Durability test of a flowing-water target for isotope harvesting

Highlights

• A non-destructive target durability test was performed on an isotope harvesting target with proton irradiation.

• Radionuclides produced in the target shell acted as radiotracers to measure target degradation.

• Estimates were made for isotope harvesting target lifetimes based on the measured target corrosion rate.

• Radiolysis products were measured at much lower levels than are predicted with literature primary production yields.

• The level of H₂O₂ measured was found to correlate with beam intensity.

Abstract

A high intensity proton irradiation was performed with the flowing-water isotope harvesting target at the University of Wisconsin-Madison Cyclotron Laboratory to measure the rate of degradation of the target shell during irradiation conditions. The beam reached an intensity of 34 µA by the end of the irradiation and covered an area of 0.7 cm² on the target. Radiolysis products, such as H₂O₂, H₂, and O₂, were measured in the bulk water of the system and found to be present at much lower levels than predicted by literature escape yields. Radionuclides formed in the target shell were measured in the system water as a radiotracer for target degradation. Using a simple, beam intensity dependent model, a corrosion rate of 1.5E-6 μm/(μA*s) was found to match the measured radiotracer activities at various points in the irradiation. This rate was used to extrapolate the lifetime of future isotope harvesting targets at the NSCL and FRIB, using the areal power density of different ion beams to scale the corrosion rate.

Introduction

At the National Superconducting Cyclotron Laboratory (NSCL), a heavy, stable particle beam is accelerated and impinged on a thin target to produce exotic, radioactive particle beams for nuclear physics experiments. Upwards of 90% of the stable beam does not react at the target and is separated from the radioactive secondary beams and sent into a solid block of tungsten, known as a beam blocker. The blocked beam reacts in the beam blocker, producing radionuclides trapped in this metal matrix.

Radiochemistry efforts at the NSCL and collaborating institutions [1], [2], [3], [4] allowed for the development of a flowing-water target to replace this solid metal beam blocker [5]. When irradiated with the leftover accelerated beam, radionuclides can be produced and extracted from this new flowing-water target through a process called isotope harvesting. This process is facilitated by a system involving tubing, sensors, a pump, a water reservoir, ion exchange resins, and gas traps, among other components. Together, this flowing-water target and isotope harvesting system allow for the production and collection of radionuclides. An overview of this process at the NSCL is depicted in Fig. 1. The ability to utilize leftover beam for radionuclide production will be even more fruitful when the Facility for Rare Isotope Beams (FRIB) comes online with much higher beam intensities than the current NSCL [6].

While this system may enable the production of rare radionuclides, the durability of the Ti64 alloy (6% aluminum, 4% vanadium, mass balanced by titanium) target shell must be assessed before the harvesting system is used for routine production. During irradiation, three main conditions might lead to degradation of the target shell: 1) displacements of the atoms in the target shell caused by the energetic beam passing through the shell material, 2) the corrosive radiolysis environment in the target water produced as the accelerated beam deposits energy in the water, and 3) erosion of the inside of the target shell from the rapid flow of water over the interior target face.

The first two of these conditions may become more problematic as the beam intensity of an irradiation increases. Previous isotope harvesting experiments, described in Table 1, have been performed at the NSCL using a target shell in part or entirely made of Ti64 using ⁴⁰Ca²⁰⁺, ⁴⁸Ca²⁰⁺, ⁷⁸Kr³⁵⁺, and ⁷⁸Kr³⁶⁺ beams with power depositions from 0.42 to 22.2 W [7]. An increasingly intense fast, heavy ion beam was used in these experiments with no observed degradation, but even still, the target will be expected to receive much higher beam intensities when installed at the beam blocker position at the NSCL and in the future, at FRIB.

There are published techniques for investigating irradiation accelerated corrosion at a metal-water interface. For example, Wang and Was [8], [9] studied the interface between a zirconium alloy and super-heated water by irradiation with protons. After irradiation, surface alterations and corrosion rates were determined electrochemically. For the case of the NSCL beam-blocker, and eventually the beam dump at FRIB, the design of the target results in a fully enclosed interior surface, making the use of similar post-irradiation analysis impossible without destruction of the target.

To approach the high-power density scenario expected at NSCL and FRIB, and to assess material degradation non-invasively, the harvesting beam blocker was temporarily installed at the University of Wisconsin-Madison Cyclotron Laboratory. There, a GE PETtrace cyclotron was used to implant a 16 MeV proton beam at currents up to 34 µA into the Ti64 flowing water target and isotope harvesting system. The purpose of the irradiation was to use the radionuclides produced in the target shell to measure the degradation rate of the target material in a comparable power density setting to that expected at the heavy ion facilities. The UW proton beam deposited 540 W of total beam power into the target face and the interior water at the maximum beam intensity used. This power deposition is comparable to that for beams potentially implanted in the beam blocker at the NSCL (e.g. 540 W of power deposited in the beam blocker by an NSCL-standard 80 pnA, 140 MeV/nucleon ⁴⁸Ca beam and 517 W of power deposition by an NSCL-standard 45 pnA, 140 MeV/nucleon ⁸²Se beam). The irradiation also allowed measurement of beam-induced radiolysis product concentrations with a high intensity beam. The results of this experiment as well as the implications for the future of isotope harvesting at FRIB are discussed here.