Study of Proton Radiation Damage Effects in Si microstrip Detector for New and Fast Tracking System of pCT

A new and fast radiation hard p-Si micro strip detector requires for the high proton imaging performance using the Proton Computed Tomography (pCT) system. Silicon trackers of the pCT have been measured the actual proton dose delivered to the cancer patient during the medical treatment. In this paper, several proton cancer doses up to 9.62x 1014 neq,/cm2 are used for the SRH modelling of the full depletion voltage and leakage current at 293 K of the Si micro-strip detector using microscopic radiation damage model. Radiation hard thin (150μm) p-type MCz Si micro strip detector design is proposed for the pCT that can work up to 800V for very high-irradiated proton cancer doses.


Introduction
In present day, Proton Computed Tomography (PCT) therapy is becoming popular for treating tumours.The important aim in medical science is to exactly identify the path of beam to the cancer cells, positioning of patient, and the actual accurate proton dose delivered to the patient.Proton Computed Tomography (PCT) can improve the accuracy of both patient positioning and dose calculation in proton therapy [1][2][3][4][5].However, it is the challenging task to identify the exact amount of dose to be delivered to the patient, and it is in range of total accumulated dose of 40-80Gy that is required to treat cancer as per the size of the cancer.It is found that a new and fast silicon micro-strip detector is an optimal choice for the pCT system that provides information about the primary particle track from the strip hit-information as well as particle's energy which can be determined in a specific energy deposition measured with each detector plane.
To find the fast Si tracking system for the pCT system, several groups in the world has done experiments on the Si, and Scintillation detectors.But still, we need it for the fast pCT system as per the requirements.
Within CERN RD50 Collaboration, n/p-MCz Si material was identifies as a new and fast detector for the collider experiments [6].
We know that radiation damage degrades the operation of the detector, and thus a new and fast thin n/p-MCz Si micro-strip detector can be explored for the system [7].
In this paper, we have used the experimentally verified microscopic model to investigate the effect of the proton radiation damage on the full depletion voltage (Vfd) and leakage current of the detector using SRH (Shockley Read Hall) statistics modelling for the different proton cancer doses.

Device Modelling
The cross section of an AC coupled p-MCz silicon micro strip detector used in the present work is shown in figure 1.The detector model consists of a p-type Si bulk of doping Concentration (NA) of 5.55 x 10 12 cm -3 .The detector is doped with p + implant at the front side and a n + implant at the backside of the detector is 80 m.At the front side of the detector, p spray isolation technique is used to avoid the shortening of collecting n + electrodes.In the proton irradiated (it is overestimated up to 9.62x 10 14 neq,/cm 2 for the detector) p-MCZ silicon micro-strip detector is operated under reverse bias condition therefore, the negative bias is applied at the p + side and n + side of the detector is grounded as per standard Dirichlet and Neumann boundary condition.

SRH modelling in proton irradiated detector
Using experimentally verified radiation damage model, the macroscopic results of the detector viz; Full Depletion Voltage (Vfd), Leakage Current (IL) can be extracted using SRH modelling.The two level deep trap proton irradiated radiation damage model for p-Fz (consider firstly as a baseline Fz Si material) Si micro strip detector is shown in table 1 for the SRH modelling as a first step [4].It is noted that the proton radiation damage model is valid for fluences higher than 10 14 cm -2 .
Table 1.Experimentally verified two deep trap radiation damage model [4].The macroscopic parameters of the detectors can be extracted from the given expressions; Full depletion voltage, Acceptor concentration, = where, W N =device depth, μ h = mobility of holes, q= charge of an electron.Effective Doping Concentration, where, = intrinsic doping concentration, = steady state occupancy of defects level, = defect concentration Leakage current, =qAd (Ʃ + Ʃ ) where, IL = Leakage current, q= charge of electron, A =Area of the detector, d =thickness of the detector, en,p= emission rate of electron or holes

Results and Discussions
In this paper, we have used the experimentally verified microscopic model to investigate the effect of the proton radiation damage on the full depletion voltage (Vfd) and leakage current of the detector using SRH (Shockley Read Hall) statistics modelling for the different proton cancer doses.

