TRIUMF Cyclotron Research Articles

Redundant field survey data of cyclotron with imperfect median plane

An accurate and detailed field map is important for cyclotron beam dynamics studies. During the long history of cyclotron studies, many techniques have been developed by cyclotron pioneers for the treatment of median plane field map. In this paper, we take the TRIUMF 500 MeV cyclotron as an example to study the asymmetric field resulting from imperfect median plane symmetry. The “Gordon approach” and a highly accurate compact finite differentiation method are used to investigate the historical field survey data. The redundancy in the survey data is revealed by the expansion method, which also makes it possible to correct the error in the measurement. Finally, both the azimuthal field Bθ and the axial gradient of the axial field dBz /dz in the median plane are corrected using the radial field map Br . The influence of the correction is examined by recalculating the equilibrium orbit properties of the TRIUMF cyclotron. The result shows significantly increased vertical centering errors of the closed orbits. A further simulation study suggests that these centering errors can be reduced to below 1.5 cm by adjusting the trim coils' Br field within the output limits of our trim coils' power supplies. The error in the measurement field data may explain why the calculated trim coils' settings during the cyclotron commissioning in 1974 encountered difficulty.

An overview of the shielding optimization studies for the TRIUMF-ARIEL facility

The Advanced Rare IsotopE Laboratory (ARIEL) facility, dedicated to Radioactive Ion Beam (RIB) production via the Isotope Separation On-Line (ISOL) technique, is currently under construction at TRIUMF. The facility is to host dual target stations that will operate simultaneously, including a 500 MeV proton beam from the main TRIUMF cyclotron and a 50 MeV electron beam from an Electron Linear Accelerator (e-linac). The proton target station will operate a symbiotic medical target in series with the ISOL target.The ARIEL facility features a compact shielding envelope and as a result its optimization relied extensively on simulations generated using the Monte Carlo particle transport and interaction code FLUKA. Dose rate maps were produced of prompt radiation associated with beam losses and expected normal operations. Maps of transient fields were also generated considering radiation sources requiring transport within the facility which include radioactive waste generated in the course of normal operations. Activation studies were also conducted to estimate the amount of residual radioactivity contained in shielding materials at the time of decommissioning. Simulation results are primarily used to inform the ongoing design and construction of ARIEL; at present, the entirety of the proposed facility shielding has been validated by FLUKA simulation data. The specification of shielding geometry and the selection of materials relied extensively on engineering support through verification of CAD models and material sampling to optimize solutions for design simplicity and economy. Normal operation and failure scenarios were both evaluated, with optimized shielding materials and thicknesses selected considering the nature of the radiation fields present and their impact on room occupancy. Finally, a decommissioning plan has been defined based on the anticipated ARIEL lifetime, the characterization of structural activation, and a general disposal strategy.While ARIEL is still under construction and significant progress has been made, the ongoing radiation modelling has enabled validation of the current shielding design for nominal operation. Moreover, the systematic studies performed were paramount in defining the optimal strategies to adopt from target storage to that for decommissioning of the facility.

Fast-switching magnet serving a spallation-driven ultracold neutron source

A fast-switching, high-repetition-rate magnet and power supply have been developed for and operated at TRIUMF, to deliver a proton beam to the new ultracold neutron (UCN) facility. The facility possesses unique operational requirements: a time-averaged beam current of 40~$\mu$A with the ability to switch the beam on or off for several minutes. These requirements are in conflict with the typical operation mode of the TRIUMF cyclotron which delivers nearly continuous beam to multiple users. To enable the creation of the UCN facility, a beam-sharing arrangement with another facility was made. The beam sharing is accomplished by the fast-switching (kicker) magnet which is ramped in 50~$\mu$s to a current of 193~A, held there for approximately 1~ms, then ramped down in the same short period of time. This achieves a 12~mrad deflection which is sufficient to switch the proton beam between the two facilities. The kicker magnet relies on a high-current, low-inductance coil connected to a fast-switching power supply that is based on insulated-gate bipolar transistors (IGBTs). The design and performance of the kicker magnet system and initial beam delivery results are reported.

A beamline for fundamental neutron physics at TRIUMF

This article describes the new primary proton beamline 1U at TRIUMF. The purpose of this beamline is to produce ultracold neutrons (UCN) for fundamental-physics experiments. It delivers up to 40µA of 480MeV protons from the TRIUMF cyclotron to a tungsten spallation target and uses a fast kicker to share the beam between the Center for Molecular and Materials Science and UCN. The beamline has been successfully commissioned and operated with a beam current up to 10µA, facilitating first large-scale UCN production in Canada.

