Eu Scintillator Research Articles

Characteristics of the neutron field of the facility at DIN-UPM

A new source facility (241Am-Be) has been installed in a bunker-type room of large dimensions. To characterise the neutron fields in the facility, detailed calculations have been made with MCNP-4C, showing the different components of the neutron radiation reaching the reference points (direct, inscattered, backscattered). The contribution from neutrons scattered in the walls to the total ambient dose equivalent remains reasonably low (<10%) in the reference points. Additionally, spectra measurements have been performed with a Bonner spheres spectrometer with a 6LiI(Eu) scintillator (0.4 phi x 0.4 cm2), UTA4 response matrix and BUNKIUT unfolding code. The calculated and experimentally obtained spectra are compared, with small differences found in the epithermal and thermal region, attributable to the concrete composition used in the calculations. The H*(10) rate has been determined from the spectra, and then compared to the reading of an active dosemeter (LB6411), with differences found lower than 8%.

Measurement of the Neutron Spectra Inside and Outside the Target Room of the 65 MeV Electron LINAC using an Extended Bonner Sphere

An extended Bonner Sphere (BS) set consisting of four polyethylene (PE) spheres and three PE spheres with an inserted lead shell to increase the detection efficiency for high energy neutron was used to measure the neutron spectra from the thick Pb target bombarded by 65 MeV electron at the Pohang Accelerator Laboratory (PAL). The response function of the BS was calculated using the MCNPX code and calibrated using a 252Cf neutron source. Active BS using the LiI(Eu) scintillator for detecting of the thermal neutron was implemented outside the target room and passive BS with gold foils was used as the thermal neutron detector due to a high intense field of gamma radiation inside the room. With the results of the spectral measurement, the design and calculation of the response function of a newly modified BS of the Korea Atomic Energy Research Institute (KAERI) are also presented.

Background neutron spectrum at 2420 m above sea level

The ambient neutron spectrum was measured in-doors at ground level in Zacatecas Mexico at 2420 m above sea level. A Bonner sphere spectrometer with a 6LiI(Eu) scintillator was used to obtain the neutron spectrum. With the spectrum the ambient dose equivalent was calculated using the ICRP 74 neutron fluence-to-dose conversion factors. The neutron fluence rate was 65±3 cm −2 h −1, producing 13.7±0.6 nSv h −1 due to ambient dose equivalent.

Advantages of passive detectors for the determination of the cosmic ray induced neutron environment.

Due to the pronounced energy dependence of the neutron quality factor, accurate assessment of the biologically relevant dose requires knowledge of the spectral neutron fluence rate. Bonner sphere spectrometers (BSSs) are the only instruments which provide a sufficient response over practically the whole energy range of the cosmic ray induced neutron component. Measurements in a 62 MeV proton beam at Paul Scherrer Institute, Switzerland, and in the CERN-EU high-energy reference field led to the assumption that conventional active devices for the detection of thermal neutrons inside the BSS, e.g. 6Lil(Eu) scintillators, also respond to charged particles when used in high-energy mixed radiation fields. The effects of these particles cannot be suppressed by amplitude discrimination and are subsequently misinterpreted as neutron radiation. In contrast, paired TLD-600 and TLD-700 thermoluminescence dosemeters allow the determination of a net thermal neutron signal.

MCNP analysis and optimization of a triple crystal phoswich detector

Researchers at the University of Missouri-Columbia have designed a triple crystal phoswich detector that allows for simultaneous detection of alpha, beta, and gamma radiation. A ZnS:Ag layer detects alpha particles, a CaF 2:Eu scintillator preferentially interacts with beta particles, and a NaI:Tl cell is used for gamma detection. The detector output is digitally collected, processed, and analyzed by a personal computer using custom software. Monte Carlo N-Particle version 4C simulations of this detector found that the phoswich design has inherent minimum energy limits of 250 keV E max for beta particles and 50 keV for gamma-rays. For a 2.54 cm thick NaI:Tl crystal, intrinsic gamma efficiency for photons ranges from a maximum of 80% at 100 keV to 26% for 2 MeV photons. Mischaracterized gamma events in the CaF 2:Eu crystal above 175 keV can be corrected by subtracting 26±4% of the total number of counts in the NaI:Tl crystal from the CaF 2:Eu response. Beta induced events in the NaI:Tl crystal primarily result from Bremsstrahlung interactions, and can be estimated by multiplying the CaF 2:Eu energy spectrum by a fourth order polynomial.

