**3. An accelerator-based neutron small animal irradiation facility**

### **3.1. Overview of IND-related radiation protection**

Several scenarios of large-scale radiological events include the use of an improvised nuclear device (IND) that may produce a significant neutron component with the prompt radiation exposure [14]. Specifically, the prompt radiation from this type of detonation is expected to be qualitatively similar to that of the gun-type 15 kT device exploded over Hiroshima [15]. In order to assess the significance of the neutron exposure in dose reconstruction for this type of scenario and to allow characterization of novel neutron-specific biodosimetry assays, a new broad-energy neutron irradiator was designed [9] at the Columbia University RARAF.

This accelerator-driven neutron irradiator provides a broad-spectrum neutron field with energies from 0.2 to 9 MeV that mimics the evaluated energy spectrum produced in the detonation of the atomic bomb at Hiroshima at 1–1.5 km distance from ground zero [15]. At this distance, both survival and radiation exposure are expected to be sufficiently high to require triage for allocation of medical efforts; based on the Hiroshima data, the most survivors around this distance receive an appreciable neutron dose (up to 0.25 Gy [16]). However, the spectrum observed at this distance is significantly different from a standard reactor spectrum due to transport in the air, and has a larger component of low-energy neutrons. It is expected that this difference would have a significant impact on biodosimetric dose reconstruction.

The neutron field is produced by a mixed beam and composed of 5 MeV atomic and molecular ions of hydrogen and deuterium that is used to bombard a thick beryllium (Be) target. The latter is a well-known neutron-producing material not only because of its high neutron yield but also because of its stability and high specific heat. This mixed beam produces a neutron spectrum which is the sum of the spectra from the 9Be(d,n)10B and 9Be(p,n)9B reactions for all the incident ions (monatomic, diatomic and triatomic) and for energies from 5 MeV and down. In general, for monatomic 5 MeV projectiles, the 9Be(d,n)10B reaction provides a spectrum with higher-energy neutrons (above 1 MeV), while the 9Be(p,n)9B reaction primarily yields neutrons below 1 MeV. These nuclear reactions generate a combined neutron spectrum with a wide range of energies, which can then be used to irradiate biological samples and small animals (e.g., mice) for radiobiology studies. The beam composition in the present setup is approximately a 1:2 ratio of protons to deuterons. However, for other scenarios, the spectrum shape can be modified by adjusting the ratio of protons to deuterons and the incident beam energy.

different probes. The small changes in oxygen consumption could be extracted from the background oxygen concentration with the self-referencing method. A similar but relative small change was observed after nucleus irradiation. Without radiation, any of these large spikes cannot be seen on a long-time background measurement. Twelve measurements were conducted resulting in a success rate of ~30% as determined by the individual cell flux test because of the cell-to-cell variations and the uncertainty of the probe locations. The results indicate a role for mitochondrial damage following irradiation and enable further evaluations of the radiation-induced bystander effect. This effect is hypothetically due to the result of damage signals received by non-hit cells from hit cells. Establishing the mechanistic basis for such responses in the form of damage signaling from hit to non-hit cells and continued signaling has proven to be elusive. However, evidence for both oxygen- and nitrogen-based small molecules and mitochondrial dysfunction has been produced. The approach outlined in this study suggests biosensor mediators in the form of oxygen radicals, nitric oxide, and hydrogen peroxide can be directly measured at a single-cell level in both hit cells and bystander cells providing an incisive method of evaluating such evidence. This establishment of a noninvasive, self-referencing biosensor/probe system in conjunction with the RARAF microbeam provides an additional means for probing biological responsiveness at the level of individual

**Figure 5.** Self-referencing oxygen ion–selective probe result with alpha microbeam irradiation (DC).

118 Radiation Effects in Materials

cell, after precise sub-cellular targeting in hit cells and bystander cells.

