order to protect the electromagnetic waves, we used a lead shield. We connected the microdosimeter to electronic equipment collection, including a preamplifier, a high voltage source for the anode and helix, an amplifier, a signal generator, an oscilloscope, and an MCA, and obtained the experimental microdosimeter spectrum. We compared this spectrum with the simulation results, which shows a good agreement. Therefore, without changing the microdosimetric spectra, other alternative materials and mextals can be used in the normal configuration of microdosimeters. This new design can be used as an accurate and accessible device without changing the microdosimeter spectra in neutron microdosimeters.