Cerenkov-light Threshold Research Articles

Measurements of intensity of produced light in water during irradiations of electron beams with energies above and below the Cerenkov-light threshold

Luminescence of water during irradiation with particles having energies below the Cerenkov-light threshold was recently found for various types of radiations. However, the relation between the intensities of Cerenkov light and of the luminescence of water at the beam energy below the Cherenkov threshold is not well known. To clarify this point, we measured the produced light irradiating a water sample with electron beams having maximum energies above and below the Cerenkov-light threshold. The intensities of light during irradiation of electron beams with different energies increased rapidly at higher energy than ∼260 keV while very small intensity was observed at the beam energy below the Cerenkov-light threshold. We conclude that the intensities of light produced in water during irradiation of electron beams with energies below the Cherenkov threshold from the luminescence of water are much lower than those of Cerenkov light.

Estimation of the origins and fractions of emitted light from acrylic block during irradiation of carbon-ions

Although light emission from an acrylic block during irradiation of carbon ions at lower energy than the Cerenkov-light threshold was found recently, the origins and fractions of the light in the images have not yet been clarified. Since light spectra may provide information to estimate the origins and fractions of the light emission, we conducted optical imaging of an acrylic block during irradiation of carbon ions using a charge-coupled device (CCD) camera with optical filters. We measured the light images of the acrylic block with optical filters of different wavelengths using an ultraviolet (UV)-sensitive CCD camera during carbon-ion irradiation with slightly higher energy to produce secondary electrons to emit Cerenkov light (241 MeV/u). From the images, we derived depth profiles with different wavelengths and calculated the spectra of the emitted light from the acrylic block. The depth profiles showed higher intensity due to Cerenkov light at the shallow area distributed in the images of longer wavelengths. By calculating between the depth profiles of different wavelengths, we could derive the depth profiles for possible origins of the produced light: luminescence, Cerenkov light, and scintillation of the acrylic block. Using the derived fractions of these components, we could estimate their depth profiles at different wavelengths.

Observation of water luminescence for diagnostic 120-kV X-rays by using PMT and CCD camera

Luminescence emitted from water was recently observed by Yamamoto et al. for X-rays with lower energy than the Cerenkov-light threshold. This phenomenon contradicts the conventional consensus that such low-energy photons cannot generate luminescence. Because properties and mechanism of this phenomenon have not been revealed yet, we carried out a series of observations of water luminescence during irradiation of diagnostic 120-kV X-rays by using a photomultiplier tube (PMT) and a charge-coupled device (CCD) camera. As a result of PMT measurement, the light yields owing to luminescence per unit dose deposition to water decreased almost linearly with the increase in the pulse duration time; approximately 25 % reduction of light yield was observed as the duration time increased from 0.025 s to 0.5 s. Therefore, the luminance intensity was not simply proportional to the dose deposition to water. In addition to water, sulfuric acid was used for luminescence imaging by a CCD camera for comparison. The profile curve of luminance along the beam center axis for sulfuric acid showed a somewhat different tendency in comparison to that for water; a notable bump appeared in the entry region of the X-rays. Based on these observations, the possibility of the contribution of sonoluminescence to the water luminescence can be discussed.

Measurements of temporal response of luminescence of water at lower energy than Cerenkov-light threshold during carbon-ion irradiation

Although the luminescence of water at lower energy than the Cerenkov-light threshold during carbon-ion irradiation was found and imaging was possible, the temporal response has not been measured, and so the difference from Cerenkov-light remains unclear. To clarify this point, we measured the temporal response of the luminescence of water at lower energy than the Cerenkov-light threshold and compared it with that of Cerenkov-light. We used silicon photomultiplier (Si-PM) modules to measure the temporal response at the Bragg peak area of a water phantom during irradiation of the carbon ion where the Cerenkov-light was not included. We also measured the temporal response at the shallow depth of the water phantom where the Cerenkov-light was included. In both areas, we measured the temporal waveforms of the light produced by the irradiation of the carbon ions in which the ripples of spills were clearly observed. We found no difference in the waveforms between the Bragg peak and the shallow depths of water. Our results do not contradict the hypothesis that the luminescence of water and Cerenkov-light are produced by the same mechanism.