Neutron and gamma-ray spectra of 239PuBe and 241AmBe

Neutron and gamma-ray spectra of 239PuBe and 241AmBe were measured and their dosimetric features were calculated. Neutron spectra were measured using a multisphere neutron spectrometer with a 6LiI(Eu) scintillator. The 239PuBe neutron spectrum was measured in an open environment, while the 241AmBe neutron spectrum was measured in a closed environment. Gamma-ray spectra were measured using a NaI(Tl) scintillator using the same experimental conditions for both sources. The effect of measuring conditions for the 241AmBe neutron spectrum indicates the presence of epithermal and thermal neutrons. The low-resolution neutron spectra obtained with the multisphere spectrometer allows one to calculate the dosimetric features of neutron sources. At 100 cm both sources produce approximately the same count rate as that of the 4.4 MeV gamma-ray per unit of alpha emitter activity.

TLD pairs, as thermal neutron detectors in neutron multisphere spectrometry

Pairs of thermoluminescence dosimeters, TLD600 and TLD700, have been used as a thermal neutron detector in a multisphere neutron spectrometer. The neutron spectra from a bare 252Cf and a D 2O moderated 252Cf sources were measured with the spectrometer with thermoluminescence dosimeters and with a 6LiI(Eu) scintillator. With the neutron spectra the total flux, average neutron energy, dose and equivalent dose were calculated. The neutron spectra and the calculated features are in agreement with the neutron sources characteristics.

Neutron energy spectra inside a PET cyclotron vault room

Neutron energy spectra were measured at three locations inside the vault room of a PET cyclotron. Measured neutron fields were produced during 17.2 MeV protons and 8.6 MeV deuterons colliding with Faraday cup, and 8.6 MeV deuterons colliding with 14N. Measurements were performed using a Bonner sphere spectrometer whose active 6LiI(Eu) scintillator was replaced by two pairs of thermoluminscent dosimeters. Modified neutron spectrometer was calibrated using bare 252Cf and D 2O moderated 252Cf neutron sources. Thermoluminiscent dosimeters were calibrated using γ-rays from 137Cs and 60Co sources. From this calibration, a single factor was derived that allow us to obtain the thermal neutron net signal used to unfold neutron spectra. MCNP 4A code was used to calculate neutron spectra at those sites were measurements were taken. For MCNP calculations a source term for protons colliding with graphite was calculated using a semiempirical model. Due the lack of source term for neutrons produced during 8.6 MeV deuterons nuclear reactions, with carbon and 14N, several models were tried. From experimental and calculated results a systematic behavior of neutron spectra was observed regardless the primary neutron source. This behavior was a peak indicating the presence of evaporation neutrons due to the large amount of iron in cyclotron.

Spectral and scintillation characteristics of compound x-ray detectors based on calcium iodide crystals

The results of an investigation of the spectral and scintillation properties of pure and Tl+, Eu2+, Fe2+, Co2+, and Ni2+ activated calcium iodide crystals as well as the results of measurements of the scintillation characteristics of compound x-ray detectors based on calcium iodide crystals are reported. It is shown that paired CaI2 and CaI2:Tl crystals can be used to fabricate compound detectors with different fluorescence times. On account of their high light output and good energy resolution CaI2 and CaI2:Eu crystals are suitable for compound detectors with different technical light output. CaI2 or CaI2:Eu scintillators together with scintillators based on calcium iodide with iron-group luminescence quenching impurity (Fe2+, Co2+, and Ni2+) can be used to obtain compound detectors with different physical light output.