**3.1. Overview of IND-related radiation protection**

**3. An accelerator-based neutron small animal irradiation facility**

Several scenarios of large-scale radiological events include the use of an improvised nuclear device (IND) that may produce a significant neutron component with the prompt radiation exposure [14]. Specifically, the prompt radiation from this type of detonation is expected to be qualitatively similar to that of the gun-type 15 kT device exploded over Hiroshima [15]. In order to assess the significance of the neutron exposure in dose reconstruction for this type of scenario and to allow characterization of novel neutron-specific biodosimetry assays, a new broad-energy neutron irradiator was designed [9] at the Columbia University RARAF.

As described elsewhere [17], the neutron spectra were evaluated by making combined measurements with a proton-recoil proportional counter [18] and liquid scintillator detector [19]. The measured recoil spectra were unfolded using maximum entropy deconvolution [20], based on Monte Carlo simulated detector response functions [21].

The dosimetry for the irradiations was performed using a custom tissue-equivalent (TE) gas ionization chamber, placed on the sample holder wheel. This chamber measures the total dose

**Figure 6.** Self-referencing oxygen ion-selective probe result with alpha microbeam irradiation (AC).

in the mixed neutron and γ-ray field. To evaluate the ratio of neutron and γ doses, gammaray dosimetry was performed separately by replacing the ionization chamber with a compen‐ sated Geiger–Mueller dosimeter, which has a very low neutron response [22]. These measurements indicated that all neutron exposures, using this spectrum, are accompanied by a parasitic photon dose of about 21% of the total dose delivered.

In order to account for possible variations in the dose rate during irradiations, a second TE gas ionization chamber is placed in a fixed location on the beam axis, directly downstream of the neutron target and used as a monitor. All measurements are normalized to the signal from the monitor, which is used to determine the dose during irradiation.

In a realistic scenario, it is expected that the neutron dose will only be a small fraction of the total exposure (e.g., DS02 reported only 2% neutron dose at 1.5 km from the Hiroshima epicenter [16]). In order to mimic such a mixed field, a 250 kVp orthovoltage X-ray machine is located on site to allow irradiating samples with the necessary additional dose of photons.

To demonstrate the use of this facility, we present results from initial test using the in vitro cytokinesis-block micronucleus assay (CBMN, [23]) to measure the induction of micronuclei in peripheral human blood lymphocytes exposed to a range of neutron doses up to 1.5 Gy. The dose-response curves generated for micronuclei frequency indicated that the RBE of this neutron spectrum is between 3 and 5 compared to micronucleus yields induced by 250 kVp X-rays. As expected, these values fall between those for accelerator-generated energetic neutrons [24] and those for a reactor-based uranium fission spectrum [25].

#### **3.2. Formation of a broad neutron spectrum**

The key feature of our broad-energy neutron irradiator is its use of a mixed-gas ion source, with the spectrum depending on the ratio of gases in the mixture fed into the ion source. For this work, hydrogen and deuterium were combined at a ratio of 1:2 in one of the ion source gas supply cylinders and placed in the accelerator terminal. The ionization process of the mixed gas is complicated, as it generates many different ion combinations. To identify the actual ion beam ratios and to optimize the beam current, two different values of the gas valve control voltage were tested. The selected gas input parameter (percentage of maximum valve voltage) is used to control the pressure to provide sufficient beam current. To determine the ratio of the different ion species in our beam, we measured beam current at a 15° deflection, as a function of the field strength of the bending magnet. **Table 1** shows the fractions of the various atomic and molecular ions at two extremal pressures we can use (outside this pressure range, the beam current is too low or the ion source operation unstable). Several ion species (e.g., D+ and H2 + ) cannot be separated magnetically because their magnetic rigidity is very close. As can be seen, increasing the valve voltage from 65.2 to 76.6% changed the ion ratios slightly for the molecular, but not atomic ions. As the major contributions to the spectrum come from the H+ and D+ ions, which vary by less than 2% over this range, we expect the spectrum will not vary significantly over this range of ion source parameters.