TRIUMF cyclotron vacuum system refurbishing

The cyclotron at TRIUMF was commissioned to full energy in 1974. The volume of the cyclotron vacuum tank is about 100 m3 and it operates at 5×10−8 Torr pressure during beam production. The pumping is mainly based on a Phillips B-20 cryogenerator (Stirling cycle 4-cylinder engine). The cryogenerator supplies helium gas at 16 K and 70 K to cryopanels in the tank. The decreasing reliability of the B-20 and demanding maintenance requirements triggered the decision to completely overhaul or replace the cryogenerator. Replacement with the LINDE-1630 helium refrigerator was found to be the most attractive (technically and economically) option. The details of the proposal with installation of the helium refrigerator and with a continuous flow liquid nitrogen shield cooling system are presented.

Double radiative pion capture on hydrogen and deuterium and the nucleon's pion cloud

We report measurements of double radiative capture in pionic hydrogen and pionic deuterium. The measurements were performed with the RMC spectrometer at the TRIUMF cyclotron by recording photon pairs from pion stops in liquid hydrogen and deuterium targets. We obtained absolute branching ratios of $[3.02\ifmmode\pm\else\textpm\fi{}0.27(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.31(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for hydrogen and $[1.42\ifmmode\pm\else\textpm\fi{}{}_{0.12}^{0.09}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.11(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for deuterium and relative branching ratios of double radiative capture to single radiative capture of $[7.68\ifmmode\pm\else\textpm\fi{}0.69(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.79(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for hydrogen and $[5.44\ifmmode\pm\else\textpm\fi{}{}_{0.46}^{0.34}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.42(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for deuterium. For hydrogen, the measured branching ratio and photon energy-angle distributions are in fair agreement with a reaction mechanism involving the annihilation of the incident ${\ensuremath{\pi}}^{\ensuremath{-}}$ on the ${\ensuremath{\pi}}^{+}$ cloud of the target proton. For deuterium, the measured branching ratio and energy-angle distributions are qualitatively consistent with simple arguments for the expected role of the spectator neutron. A comparison between our hydrogen and deuterium data and earlier beryllium and carbon data reveals substantial changes in the relative branching ratios and the energy-angle distributions and is in agreement with the expected evolution of the reaction dynamics from an annihilation process in $S$-state capture to a bremsstrahlung process in $P$-state capture. Lastly, we comment on the relevance of the double radiative process to the investigation of the charged pion polarizability and the in-medium pion field.

ISAC Moves Beyond the Coulomb Barrier with ISAC-II Commissioning

The initial stage of the ISAC-II upgrade consisting of the installation of a heavy ion superconducting linac has been successfully commissioned. This milestone makes possible the acceleration of radioactive ions above the Coulomb barrier. In brief, the ISAC (Isotope Separator and Accelerator) facility[1], operational since 1998, includes a 500 MeV proton beam (I ≤ 100 μA) from the TRIUMF cyclotron impinging on one of two thick targets, an online source to ionize the radioactive products, a mass-separator, an accelerator complex, and experimental areas. The original accelerator chain includes a 35 MHz RFQ, to accelerate beams of A/q ≤ 30 from 2keV/u to 150keV/u and a post stripper, 106 MHz drift tube linac (DTL) to accelerate ions of A/q ≤ 6 to a final energy fully variable from 0.15 MeV/u to 1.5 MeV/u primarily for nuclear astrophysics investigations.

Recoil alignment in muon capture onN14

We report a measurement of the longitudinal alignment $A_L$ of the recoil nucleus in the $\mu^- + ^{14}$N$(1^+, 0)$ $\to$ $\nu_{\mu} + ^{14}$C$(2^+, 7012)$ transition. The experiment was performed on the M9B beamline at the TRIUMF cyclotron via the measurement of the Doppler lineshape of the subsequent $^{14}$C$(2^+, 7012)$ $\to$ $^{14}$C$(0^+, 0)$ gamma-rays. We compare our result $A_L = 0.60 \pm 0.11$ to various model calculations, and discuss the sensitivity to the induced pseudoscalar coupling, second-forbidden effects and $2 \hbar \omega$ wavefunction admixtures.