Multisphere Neutron Spectra Measurements Near a High Energy Medical Accelerator

Photoneutron spectral measurements in the vicinity of a high energy medical accelerator have been performed. A LiI (Eu) scintillator and paired thermoluminescent dosimeters were used in conjunction with moderating multispheres to measure the neutron spectra. The photoneutron fluence rate, fluence lethargy, average energy, and quality factors have been calculated using a recursive unfolding algorithm. The dose rates and dose equivalent rates received by the patient and operators due to these neutrons have been estimated from these unfolded spectra. Doses predicted by a semi-empirical dose-equivalent model were compared to the measured values at each point. The dose equivalent rate to operators was found to be 1.9 x 10(-11) Sv MU(-1).

A dual detector β-ray imaging probe with γ-ray background suppression for use in intra-operative detection of radiolabeled tumors

The basic concepts for a dual detector β+ imaging probe incorporating a method of γ-ray background suppression were evaluated. The test devices consisted of a 1.2cm2 β+ imaging CaF2(Eu) scintillator disk optically coupled to a 1.7mm thick, area-matched transparent diffuser disk, which in turn was coupled to sets of 2×2 or 4×4mm2 by 10mm long BGO, GSO or LSO scintillators, which acted as both γ-detectors as well as light guides for the CaF2(Eu) scintillation light, which in turn are attached to a fiber optic bundle coupled to a multichannel photomultiplier. Pulse shape discrimination allowed identification of the detector of interaction and coincidence between the two detectors allowed selection of β+-only events. This type imaging detector system was shown to be capable of producing high-resolution images of β+ distributions while suppressing the events due to the 511keV annihilation radiation background. GSO was found to be the detector of choice for this application.

Development of a high resolution beta camera for a direct measurement of positron distribution on brain surface

We have developed and tested a high resolution beta camera for a direct measurement of positron distribution on the brain surface of animals. The beta camera consists of a thin CaF/sub 2/(Eu) scintillator, a tapered fiber optic plate (tapered fiber) and a position sensitive photomultiplier tube (PSPMT). The tapered fiber is the key component of the camera. We have developed two different sizes of beta cameras. One is 20 mm diameter field of view camera for imaging the brain surface of cats. The other is 10 mm diameter camera for that of rats. Spatial resolutions of the beta camera for cats and rats were 0.8 mm FWHM and 0.5 mm FWHM, respectively. We demonstrated that developed beta cameras may overcome the limitation of the spatial resolution of positron emission tomography (PET).

Improved limits on WIMP-19F elastic scattering and first limit on the 2EC2ν 40Ca decay by using a low radioactive CaF2(Eu) scintillator

Improved limits on WIMP-F-19 elastic scattering and first experimental limit on the 2EC2 nu Ca-40 decay have been obtained by using a low radioactive CaF2(Eu) scintillator at the Gran Sasso National Laboratory (LNGS). (C) 1997 Elsevier Science B.V.

Development of a high-resolution alpha camera

We have developed and tested a high-resolution alpha camera. The alpha camera consists of a thin CaF 2(Eu) scintillator, a tapered fiber optics plate (tapered fiber), a position-sensitive photomultiplier tube (PSPMT) and a position calculation circuit. Though this configuration was originally planned as a high-resolution beta camera for direct measurement of positron distribution over brain surface of animals, we found that, with some modifications, it could also image alpha particles. The scintillator is covered by a thin aluminized mylar to protect it from light while allowing the transmission of alpha particles. The tapered fiber is a key component of the camera. The alpha-induced scintillation image is magnified by the tapered fiber with little light loss. Resolution of the alpha camera can be improved by approximately the magnification factor of the tapered fiber, though the field of view of the camera is decreased. We developed a 20 mm diameter field of view alpha camera. Energy resolution of the camera was 30% full-width at half-maximum (FWHM) for 241Am alpha particles (5.5 MeV). Spatial resolution for 241Am alpha particles was better than 0.6 mm. We confirmed that this alpha camera can be used for high-resolution alpha autoradiography or other alpha imaging applications.