Total Proton accumulated dose: Impact of proton energy on RSP
It has been found in many literature [2][3][4][5], it is a challenging task to identify the accurate amount of proton dose delivered to the cancer patient based on the size of the cancer.Therefore, it is required to estimate the dose implant as per the cancer size.
After estimating this, = 1.6x10 -8 (6) It is known that an absorbed dose is usually measured in rad.So, the proton dose is in rad (1Gy=100 Rad).
We know that the Dose (rad) = Fluence * LET * 1.6 x 10 -8 (7) where LET is energy absorbed in cancer tissues per unit length.For the proton dose in (rad/Gy) for the different energy of the proton beams, the proton fluencies (p/cm 2 ) can be extracted from (7).For 160 MeV proton beam energy, LET is taken 5.2 MeV/gm.-cm in the present calculation (hardness factor is 1 consider here, for the 1 MeV neutron equivalent fluence. Multiple Coulomb Scattering (MCS) leads to a lateral broadening of the beam as a function of depth.Table 2 shows the range of a proton beam as a function of the beam energy, which is typically between 70 MeV and 250 MeV for most treatment facilities [2].
Relative stopping power (RSP) is energy loss per distance, and it can be estimated for different energies with different ranges of cancer sizes, RSP= -  Figure2 shows that with the increasing energy, proton beam goes deeper in cancer tissues.Therefore, it observed that with an increasing energy of the proton beams, the range of the beam will increase into the tissue, and relative stopping power will decrease at the same time.

Vfd as a function of proton fluencies
Figure 3 shows the Vfd of the thin detector as a function of the proton fluencies for the different doping concentration.It has been observed that the Vfd is linearly increasing for high proton doses up to 8000Gy for every doping concentration.For the doping concentration of 5.55x10 12 cm -3 , Vfd is up to 1100V, and it is significantly high, which is due to the fact that the damage is accumulated in the Fz material.To overcome this effect, the baseline material can be chosen (MCz) that can compensate the effect of damage on the Vfd up to 800V and fast response of the detector.The Vfd is having in uncertainty of ± 15% (not shown in figure .2).

Leakage current as a function of the proton fluencies
Figure 4 shows that leakage current as a function of the proton fluencies for the doping concentration of the 5.55x10 12 cm -3 at 293K.The leakage current is increases linearly as expected in the detector.It is 1.1μA at a dose of 9.62x 10 14 neq,/cm 2 .This can be controlled by cooling of the detector, or reducing the dose to the detector.To find the detector that can operate the detector up to 800V with fast response of the system, we need a prime material n/p-MCz for the pCT system [6][7].

Conclusions
In this paper, we have used the experimentally verified microscopic model for the base line material based detector to investigate the effect of the proton doses on the Vfd, and leakage current performance of the detector.For the high doses up to 8000Gy, more than 1000V Vfd and a leakage current up to around 1μA is observed.To overcome this effect, a new and fast radiation hard thin (150μm) p-MCz design is proposed as a first option to reduce MCS and reduce dead edge < 500μm for the further study on the development of the detectors for the pCT system that can work up to 800V for very high irradiated proton doses up to 9.62x 10 14 neq,/cm 2 .

Figure 1 .
Figure 1.Cross-section of a schematic of n in p-MCz Si microstrip detector used in the present work.

Figure 2 .
Figure 2. Energy and relative Stopping Power as a function of range with different proton energies.

Figure 3 .
Figure 3. Full depletion voltage as a function of equivalent (proton) fluencies for the p-Fz Si micro-strip detector having device depth is 150µm.

Figure 4 .
Figure 4. Leakage current as a function of equivalent (proton) fluencies for the p-Fz Si detector for different fluencies at 293K.

Table 2 .
RSP and proton beam energy as a function of range.