A multiwire chamber system for measurements of charged particle spectra

A large solid angle, high counting rate detection system for p, d t identification and energy measurement in the range of 15 to 65 MeV has been developed. The detector consists of two position-sensitive delay-line multiwire chambers, a thin plastic scintillator ΔE, and two large area NaI E detectors. Measurements of energy spectra of protons from ¹²C(n, p) are presented.     

Ion chamber system for neutron flux measurements

A helium-filled ion chamber detector for intensity measurements of high-intensity epithermal neutron bursts with instantaneous rates as high as 10¹¹ Hz is presented. This system consists of an ion chamber to detect a portion of the neutron beam, a current-to-frequency converter and CAMAC scalers to readout the chamber. The chambers and readout electronics have a small temperature sensitivity and have high noise immunity. The statistical precision of the system is measured to be 10⁻³ for each neutron beam pulse.     

Spectrometer equipment for neutron spectra measurements in mixed neutron/photon fields

The paper describes spectrometer equipment in the IPPE experimental laboratory for neutron spectra measurement in mixed (n,γ) fields. The laboratory was founded in 1957 and it occupies a leading position in the field of nuclear facilities radiation spectrometry and benchmark experiments in Russia. Spectrometer equipment includes spectrometers based on the organic stilbene scintillator, hydrogen counter and Bonner balls. Basic fields of spectrometer application are mixed radiation neutron spectra measurement of radionuclide sources, of nuclear reactors and accelerators; study of neutron transfer through the material, including benchmark experiments and measurement of neutron spectra in the rooms of nuclear facilities.     

Self-shielding effects in neutron spectra measurements for neutron capture therapy by means of activation foils

The design and optimisation of a neutron beam for neutron capture therapy (NCT) is accompanied by the neutron spectra measurements at the target position. The method of activation detectors was applied for the neutron spectra measurements. Epithermal neutron energy region imposes the resonance structure of activation cross sections resulting in strong self-shielding effects. The neutron self-shielding correction factor was calculated using a simple analytical model of a single absorption event. Such a procedure has been applied to individual cross sections from pointwise ENDF/B-VI library and new corrected activation cross sections were introduced to a spectra unfolding algorithm. The method has been verified experimentally both for isotropic and for parallel neutron beams. Two sets of diluted and non-diluted activation foils covered with cadmium were irradiated in the neutron field. The comparison of activation rates of diluted and non-diluted foils has demonstrated the correctness of the applied self-shielding model.     

A measuring system with a recombination chamber for neutron dosimetry around medical accelerators

A measuring system for dosimetry of neutrons generated around medical electron accelerators is proposed. The system consists of an in-phantom tissue-equivalent recombination chamber and associated electronics for automated control and data acquisition. A second ionization chamber serves as a monitor of photon radiation. Two quantities are determined by the recombination chamber--the total absorbed dose and the recombination index of radiation quality. The ambient dose equivalent, H*(10), or neutron absorbed dose in an appropriate phantom, can be then derived from the measured values. Tests of the system showed that a 0.5% dose contribution of neutrons to the absorbed dose of photons could be detected and estimated under laboratory conditions. Preliminary tests at the 15 MV Varian Clinac 2300C/D medical accelerator confirmed that the measuring system could be used under clinical conditions. The H*(10) of the mixed radiation was determined with an accuracy of approximately 10%.     

Spallation neutron spectra measurements Part II: Proton recoil spectrometer

We present the experimental method conceived to measure high energy neutrons in the range (200 ≤ E ≤ 1600 MeV). The neutrons produce recoil protons in a liquid hydrogen converter. Momentum evaluation and identification of these protons are made by using a magnetic spectrometer equipped with plastic scintillators and three double-plane (X-Y) wire chambers. The response functions of the apparatus are determined using quasi-monoenergetic neutron beams produced by the break-up of deuterons or ³He on a Be target. The performance of the apparatus is illustrated in the form of a preliminary neutron spectrum.     

The performance of a UV sensitive multiwire proportional chamber filled with TMAE

We tested a low pressure multiwire proportional chamber filled with TMAE under various conditions. The asymptotic quantum efficiency (infinite thickness) for gaseous TMAE was measured to be 8% at 210 nm. No signal could be extracted from an adsorbed or condensed layer of TMAE. However, we expect that such a device with gaseous TMAE could be very efficient (> 50%) in the 100–180 nm range for detecting UV light, especially for astrophysical applications.     

Clinical PET with a large area multiwire proportional chamber PET camera

A large-area multiwire proportional chamber positron camera (MUP-PET) is under evaluation for clinical nuclear medicine use remote from a medical cyclotron. A preliminary physical evaluation shows that the detector has a sensitivity 5–10 times that of a conventional gamma camera but one tenth that of a multicrystal, multiring PET system. The spatial resolution is 6 mm throughout an imaging volume 40 cm diameter by 20 cm axially. The electronic readout presently limits the data acquisition rates to ∼ 2–3 kcps. The system has potential to acquire data at 50 kcps with a further increase in sensitivity of 5–10 and an improved resolution of 3 mm. The camera is under evaluation for clinical use in oncological studies using radiopharmaceuticals labelled with ⁶⁸Ga from an in-house generator plus longer-lived radionuclides (¹⁸F, ¹²⁴I, ⁵⁵Co, ⁶⁸Ga) from off-site cyclotrons. In particular quantitative measurements of dosimetry in both chemotherapy and radiotherapy will be an important part of the work.     