Search for exotic baryons in double radiative capture on pionic hydrogen

We report a search for low-lying exotic baryons via double radiative capture on pionic hydrogen. The data were collected at the TRIUMF cyclotron using the RMC spectrometer by detecting gamma-ray pairs from pion stops in liquid hydrogen. We found no evidence to support an earlier claim for exotic baryons of masses 1004 and 1044 MeV/c2, and set upper limits on exotic baryon mediated capture of <3×10−6 and <4×10−6, respectively. We also obtained bounds on the couplings between exotic baryons and ordinary nucleons.

High power target developments at ISAC

TRIUMF, Canada’s national research facility for particle and nuclear physics is currently operating the ISAC facility. A high-energy proton beam from the H - TRIUMF cyclotron is used to generate short-lived radioactive species in a thick target. An ion source at the target creates a radioactive beam, which is then injected into the ISAC beam lines and accelerator system. The ISAC facility is designed to accept proton beam intensity up to 100 μA at 500 MeV. At present our target design can only sustains 40 μA at maximum. Beyond this point the target has to be cooled. A new target equipped with fins has been developed that may sustain proton beam up to 100 μA. The fined target has been tested off-line and a thermal simulation using ANSYS ® has been conducted and the results are reported here.

Observation of double radiative capture on pionic hydrogen.

We report the first observation of double radiative capture on pionic hydrogen. The experiment was conducted at the TRIUMF cyclotron using the RMC spectrometer and detected gamma-ray coincidences following pi(-) stops in liquid hydrogen. We found the branching ratio for double radiative capture to be [3.05+/-0.27(stat)+/-0.31(syst)]x10(-5). The measured branching ratio and angle-energy distributions support the theoretical prediction of a dominant contribution from the pipi-->gammagamma annihilation mechanism.

Search for [formula omitted] dibaryon by double-radiative capture on pionic deuterium

We report a search for d∗ dibaryon production by double-radiative capture on pionic deuterium. The experiment was conducted at the TRIUMF cyclotron using the RMC cylindrical pair spectrometer, and detected γ-ray coincidences following pion stops in liquid deuterium. We found no evidence for narrow dibaryons, and obtained a branching ratio upper limit, BR<6.7×10−6 (90% C.L.), for narrow d∗ production in the mass range from 1920 to 1980 MeV.

Radiative muon capture on O, Al, Si, Ti, Zr, and Ag

The photon spectra from radiative muon capture (RMC) on O, Al, Si, Ti, Zr, and Ag have been measured for photon energies greater than 57 MeV using a cylindrical pair spectrometer at the TRIUMF cyclotron. Values of ${R}_{\ensuremath{\gamma}},$ the ratio of the radiative rate for photon energies above 57 MeV, to the nonradiative rate, are $1.67\ifmmode\pm\else\textpm\fi{}0.18,$ $1.40\ifmmode\pm\else\textpm\fi{}0.11,$ $2.09\ifmmode\pm\else\textpm\fi{}0.20,$ $1.30\ifmmode\pm\else\textpm\fi{}0.12,$ $1.31\ifmmode\pm\else\textpm\fi{}0.15$, and $1.12\ifmmode\pm\else\textpm\fi{}0.13$, respectively, in units of ${10}^{\ensuremath{-}5}$. The Al/Si rate difference confirms an earlier result. The Ti/Ca rate difference and the rate suppression in Zr and Ag are new results which confirm that the RMC rate is a much smoother function of neutron excess than of atomic number. This suggests that Pauli blocking is relatively more important for radiative capture than for nonradiative capture. The value of the ratio of the induced weak pseudoscalar to axial coupling constants, ${g}_{p}{/g}_{a},$ for O was found to be $4.9\ifmmode\pm\else\textpm\fi{}0.6,$ $6.3\ifmmode\pm\else\textpm\fi{}1.1$, or ${8.1}_{\ensuremath{-}2.1}^{+1.8}$, depending on the theory used to extract it. These values are in good or fair agreement with the partially conserved axial-vector current (PCAC) hypothesis. For the other nuclei studied, large model dependences or a lack of detailed RMC calculations made tests of the PCAC hypothesis difficult.