**Table 1.** Ion species percentage for two different percentages of maximum gas control voltage.

in the mixed neutron and γ-ray field. To evaluate the ratio of neutron and γ doses, gammaray dosimetry was performed separately by replacing the ionization chamber with a compen‐ sated Geiger–Mueller dosimeter, which has a very low neutron response [22]. These measurements indicated that all neutron exposures, using this spectrum, are accompanied by

In order to account for possible variations in the dose rate during irradiations, a second TE gas ionization chamber is placed in a fixed location on the beam axis, directly downstream of the neutron target and used as a monitor. All measurements are normalized to the signal from the

In a realistic scenario, it is expected that the neutron dose will only be a small fraction of the total exposure (e.g., DS02 reported only 2% neutron dose at 1.5 km from the Hiroshima epicenter [16]). In order to mimic such a mixed field, a 250 kVp orthovoltage X-ray machine is located on site to allow irradiating samples with the necessary additional dose of photons.

To demonstrate the use of this facility, we present results from initial test using the in vitro cytokinesis-block micronucleus assay (CBMN, [23]) to measure the induction of micronuclei in peripheral human blood lymphocytes exposed to a range of neutron doses up to 1.5 Gy. The dose-response curves generated for micronuclei frequency indicated that the RBE of this neutron spectrum is between 3 and 5 compared to micronucleus yields induced by 250 kVp X-rays. As expected, these values fall between those for accelerator-generated energetic

The key feature of our broad-energy neutron irradiator is its use of a mixed-gas ion source, with the spectrum depending on the ratio of gases in the mixture fed into the ion source. For this work, hydrogen and deuterium were combined at a ratio of 1:2 in one of the ion source gas supply cylinders and placed in the accelerator terminal. The ionization process of the mixed gas is complicated, as it generates many different ion combinations. To identify the actual ion beam ratios and to optimize the beam current, two different values of the gas valve control voltage were tested. The selected gas input parameter (percentage of maximum valve voltage) is used to control the pressure to provide sufficient beam current. To determine the ratio of the different ion species in our beam, we measured beam current at a 15° deflection, as a function of the field strength of the bending magnet. **Table 1** shows the fractions of the various atomic and molecular ions at two extremal pressures we can use (outside this pressure range, the beam current is too low or the ion source operation unstable). Several ion species (e.g., D+

) cannot be separated magnetically because their magnetic rigidity is very close. As can

be seen, increasing the valve voltage from 65.2 to 76.6% changed the ion ratios slightly for the molecular, but not atomic ions. As the major contributions to the spectrum come from the H+ and D+ ions, which vary by less than 2% over this range, we expect the spectrum will not vary

a parasitic photon dose of about 21% of the total dose delivered.

120 Radiation Effects in Materials

monitor, which is used to determine the dose during irradiation.

neutrons [24] and those for a reactor-based uranium fission spectrum [25].

**3.2. Formation of a broad neutron spectrum**

significantly over this range of ion source parameters.

and H2 + Neutrons are generated as the particle beam impinges on a thick (500 μm) beryllium target. At this thickness, the 5 MeV deuterons and protons are completely stopped in the beryllium. The neutron energy spectrum obtained by this configuration was previously [9] modeled using MCNPX and more recently validated experimentally [17]. Briefly, an EJ-301 liquid-filled scintillation detector and a gas proportional counter filled with 3 atm of hydrogen were used for measuring neutron energies above and below 1.0 MeV, respectively. The combination of the two detection systems covers a wide-energy range, from 0.2 to >9 MeV. The recoil pulse height spectra acquired by the detector systems were carefully evaluated using different quasimonoenergetic neutron beams (0.2–9 MeV) available at the RARAF accelerator, discriminating the γ-ray signals from the raw acquisition data with pulse rise time.

The two portions of the spectrum (obtained from the two detectors) were combined to form a kerma-weighted total spectrum (**Figure 7**). Overall, the obtained spectrum is similar to the one evaluated for Hiroshima [15], although it is slightly flatter. A more detailed discussion of the differences between our spectrum and Hiroshima appears elsewhere [17].

**Figure 7.** A comparison of the kerma-weighted neutron spectrum generated by us (hashed) with the one at 1.5 km from Hiroshima ground zero (gray).