Optical range determination of clinical proton beams in water. A comparison with standard measurement methods

As recently discovered, water emits a weak luminescence when it is irradiated with protons even with energies below the Cerenkov light threshold. In this work it was investigated if this phenomenon could be exploited for range measurements in proton therapy. A measurement setup based on a scientific CMOS camera that can be operated under normal room light was built and tested in a proof-of-principle experiment at the West German Proton Therapy Center, Essen. The luminescence depth profiles were analyzed to obtain the range information and the method was compared with ionization chamber based depth dose measurements. The noise caused by scattered radiation hitting the camera chip could be removed with a simple threshold-based median filter. The influence of Cerenkov radiation produced by delta electrons was analyzed by FLUKA simulations and it was shown that it does not affect the range measurements. It could be shown that the luminescence method is as fast as the multi-layer ionization chamber measurement (a few seconds) but with a higher depth resolution that is comparable with the Bragg peak chamber method. The proton ranges determined with the luminescence method agree with the reference methods better than 0.2% over the whole energy range 100–226MeV. The sensitivity of the method regarding detectable range shifts was tested. It was shown, that energy shifts of 0.5MeV (at 151MeV), leading to a range shift of ∼0.9mm, were clearly detectable.

Optical imaging of produced light in water during irradiation of gamma photons lower energy than the Cerenkov-light threshold

It is commonly believed that gamma photon irradiation to water does not produce light in water if the gamma photon energy is lower than the Cerenkov-light threshold and imaging of gamma photon is impossible because the produced electrons from gamma photons do not emit light. However, we found that the light is produced in water and the imaging of produced light in water was possible with gamma photon irradiation even at lower energy than the Cerenkov-light threshold. We irradiated gamma photons with lower energy than the Cerenkov-light threshold (260 keV for electrons in water) from 99mTc (gamma photon energy of 140.5 keV) to water phantom, optical imaging was carried out with a charge-coupled device (CCD) camera. Also we conducted imaging of the gamma photons irradiation to acrylic and plastic scintillator blocks. The optical images of water were observed from 3600 s exposure time, and the intensity of the luminescence was linearly increased with time although the intensity was ~0.001% of that with the plastic scintillator block. The optical imaging of water during irradiation of gamma photons lower energy than the Cerenkov-light threshold was possible and it could be a new method for distribution measurements of gamma photons in water.

Estimation of the fractions of luminescence of water at higher energy than Cerenkov-light threshold for various types of radiation.

.Although the luminescence of water at a lower energy than the Cerenkov-light (CL) threshold has been found for various types of radiation, the fractions of the luminescence of water to the total produced light have not been obvious for radiations at a higher energy than the CL threshold because it is difficult to separate these two types of light. Thus, we used a Monte Carlo simulation to estimate the fractions of the luminescence of water for various types of radiation at a higher energy than the CL threshold to confirm the major component of the produced light. After we confirmed that the estimated light production of the luminescence of water could adequately simulate the experimental results, we calculated the produced light photons of this luminescence and the CL from water for protons (170 MeV), carbon ions (330 MeV/n), high-energy x-ray (6 MV) from a linear accelerator (LINAC), high-energy electrons (9 MeV) from LINAC, positrons (F-18, C-11, O-15, and N-13), and high-energy gamma photon radionuclides (Co-60). For protons, the major fraction of the produced light was the luminescence of water in addition to the CL from the prompt gamma photons produced by the nuclear interactions. For carbon ions, the major fraction of the produced light was the luminescence of water and the CL produced by the secondary electrons in addition to the prompt gamma photons produced by the nuclear interactions. For high-energy x-ray and electrons from LINAC, the fractions of luminescence of water were to 0.2%. The fractions of luminescence of water for positrons were 0.2% to 1.5% and that for Co-60 was 0.4%. We conclude that the major fractions of light produced from x-ray and electrons from LINAC, positron radionuclides, and the Co-60 source are CL, with fractions of the luminescence of water from to 1.5%.