Isotope dependence of radiative muon capture on the58,60,62Ni isotopes

We report measurements of the photon rates and energy spectra from inclusive radiative muon capture on three nickel isotopes using a photon pair spectrometer at the TRIUMF cyclotron. The values of ${R}_{\ensuremath{\gamma}},$ the partial branching ratios of radiative muon capture for photon energies $&gt;57$ MeV, for ${}^{58,60,62}\mathrm{Ni}$ were found to be (in units of ${10}^{\ensuremath{-}5})$ $1.48\ifmmode\pm\else\textpm\fi{}0.08,$ $1.39\ifmmode\pm\else\textpm\fi{}0.09,$ and $1.05\ifmmode\pm\else\textpm\fi{}0.06,$ assuming a Fermi-gas photon spectral shape. The results reveal a significant isotope effect in the nickel branching ratios and a simple empirical scaling of the present nickel and earlier nuclear ${R}_{\ensuremath{\gamma}}$ data with neutron excess. Furthermore, the observed atomic mass and neutron excess dependence of the entire nuclear ${R}_{\ensuremath{\gamma}}$ data set is well reproduced by a relativistic Fermi-gas calculation. The ability of the model to reproduce the variation of the ${R}_{\ensuremath{\gamma}}$ data using the Goldberger-Treiman formula for ${g}_{p}$ suggests there is no compelling reason to invoke a more exotic $A$-dependent renormalization of ${g}_{p}.$

The TRIUMF optically pumped polarized H− ion source

The TRIUMF dc optically pumped polarized H− ion source (OPPIS) produces a 200 μA dc H− current at 85% polarization within a normalized emittance (90%) of 0.8 π mm mrad, for operation at the TRIUMF cyclotron. As a result of development of the electron cyclotron resonance primary proton source, a 1.6 mA dc polarized H− current is produced within a normalized emittance of 2 π mm mrad, suitable for high energy accelerators. The OPPIS has also been developed for use in a parity nonconservation experiment which has very severe limits on permissible helicity- correlated changes in beam current and energy.

TRIUMF injection line nonintercepting beam energy monitoring through phase measurement

A nonintercepting beam velocity monitoring system has been installed in the 300 keV injection line of the TRIUMF cyclotron to reproduce the injection energy for different ion source beams and monitor any beam energy fluctuations. The monitor is tuned to resonate at 46 MHz and is 3.25 m downstream from the first 23 MHz buncher. The phase difference between the first buncher and the pickup is related to the average beam velocity, i.e., beam energy. The energy sensitivity of the measurement is 1×10−4. The dynamic range of the monitor with beam current is 60 dB, and the measured instantaneous beam intensity can go down to 100 nA. The theory behind the device and the results of beam tests will be described.

Optimization studies of proton polarization in the TRIUMF optically pumped polarized H− ion source

The TRIUMF optically pumped polarized ion source presently produces a 15 μA dc beam of up to 80% polarized H− ions within a calculated normalized emittance of ≲ 0.16 π cm mrad. Extracted beam parameters, after acceleration by the TRIUMF cyclotron to 230 MeV, are ∼ 4 μA with a proton polarization of 75%. The source is based on optical pumping of Rb vapor by Ti:sapphire lasers, resulting in improved polarization compared to previous results using Na as the polarized medium. The instrumentation and results of a detailed study of parameter dependences are reported.

In-beam tests of proximity mesh dynode tubes for the STAR TOF subsystem

The performance of Bicron BC404, BC408 and BC420 scintillators coupled to a Hamamatsu R3432 proximity mesh dynode photomultiplier tube was studied using pion beams from the TRIUMF cyclotron and the Brookhaven National Laboratory AGS accelerator. A system resolution of better than 100 ps seems feasible. This will satisfy the requirement for charged particle identification in the STAR detector at RHIC.

Production and monitoring of short duration beam bursts from the TRIUMF cyclotron

A stable, phase restricted tune has been developed for the TRIUMF cyclotron. By use of a radially adjustable foil or skimmer on the first turn and a slit on the tenth turn of the machine, the phase acceptance may be reduced from its normal value of approximately 35° to typically 3.7°, corresponding to a beam burst duration of 450 ps FWHM. The beam intensity is reduced by a factor of ten. The time structure of the beam is measured by a time pickoff near the experimental target location. Timing information is derived from protons elastically scattered in a thin in-beam target. The timing pickoff was found to have an intrinsic resolution of 50 ps rms.

A direct current polarized H− ion source based on optically pumped rubidium

The TRIUMF optically pumped polarized H− ion source has been modified to operate with rubidium as the optically pumped medium. The intrinsic characteristics of rubidium as well as an increase in laser power, compared with the previous system based on sodium, have led to a large increase in beam polarization to over 75%. A proton current of 2.5 μA extracted from the TRIUMF cyclotron at 230 MeV was easily attained during initial operations.