1 INTRODUCTION

Calibration measurements of two Cherenkov monitors (Ch1, Ch2) of the proton beam [1] were carried out on the medical channel of the Institute for Nuclear Research, Russian Academy of Sciences, shared use center in comparison with the ionization chamber (IC) Semiflex Ionization Chamber Type 31010 [2] with a volume of  0.14 cm3 used in conjunction with the PTW Multidos elektrometer clinical dosimeter and the Gafchromic EBT3 calibrated plastic monitor (RF) [3].

The proton accelerator with an energy of 169−209 MeV operates in a frequency range of 1−50 Hz, over a wide range of proton beam intensities–from small pulsed currents of 0.1 μA to 10 mA–while the duration of the proton pulse is in the range from 0.3 to 100 μs. The Cherenkov monitor has the fundamental ability to operate in this range of proton beam intensities under conditions of a strong low-energy electromagnetic background. Known detectors, such as ionization chambers [4–6] and inductive current sensors [7, 8], cannot operate in such a wide range of intensities. Ionization chambers work well in the region of relatively low currents, up to 10 μA. At high currents in ionization chambers, nonlinear effects arise associated with the space charge and recombination of ions in the chamber gas. Inductive current sensors work well in the region of relatively high currents, above 10 µA. An ionization chamber and a plastic monitor measure the absorbed dose of ionizing radiation. The Cherenkov monitor measures the Cherenkov light emitted by protons in the thin glass radiator of the monitor. Protons with an energy of 209 MeV emit an average of 0.5 photons/proton [1] in the sensitivity region of the XP2020 photomultiplier spectrum. The monitor does not detect photons and electrons with energies below 170 keV, which is important for low-energy background suppression. The absorbed dose in the substance of the detectors and the Cherenkov radiation are proportional to the number of detected protons.

2 CALIBRATION MEASUREMENTS ON THE PROTON BEAM

The layout of the detectors on the proton beam with an energy of 169 MeV is shown in Fig. 1. The proton beam was completely absorbed in the MR3-R T41029 water phantom 448 × 480 × 350 mm in size, in which the ionization chamber IC was placed in the region of maximum energy release (Bragg peak) of the proton beam. Cherenkov Ch1, Ch2, and plastic RF monitors were installed at the center of the proton beam in front of the phantom in the air gap.

Fig. 1.
figure 1

Layout of detectors and water phantom MR3-R T41029 on a proton beam with an energy of 169 MeV. Ch1, Ch2—Cherenkov detectors; RF—calibrated plastic detector; IC—ionization chamber.

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The measurements were carried out on a proton beam with an energy of 169 MeV and a proton pulse duration of 35 µs for three modes of accelerator operation: cycle D1 of 140 pulses, D2 of 943 pulses, and D3 of 1561 pulses. The IC and RF detectors measure the total dose per cycle from all proton pulses. Detectors Ch1 and Ch2 measure the current pulse over time for each proton pulse. Integrating the current pulse over time gives a charge proportional to the number of protons for each pulse.

Figure 2a shows the dependences of the signal amplitude on time for three sample pulses. The signal amplitude from detectors Ch1 and Ch2 was measured using a CAEN DT5720 transducer [9]. The distribution histogram of the number of protons in each pulse for the D1 cycle, which consists of 140 pulses, is shown in Fig. 2b.

Fig. 2.
figure 2

(a) Dependence of the Ch1 detector signal amplitude on time for three sample pulses (small, medium, large) of the proton beam from the D1 cycle; (b) histogram of the charge distribution in each pulse for the D1 cycle, which consists of 140 pulses.

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The results of measurements of the absorbed dose by the Ch1, IC, and RF detectors in three accelerator operating modes are presented in Table 1. The ionization chamber IC and the plastic monitor RF are calibrated in gray. The measured dose by the Cherenkov monitor is obtained by integrating the current pulse (see Fig. 2a) to determine the charge in each pulse (see Fig. 2b). The total values of the detector charge Ch1 for the entire measurement cycle in Table 1 are presented in arbitrary units.

Table 1. Results of measurements of the absorbed dose of ionizing radiation by detectors (Ch1, IC and RF) in three operating modes of the accelerator
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Figure 3a shows the results of dose measurements by the RF plastic monitor as a function of the total dose measured by the IC ionization chamber in three accelerator operating modes. The solid line represents the result of fitting with a linear dependence. The result shows good agreement between the data obtained by the IC and RF detectors. This demonstrates the good calibration of the IC and RF detectors. The linear dependence of the total dose measured by the Cherenkov monitor Ch1 on the total dose received by the ionization chamber IC obtained as a result of fitting indicates that the detectors are in good agreement with each other (Fig. 3b). Thus, it is shown that the Cherenkov detector can measure the dose in the water phantom in the region of maximum energy release (Bragg peak) of the proton beam.

Fig. 3.
figure 3

(a) Dependence of the total dose measured by the RF plastic monitor on the total dose measured by the ionization chamber; (b) calibration graph of the relationship between the results of measurements of the total dose by the Cherenkov monitor Ch1 and the ionization chamber IC. Solid lines are the results of fitting by linear dependencies.

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The measurements with Cherenkov monitors Ch1 and Ch2 are in good agreement with each other (Fig. 4). The relative accuracy of the Ch monitor dose measurements can be estimated from the point spread (see Fig. 4a) in the region of 20–25 Gy; its value is ~3%.

Fig. 4.
figure 4

(a) Dependence of the total dose measured by the Ch2 Cherenkov monitor on the dose measured by the Ch1 Cherenkov monitor, the solid line is the result of fitting with a linear dependence; (b) two-dimensional histogram of the results of measurements by monitors Ch1 and Ch2 showing the degree of correlation and scatter of independent results of dose measurements by Cherenkov monitors.

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3 CONCLUSIONS

The results of calibration measurements with a Cherenkov proton beam monitor are presented. A good agreement was obtained between the results of measurements by the monitor and the readings of the ionization chamber and the plastic monitor. The Cherenkov monitor does not register photons and electrons with energies below 170 keV, which is important for low-energy background suppression.