Imaging of the scintillation light of float and silica glasses during irradiation of radiations

Although luminescence of water during radiation irradiations with energy lower than the Cerenkov-light threshold were found recently, it was not still obvious whether the luminescence or scintillation is produced or imaging is possible in commonly used float or silica glass. Thus we tried to image the produced light from plates and a block made of float or silica glasses during irradiations of radiations. We clearly detected the images of the produced light of glass plates by using an electron multiplied (EM) CCD camera during irradiation of alpha particles for silica glass and float glass. The silica and float glass plates produced 13 photons/MeV and 258 photons/MeV for alpha particles, respectively. The light spectrum showed high intensity for longer wavelength in silica glass plate and it was relatively broad for float glass plate. By combining the float glass plate with a photomultiplier tube (PMT), energy spectrum could be measured for 5.5 MeV alpha particles with the energy resolution of 65% FWHM. We also measured the scintillation image during irradiation of proton to a float glass block. The image in float glass block during irradiation of proton showed light distribution similar to the dose in the glass. These results suggest float or silica glass plates or blocks are promising materials for radiation detection and imaging.

Source of luminescence of water lower energy than the Cerenkov-light threshold during irradiation of carbon-ion

Although luminescence of water during irradiations of proton and carbon-ion lower energy than the Cerenkov-light threshold were found recently, the sources of the luminescence were not yet obvious. To estimate the sources of the luminescence, we measured the light spectrum of the luminescence of water during carbon-ion irradiations and estimated the sources of the luminescence. Using an ultraviolet (UV) light sensitive charge coupled device (CCD) camera, we measured the luminescence images of water during carbon-ion beam irradiations by changing optical filters, derived the light spectra of the luminescence of water and compared with the calculated results. The intensity of the measured light spectrum of the luminescence of water at the Bragg peak region was decreased as the wavelength of light proportional to ∼λ−2.0 where λ is the wavelength of the light, indicating the source of the luminescence of water can be electromagnetic pulse produced by the dipole displacement inside the water molecules. In the shallow part of the water prior to the Bragg peak, where the Cerenkov-light is included, the spectrum showed steeper curve that is proportional to ∼λ−2.6, which was similar to the calculated spectrum of Cerenkov-light including the refractive index changes of water with the wavelength of light. From these results, the luminescence of water is thought to be mainly come from electromagnetic pulse produced by the dipole displacement inside the water molecules.

Addition of luminescence process in Monte Carlo simulation to precisely estimate the light emitted from water during proton and carbon-ion irradiation

Although luminescence of water lower in energy than the Cerenkov-light threshold during proton and carbon-ion irradiation has been found, the phenomenon has not yet been implemented for Monte Carlo simulations. The results provided by the simulations lead to misunderstandings of the physical phenomenon in optical imaging of water during proton and carbon-ion irradiation. To solve the problems, as well as to clarify the light production of the luminescence of water, we modified a Monte Carlo simulation code to include the light production from the luminescence of water and compared them with the experimental results of luminescence imaging of water. We used GEANT4 for the simulation of emitted light from water during proton and carbon-ion irradiation. We used the light production from the luminescence of water using the scintillation process in GEANT4 while those of Cerenkov light from the secondary electrons and prompt gamma photons in water were also included in the simulation. The modified simulation results showed similar depth profiles to those of the measured data for both proton and carbon-ion. When the light production of 0.1 photons/MeV was used for the luminescence of water in the simulation, the simulated depth profiles showed the best match to those of the measured results for both the proton and carbon-ion compared with those used for smaller and larger numbers of photons/MeV. We could successively obtain the simulated depth profiles that were basically the same as the experimental data by using GEANT4 when we assumed the light production by the luminescence of water. Our results confirmed that the inclusion of the luminescence of water in Monte Carlo simulation is indispensable to calculate the precise light distribution in water during irradiation of proton and carbon-ion.