Gamma and beta intra-operative imaging probes

Small area (∼1.5 cm 2) scintillation cameras for imaging gammas and betas using inter-changeable detector front ends were built and characterized. Components common to both emission imaging cameras include: (1) fiber optic bundles 2–3 m long, comprised of multi-clad fibers which connect the scintillation detector to (2) an MC-PMT; (3) parallel MC-PMT outputs feed a resistive positioning network and i- V converter/line driver network which produce balanced + X, − X, + Y, and − Y outputs; and (4) four ADCs and a Macintosh PC for system control and image display. The beta and gamma devices used distinct scintillation detectors which were characterized by both simulation and measurement. The beta camera utilized a 0.5 mm by 1.25 cm φ CaF 2(Eu) scintillation crystal coupled, through a diffusing light guide, to 19 2-mm φ optical fibers. These front-end fibers are in turn coupled by a more flexible fiber bundle to the MC-PMT. CaF 2(Eu) has high light output, high beta sensitivity, and low gamma sensitivity. Image signals are histogrammed and displayed after Anger logic computations are performed on digitized signals. The beta camera has <0.6 mm FWHM intrinsic resolution. The gamma camera concept was tested with matrices of discrete 1 × 1 mm 2 and 2 × 2 mm 2 CsI(Tl) and NaI(Tl) crystals of various lengths, and 3 mm thick continuous crystals. Configurations using 4 × 4 element matrices with one-to-one coupling between crystals and fiber channels, and light diffusers between each crystal matrix and fibers were evaluated. The continuous crystals were coupled directly to the fiber optics with signal and data processing analogous to the beta camera. Coupling of discrete crystals to fiber optics by both methods gave essentially perfect identification of the crystal of interaction, allowing spatial resolution to be defined by the crystal size and collimator. The continuous crystal gamma camera gave intrinsic resolution of ∼1.4 mm FWHM.

A performance study on a phoswich detector consisting of an inner NE213 scintillator and an outer CaF 2(Eu) crystal wall

We developed a new phoswich detector. The detector consists of an inner NE213 organic liquid scintillator and an outer thin CaF 2(Eu) crystal which are coupled to a photomultiplier. The charged particles produced in the NE213 scintillator by the interaction of radiation with the detector either stop in the NE213 scintillator or pass through both the NE213 scintillator and the outer CaF 2(Eu) crystal. The fast output pulses of a few ns from the NE213 scintillator can easily be separated from the slow output pulses of about 1 μs from the outer CaF 2(Eu) scintillator. From this difference, the events that the charged particle stops inside NE213 scintillator can be separated out from the events that the charged particle escapes from the inside NE213 detector. The phoswich detector response functions for gamma rays and neutrons measured with the radioactive gamma-ray sources and the neutrons produced by a Be + Cu target bombarded by protons from the cyclotron, and compared with Monte Carlo calculations. It was found that the main characteristics of the NE213 scintillator were not degraded by assembling as a phoswich detector using the CaF 2(Eu) crystal wall. The n-γ discrimination property is slightly degraded but neutrons can be separated from gamma rays. Further we used to test the phoswich detector for neutron measurements under the intense charged particles, mainly protons, with considering the possibility of neutron detection in space crafts. It was shown that the phoswich detector is capable of detecting neutrons in the mixed field.

Design considerations and initial performance of a 1.2 cm/sup 2/ beta imaging intra-operative probe

A novel small area beta (/spl beta//sup /spl plusmn//) detector is under development for nuclear emission imaging of surgically exposed, radiolabeled tumor beds. The imaging device front-end consists of a 0.5 mm thick by 1.25 cm diameter CaF/sub 2/(Eu) scintillator disk coupled to a rigid bundle of 2 mm diameter double clad optical fibers through a polystyrene light diffuser. The detector area (1.2 cm/sup 2/) was determined by the requirement of introducing the probe into small cavities, e.g. during neuro-surgical lesion resection, but large enough to produce images of clinical significance. Flexible back-end optical fibers (1.9 m long) were coupled to the front-end components allowing /spl sim/75 photo-electrons to be detected for mean beta energies of 250 keV, indicating that sufficient signal can be obtained with clinical beta emitters (e.g. /sup 18/F, /sup 131/I). The long flexible fibers guide the scintillation light to a Philips XP1700 series, fiber optic faceplate, Multi-Channel PMT. The parallel MC-PMT outputs are fed into a variable gain, charge divider network and an i-V pre-amplifier/line driver network, whose resulting four outputs are digitized and histogrammed with standard Anger positioning logic. The various components in the imaging chain were evaluated and optimized by both simulations and measurements. Line spread functions measured in the 10.8 mm FOV were 0.50 mm /spl plusmn/0.038 mm and 0.55 mm /spl plusmn/0.065 mm FWHM in X and Y, respectively. A 20% variation in pulse height and minimal variation in spatial resolution was observed. The differential image uniformity was measured to be /spl plusmn/15.6% with /spl sim/150 cts/pixel. Preliminary images show excellent reproduction of phantom activity distributions.

