A. Detector configuration

Benjamin design [11, 12] is employed in the spherical TEPC to keep multiplication process spatially uniform around the anode wire. Figure 1 shows a sketch and picture of the TEPC. With an inner diameter of 44.8 mm, the TEPC is made of A-150 (Shonka [13]) plastic in wall thickness of 3.0 mm. A single tungsten wire (Φ25 μm, gold-coated) is used as the anode. An outer stainless steel shell of 3.0 mm thickness is used for air tightness. Sealed mode, instead of gas-flow system, was employed for portability consideration. Leak rate of the system was measured at <10^-13 Pa m³/s, ensuring working stability of the TEPC. An ²⁴¹Am alpha source is attached to the outside wall for energy calibration. Most of the alpha particles can go into the cavity through a Φ1 mm hole on the cathode wall. In this work, the TEPC was temporarily flushed with methane based tissue equivalent (MTE) gas (64.4% CH₄, 32.4% CO₂, 3.2% N₂) of 2.12 kPa to simulate soft tissue of 1 μm size, that is, the deposited energies by a charged particle along the TEPC diameter are equivalent to the absorbed energies in soft tissue of 1 μm size. An absolute barometer was used to measure MTE gas pressure of the detector, from 0 to 100 kPa, with the precision of 0.01 kPa.

Fig. 1.Full-screen View

(Color online) The sketch (a) and picture (b) of the TEPC detector.

B. Experiments in reference ²⁵²Cf field

Dose equivalent response RH is defined as the ratio of dose equivalent reading of a TEPC to the ambient dose equivalent H*(10) [14] at the same point. RH of the TEPC was measured in a standard bare ²⁵²Cf neutron radiation field. Neutron emission rate of the ²⁵²Cf source is about 4.86×10⁶ s^-1. In this measurement, the TEPC was tested with the electronic system in Fig. 2. The detector pre-amplifier (ORTEC 142PC) was set in irradiation room and the TEPC was placed 30 cm away from the ²⁵²Cf source. A 40-m signal cable connecting the pre-amplifier and amplifier brought significant noise to the measuring system. This noise was measured after the experiment and subtracted from the measured neutron pulse height distribution.

Fig. 2.Full-screen View

Experimental circuit in ²⁵²Cf reference field.

C. Measurements in mono-energy neutron field

The TEPC was tested with 2.45 MeV neutrons generated by d(d,n)p reaction. A schematic layout of the experimental arrangement was shown in Fig. 3. Deuterium ions were accelerated to bombard deuterium target to produce 2.45 MeV neutrons. The TEPC detector was placed at 20 cm from the target. A Si(Au) surface barrier detector (SBD) was used to monitor total neutron yields by recording the protons at 135 to the beam incidence and at 1 m from the target. It is assumed that contamination of the photo events for an accelerator-based neutron source are negligible.

Fig. 3.Full-screen View

Experimental arrangement of mono-energetic neutron measurement.

A. Characterization of the TEPC with built-in ²⁴¹Am alpha source

Calibration of the TEPC was implemented with an ²⁴¹Am alpha source. Because of the extremely low pressure of MTE gas in the TEPC, the alpha particles lose only part of its kinetic energy in the gas cavity. A typical signal of ²⁴¹Am alpha source output from pre-amplifier (142-PC) is shown in Fig. 4. Energy deposition of 5.48 MeV alpha particles of ²⁴¹Am in the gas cavity was simulated by FLUKA codes [15]. In the simulation, the sampling alpha particles were set to fire into the TEPC along the cavity diameter. The result of energy deposition distribution was plotted in Fig. 5. It can be seen that an average alpha particle traversing 44.8 mm in the TEPC cavity deposits 85.11 keV. As a consequence, the lineal energy is yα=energy deposited/mean chord length=85.11 keV / (2/3)μm = 127.67 keV/μm. The calibration of lineal energy yI with channel I in the MTE-filled TEPC is thus given by,

Fig. 4.Full-screen View

(Color online) Output signal of pre-amplifier for ²⁴¹Am alpha source.