Imaging of Ra-223 solution using an optical method

Ra-223 has recently been introduced in alpha-targeting radionuclide therapy. According to the decay scheme of Ra-223, beta particles with higher energy than the Cerenkov-light threshold are emitted. In addition, there are reports that the alpha particles themselves emit luminescence in water and air. However, measured optical imaging results from Ra-223 have not yet been reported. Therefore, we conducted optical imaging of an Ra-223 solution and compared it with that of an F-18 solution. Ra-223 and F-18 solutions contained in transparent glass vials were imaged using a high-sensitivity CCD camera. We could obtain an image of the Ra-223 solution with radioactivity of 0.72 MBq in an acquisition time of less than 10 s. At the same level of radioactivity, the intensity of the optical signal in the solution part of the vial of Ra-223 was 21 times higher than that of F-18. Furthermore, in the air part of the vials, Ra-223 had higher luminescence than that of F-18. The light spectra for Ra-223 and F-18 solutions were similar, but those in the vials' air parts were different. The optical signals in the solution part of both Ra-223 and F-18 are attributed to Cerenkov light, while that in the air part of Ra-223 is attributed to the scintillation of N2 gas in air. No obvious luminescence of water by the alpha particles was observed. We concluded that optical imaging of the Ra-223 solution was possible, and this solution's light intensity was much higher than that of the F-18 solution and thus easier to detect. Therefore, Ra-223 is a promising radionuclide for optical imaging in distribution measurements.

Stability and linearity of luminescence imaging of water during irradiation of proton-beams and X-ray photons lower energy than the Cerenkov light threshold

Luminescence of water during irradiations of proton-beams or X-ray photons lower energy than the Cerenkov-light threshold is promising for range estimation or the distribution measurements of beams. However it is not yet obvious whether the intensities and distributions are stable with the water conditions such as temperature or addition of solvable materials. It remains also unclear whether the luminescence of water linearly increases with the irradiated proton or X-ray energies. Consequently we measured the luminescence of water during irradiations of proton-beam or X-ray photons lower energy than the Cerenkov-light threshold with different water conditions and energies to evaluate the stability and linearity of luminescence of water. We placed a water phantom set with a proton therapy or X-ray system, luminescence images of water with different conditions and energies were measured with a high-sensitivity cooled charge coupled device (CCD) camera during proton or X-ray irradiations to the water phantom. In the stability measurements, imaging was made for different temperatures of water and addition of inorganic and organic materials to water. In the linearity measurements for the proton, we irradiated with four different energies below Cerenkov light threshold. In the linearity measurements for the X-ray, we irradiated X-ray with different supplied voltages. We evaluated the depth profiles for the luminescence images and evaluated the light intensities and distributions. The results showed that the luminescence of water was quite stable with the water conditions. There were no significant changes of intensities and distributions with the different temperatures. Results from the linearity experiments showed that the luminescence of water linearly increased with their energies. We confirmed that luminescence of water is stable with conditions of water. We also confirmed that the luminescence of water linearly increased with their energies.

Luminescence Imaging of Water During Irradiation of Beta Particles With Energy Lower Than Cerenkov-Light Threshold

The luminescence imaging of water during irradiation of beta particles with energy lower than the Cerenkov-light threshold has widely been considered impossible because such particles do not emit Cerenkov light. Contrary to this consensus, we found that luminescence imaging of water is in fact possible with such relatively low-energy beta particles. Beta particles from a 500-kBq 45Ca beta source (maximum energy: 257 keV) were irradiated to pure water, and the source’s luminescence images were acquired with a high-sensitivity, cooled charge-coupled device camera. We also conducted image acquisition for an acrylic plate and a plastic scintillator. The water’s luminescence image during beta particle irradiation became visualized after a 1200-s acquisition time. The luminescence intensity with the beta irradiation of water was 0.54 photons/MeV, and the luminescence intensity with that of the acrylic plate was 3.3 photons/MeV. Consequently, we have shown that luminescence imaging of water using beta particles with energy lower than the Cerenkov-light threshold is a promising new method for beta particle detection and distribution measurements.