Influence of radionuclide adsorption on detection efficiency and energy resolution for flow-cell radiation detectors

Flow-cell and batch test experiments were performed to determine the uranyl ion adsorption onto and/or complexation with CaF/sub 2/:Eu scintillator as a function of pH. The flow-cell experiments were modeled with an energy dispersive Monte Carlo algorithm. At pH 2, the flow-cell and batch tests gave consistent results, detection efficiency /spl sim/60% and distribution coefficient (K/sub d/) /spl sim/0.3-0.7 mL/g, with the model. At pH 8, the comparison was not as good and assumed to be due to variation in scintillator particle size realized in the experiment. From the flow-cell experiments performed at pH 8, the detection efficiency was determined to be 68% which correlated reasonably well with the model (60%). Where the experiments and model differed greatly was with K/sub d/ which was determined to be 102 and 60 for the flow-cell and batch experiments, respectively, and 0.43 from the model.

Optimizing light collection from thin scintillators used in a beta-ray camera for surgical use

The authors are developing a 1-2 cm/sup 2/ area camera for imaging the distribution of beta-emitting radiopharmaceuticals at the surface of tissue exposed during surgery. The front end consists of a very thin continuous or segmented scintillator sensitive to betas (positrons or electrons) of a few hundred keV, yet insensitive to gamma rays. The light from the scintillator is piped through clear fibers to the photon detector (PD). This approach requires that a sufficient number of scintillation photons be transported from the scintillator, through the fibers to the PD. The scintillator, reflector, surface treatments, geometry, fiber light guides, and optical couplings must be optimized. The authors report here on efforts made to optimize the light collection from <3 mm thick plastic and CaF/sub 2/(Eu) scintillators into clear fibers using experimental measurements and computer simulations. The authors measured that with a 1.25 cm diameter, 0.5 mm thick optimized CaF/sub 2/(Eu) disk coupled to a 5 cm long bundle of clear optical fibers, on average, /spl sim/250 photoelectrons are produced at a PMT photocathode for a /sup 204/Tl beta flood source (E/sub max/=763 keV). This corresponds to a sufficient number of photoelectrons for <1 mm resolution imaging capabilities for the proposed camera.

A study of X-ray luminescence and spectral compatibility of europium-activated yttrium-vanadate (YVO4: Eu) screens for medical imaging applications

The X-ray luminescence efficiency of laboratory-prepared YVO4: Eu screens and their spectral compatibility to common optical detectors were studied under medical fluoroscopy conditions. YVO4: Eu screens were prepared by sedimentation and with different coating thickness. Luminescence efficiency of the YVO4: Eu screens was measured at various X-ray tube voltages (50–250 kVp) and for screens of different coating thicknesses (20–180 mg/cm2). Spectral response was also measured and spectral matching factors between the YVO4: Eu screens and some common optical detectors (photocathodes, photodiodes, photographic emulsion) were calculated. Experimental results on efficiency were fitted by formulas of the theoretical model developed by Hamaker and Ludwig in order to determine phosphor intrinsic X-ray-to-light conversion efficiency and intrinsic optical characteristics, such as coefficients related to light scattering and absorption. Although the luminescence efficiency of YVO4: Eu screens was found to be relatively low (3–11 μM s/mR m2), the matching factor of YVO4 : Eu screens with some red sensitive optical detectors was excellent, of the order of 0.96. High spectral compatibility may indicate that YVO4: Eu scintillators could be used in medical image detectors.