Fig. 5.Full-screen View

(Color online) Energy deposition of ²⁴¹Am alpha particles in TEPC simulated by FLUKA code.

where Iα is the channel number corresponding to the alpha peak of ²⁴¹Am. In the calibration, counting rate of the TEPC to the built-in ²⁴¹Am alpha source is about 1.2 cps. For an accurate Iα value, the data collection time should be no less than 20 min. At bias of 730 V, the lineal energy per channel using Eq. (4) was 0.152 keV/μm. With an MCA of 2048 channels, the measured lineal energy can be extended from 0.2 keV/μm to 300.0 keV/μm in a single measurement. The measured alpha peak with TEPC biased at 730 V is shown in Fig. 6.

Fig. 6.Full-screen View

(Color online) Alpha peak measured by TEPC biased at 730 V for detector calibration.

B. Dose equivalent response of the TEPC in ²⁵²Cf neutron field

The direct output of TEPC measurement is pulse height distribution, which is known also as energy deposition spectra. The TEPC measurement of a ²⁵²Cf source is shown in Fig. 7. After photo components [16] of ²⁵²Cf source and electronic noise subtraction, the lineal energy yi and counts ni for each channel were calculated using Eq. (4). Absorbed dose D, mean quality factor Q and dose equivalent rate were derived using Eqs. (1)–(3). Thus, measured from the ²⁵²Cf source, we had H_TEPC rate = 19.2 μSv/h and H*(10) rate = 17.6 μSv/h. It should be noted that the reference value of H*(10) rate for ²⁵²Cf neutrons at the same point was 17.6 μSv/h, so that the dose response of the TEPC to ²⁵²Cf neutrons is RH=1.1, indicating a good approximation of dose equivalent rate reading of the TEPC to H*(10) rate in ²⁵²Cf reference neutron fields.

Fig. 7.Full-screen View

(Color online) Pulse height distribution of ²⁵²Cf neutrons measured by TEPC.

C. Characterization of TEPC in mono-energetic neutron field

1. Microdosimetric spectra and mean lineal energy

Figure 8 shows the measured pulse height distribution of 2.45 MeV neutrons. Frequency-mean lineal energy ${\overline{y}}_f$ and dose-average lineal energy ${\overline{y}}_d$ were estimated from the measurement using Eqs. (5) and (6),

Fig. 8.Full-screen View

(Color online) Pulse height distribution of 2.45MeV neutrons measured by TEPC.

${\overline{y}}_f=\frac{{\sum }_{i=1}^N{y}_i\times f(y_i)}{{\sum }_{i=1}^{N}f(y_i)}\mathrm{,}$ (5) ${\overline{y}}_d=\frac{{\sum }_{i=1}^N{y}_i\times d(y_i)}{{\sum }_{i=1}^{N}d(y_i)}\mathrm{,}$ (6)

where, f(y) is the probability density function of lineal energy:

$f(y_i)=\frac{n_i}{{\sum }_{i=1}^N{n}_i}\mathrm{,}$ (7)

d(y) is the dose distribution:

$d(y)=y\times f(y)\mathrm{.}$ (8)

The measured and simulated mean lineal energy are given in Table 1. In general, the measurement results agree well with the simulation results, being just 8.8% less than the simulated ${\overline{y}}_f$ and 6.9% larger than the simulated ${\overline{y}}_d$. The measured and simulated microdosimetric spectra are shown in Fig. 9. It can be seen that the two spectra fitted well to each other for the region of > 40 keV/μm, though they differ relatively large in the region of < 10 keV/μm, probably due to the photo events, which was omitted in the FLUKA simulation. The falling slope near 100 keV/μm, referred as the proton edge, was attributed to the recoil protons generated in the A-150 plastics and stopped exactly at the border of A-150 and MTE gas after traversing a random diameter of the cavity. Because of the Bragg effect, the recoil protons deposited the maximum energy. According to energy-range relations for protons in MTE gas, the proton energy was 72.6 keV for TEPC simulating 1 μm tissue. The corresponding lineal energy was 108.9 keV/μm, which agreed well with both the experimental and simulation results.

Figure 9:Full-screen View

(Color online) Microdosimetric spectra for 2.45 MeV neutron.

TABLE 1.Full-screen View

Experimental and simulated mean lineal energy of 2.45 MeV neutrons

	Measured data	FLUKA
yf (keV/μm)	28.9	31.7
yd (keV/μm)	50.8	47.5

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