Three-dimensional dose distribution of proton beams derived from luminescence images of water

We recently found that luminescence was emitted from water during proton irradiation at lower energy than the Cerenkov-light threshold and imaging was possible by using a CCD camera. However, since the measured distributions were projection images of the luminescence, precise dose estimations from the images were not possible. If the 3 dimensional images can be formed from the projection images, more precise dose information could be obtained. For this purpose, we calculate the 3-dimensional distribution of the proton beams from the luminescence images and use them for beam width estimations. We assumed that the proton beams have circular shape and the transverse images were reconstructed from the projection images using the filtered backprojection (FBP) algorithm for positron emission tomography (PET). The reconstructed images were compared to estimate the proton-beam widths with those obtained from the projection images and simulation results. We obtained 3-dimensional distributions of the proton beams from the projection images and also the reconstructed sagittal, coronal, and transverse images as well as volume rendering images. The estimated beam widths from the reconstructed images, which were slightly smaller than those obtained from the projection images, were identical to those calculated with the simulation. The 3-dimensional distributions of the luminescence images of water of proton beams could be reconstructed from the projection images and showed improved accuracy in estimating the beam widths of the proton beams.

Luminescence imaging of biological subjects during X-ray irradiations lower energy than Cerenkov-light threshold

It is commonly thought that UV or visible-light luminescence imaging of biological subjects during X-ray irradiation at the energy below 120 keV is impossible because the secondary electrons produced in this energy range do not emit Cerenkov light. Contrary to this consensus, we found UV or visible-light luminescence imaging of the subjects were possible with X-ray irradiations of this energy range. We placed one of the biological subjects in a black box; visible-light luminescence images were measured with a high-sensitivity, cooled charge coupled device (CCD) camera during X-ray irradiation at energy below 120 keV. We also conducted the imaging of air without subjects during irradiation of the same X-ray. The biological subjects emitted visible-light luminescence, and the imaging was possible with the irradiation of the X-ray below 120 keV. The luminescence images were observed in only the X-ray irradiated areas. Also air luminescence images could be obtained and the intensity of the luminescence measured from the images was proportionally increased with the exposure dose. UV or visible-light luminescence imaging of biological subjects was possible during X-ray irradiations lower energy than the Cerenkov-light threshold. The phenomenon was different from general X-ray fluorescence because wavelength of the luminescence is UV or visible-light. The luminescence imaging method is promising for estimating the irradiated area with X-ray, which could be used for interventional radiology (IVR). Also air luminescence imaging would be applied to the exposure dose distribution measurements for X-ray of diagnostic X-ray systems.

Luminescence imaging of water during alpha particle irradiation

The luminescence imaging of water using the alpha particle irradiation of several MeV energy range is thought to be impossible because this alpha particle energy is far below the Cerenkov-light threshold and the secondary electrons produced in this energy range do not emit Cerenkov-light. Contrary to this consensus, we found that the luminescence imaging of water was possible with 5.5MeV alpha particle irradiation. We placed a 2MBq of 241Am alpha source in water, and luminescence images of the source were conducted with a high-sensitivity, cooled charge-coupled device (CCD) camera. We also carried out such imaging of the alpha source in three different conditions to compare the photon productions with that of water, in air, with a plastic scintillator, and an acrylic plate. The luminescence imaging of water was observed from 10 to 20s acquisition, and the intensity was linearly increased with time. The intensity of the luminescence with the alpha irradiation of water was 0.05% of that with the plastic scintillator, 4% with air, and 15% with the acrylic plate. The resolution of the luminescence image of water was better than 0.25mm FWHM. Alpha particles of 5.5MeV energy emit luminescence in water. Although the intensity of the luminescence was smaller than that in air, it was clearly observable. The luminescence of water with alpha particles would be a new method for alpha particle detection and distribution measurements in water.