Investigation of timing properties of a thin multiwire proportional chamber with Csl photocathode

A measurement of the time distribution of signals due to single electrons created at the CsI photocathode in a thin multiwire proportional chamber is described. The full width at half maximum (FWHM) has been measured to be 8.3 ns. Calculations, which include different drift lengths, diffusion and electronic noise, agree well with the measured distribution. A timing resolution of about 1 ns FWHM is expected for Cherenkov radiation in 1 cm of NaF crystal, giving rise to approximately 30 photoelectrons from the CsI photocathode.     

A new multiwire proportional chamber with fast single strip readout and individual analog to digital conversion

A new precision MWPC with an active length of 400 mm has been built for the Munich Q3D spectrograph. The readout method uses the individual amplitude signals of narrow cathode strips (3 mm width, 0.5 mm spacing). Each of these signals is converted into a digital word by an individual fast ADC. A newly developed hard wired logic calculates the position of the particle event by the center of gravity method with a dead time of about 108 μs. The position resolution in test measurements simulating particle events was better than 0.1 mm.     

A new bridge technique for neutron tomography and diffraction measurements

An attractive feature of neutron techniques is the ability to identify hidden materials and structures inside engineering components and objects of art and archaeology. Bearing this in mind we are investigating a new technique, “Tomography Driven Diffraction” (TDD), that exploits tomography data to guide diffraction experiments on samples with complex structures and shapes. The technique can be used utilising combinations of individual tomography and diffraction instruments, such as NEUTRA (PSI, CH) and ENGIN-X (ISIS, UK), but is also suitable for new combined imaging and diffraction instruments such as the JEEP synchrotron engineering instrument (DIAMOND, UK) and the proposed IMAT neutron imaging and diffraction instrument (ISIS, UK).     

Dosimetric measurements with a brain equivalent plastic walled ionization chamber in an epithermal neutron beam

The tissue substitute A-181 plastic, which has an elemental composition matching both the constituent hydrogen and nitrogen of brain tissue, was assessed for dosimetry in boron neutron capture therapy (BNCT). The sensitivity of an A-181 walled ionization chamber relative to photons for all neutrons in a clinical epithermal beam was calculated to vary between 0.79 +/- 0.04 in-air and 0.95 +/- 0.01 at depths of 4 cm and greater in-phantom. Differences in the total neutron doses measured with A-150 and A-181 plastic-walled chambers were attributed, within experimental error, to the dose produced by thermal neutron capture reactions from the different concentrations of nitrogen in the two tissue substitutes. The response of the A-181 chamber was converted to total neutron dose with an uncertainty increasing with depth in-phantom from 13 to 23% the magnitude of which is determined by the subtraction of a relatively large photon dose. The use of A-181 in place of A-150 plastic will no longer require partitioning the measured neutron dose by energy and should simplify dose reporting in BNCT.     

A monitor for neutron flux measurements up to 20 MeV

A liquid scintillation detector aimed for neutron energy and fluence measurements in the energy region <20 MeV has been calibrated using monoenergetic and white spectrum neutron fields. Careful measurements of the proton light output function and the response matrix have been performed allowing for the application of unfolding techniques using existing codes. The response matrix is used to characterise monoenergetic neutron fields produced by the T(d,n) at a low-energy deuteron accelerator installed at the Swedish Defense Research Agency (FOI).     

Microdosimetric spectra of the THOR neutron beam for boron neutron capture therapy

A primary objective of the BNCT project in Taiwan, involving THOR (Tsing Hua Open Pool Reactor), was to examine the potential treatment of hepatoma. To characterise the epithermal neutron beam in THOR, the microdosimetry distributions in lineal energy were determined using paired tissue-equivalent proportional counters with and without boron microfoils. Microdosimetry results were obtained in free-air and at various depths in a PMMA phantom near the exit of the beam port. A biological weighting function, dependent on lineal energy, was used to estimate the relative biological effectiveness of the beam. An effective RBE of 2.7 was found at several depths in the phantom.     

Large area multiwire chamber ΔE-E telescope for (n, Z) studies in a continuous energy neutron beam

Multiwire chambers have been constructed for detecting reaction products created in a continuous energy neutron beam ranging from 15 to 50 MeV. The charged reaction products are identified by large area plastic detectors using the ΔE-E technique.     

Ion chamber gas-to-wall conversion factors for fast neutron dosimetry

Modern ionising photon dosimetry is essentially entirely based upon gas-filled cavity determinations. For photons, ion chamber response is largely independent of photon energy almost perfectly transforming absorbed dose in the gas to the surrounding media. Absolute uncertainties are <1-2%. For fast neutron dosimetry, this is certainly not the case. Interpretation of the response of the cavity filling material, usually a gas, to the charged particle spectrum induced in the walls and interacting with the cavity gas is fraught with uncertainties. Despite these challenges, gas filled cavities surrounded by various mixtures, compounds and elements, have proved to be essential for integral determinations of the indirectly ionising neutrons, generating dosimetric quantities, such as kerma and absorbed dose. The transformation from gas response to wall dose is material dependent and varies with neutron energy. This study discusses recent advances in cavity response interpretation using the results from complex nuclear modelling of microscopic cross sections as well as estimates of secondary particle production enabling much improved cavity gas-to-wall media conversion factors.