BABYSCAN - a whole body counter for small children in Fukushima

BABYSCAN, a whole body counter for small children with a detection limit for $^{137}$Cs of better than 50 Bq/body, was developed, and the first unit has been installed at a hospital in Fukushima, to help families with small children who are very much concerned about internal exposures. The design principles, implementation details and the initial operating experience are described.

2 are about 1% for adults, and practically 0% for children (age 6-15). These results are consistent with those of many other measurements and studies conducted so far in Fukushima, e.g., rice inspection, foodstuff screening and duplicate-portion studies.
Nevertheless, there continue to be many residents, families with small children in particular, who are very much concerned about internal exposures. This is in part due to the fact the WBCs currently being used in Fukushima, such as the FASTSCAN [4], are designed for radiation workers, who are adults. Children have been successfully measured previously at Chernobyl, and in Fukushima Prefecture, by having them stand on a small stool to get their bodies into the detection zone. While this is suitable to measure larger uptakes in larger children, it is not optimum for measuring small children ( 4 y), and is not suitable for infants or children who cannot stand.
Scientifically, it is sufficient to measure parents, but worried parents strongly request to have their babies measured. We therefore launched a project in the spring of 2013 [5] to develop a whole body counter for small children called a "BABYSCAN", and have installed the first unit at the Hirata Central Hospital in Fukushima Prefecture in December 2013. The design principles, implementation and the initial operating experience are reported.

II. BABYSCAN REQUIREMENTS
About 80% of some 60 WBCs currently installed in Fukushima Prefecture are Canberra's FASTSCAN. A subject stands for two minutes in a shielding box made of iron, which houses two 7.6 × 12.7 × 40.6 cm sodium iodide (NaI) gamma-ray detectors. The detection limit for radioactive cesium is about 250-300 Bq/body (both for 134 Cs and 137 Cs), which is nearly independent of height and/or weight of the adult subject (flat within ∼ ±15%).
This detection limit is however too high for reliably measuring small children, since the biological half-life of radioactive cesium in children (∼ 13 days for 1-year old, ∼ 30 days for 5-year old) is much shorter than that in adults (∼ 110 days). As a result, children's internal contamination is harder to detect.
For example, if an adult ingested 3 Bq of 137 Cs every day, the body burden would reach an equilibrium plateau of ∼ 400 Bq/body [6]. This can be detected by the FASTSCAN. If on the other hand a 1-year-old child ingested the same amount, the resultant body burden would be ∼ 60Bq/body. Therefore, the WBC for babies must have a much lower detection limit.
Our goal was to achieve a detection limit of < 50 Bq/body for 134,137 Cs. In order to realize this high sensitivity, the BABYSCAN must be ergonomically designed so that a small child can stay still for several minutes, without feeling afraid of confinement.
From the beginning, it was recognized that the BABYSCAN's design must be reassuring to parents, and that in addition to being a measurement device, it would be expected to play an important role as a communication tool to facilitate interactions between medical staff and residents.

III. BABYSCAN DETAILS
The BABYSCAN's design principles and technologies were derived from those of FASTSCAN, but in order to realize higher sensitivity, there are some crucial differences.
As shown in Fig. 1, the subject lies down inside the measurement chamber of BABYSCAN, as opposed to standing as in the case of FASTSCAN. A child can either lie on the bed supine (on their back and face up), or prone (on their stomach, face down). During development, we discovered that older children's posture tends to be more stable in the prone position, as shown in the right-hand panel of Fig. 1. Small babies, however, tend to prefer the supine position, and are more comfortable when they can also see their mother's face through the opening. Both positions are OK, as they have essentially the same efficiency.
There are four NaI detectors (7.6 × 12.7 × 40.6 cm each), arranged in a two-by-two geometry, installed in an iron-shielded compartment placed above the subject. The bottom of the NaI compartment has a window facing the subject, made of a carbon-honeycomb plate measuring 280 mm × 860 mm. The body of a small child is therefore nearly entirely within the maximum effective range of the detectors, thereby achieving a high gamma-ray detection efficiency. The detection efficiency can be further optimized by using a height-adjustable bed. The distance from the bed surface to the bottom of the NaI detector is either 20 cm, 25 cm or 30 cm. The left panel of Fig. 1 shows a 20 cm bed with a harness used for measuring small babies, while the right panel shows a child lying on a 25 cm bed. The bed is pulled out when a child enters/leaves the measurement chamber, and it is pushed in during the measurement.
The size of the measurement chamber is 300 mm (H) ×800 mm (W) ×1400 mm (L), and the This iron structure is covered by an ergonomically designed plastic cover. The exterior surface of BABYSCAN is covered by smooth curved panels (made of glass-fiber reinforced plastic (GFRP)) colored with natural white for its gentle appearance. In order to provide a cozy space for children, the interior surface is also covered by organic surface (made of carbon-fiber reinforced plastic (CFRP), so as to avoid the radium, thorium, and 40 K background from the glass in GFRP), colored by light blue which looks like being made of soft materials. These panels are precisely assembled to eliminate the possibility of injuring baby's skin by their gaps or edges.
All the materials used to manufacture BABYSCAN, including the bed and the tablet computer, were tested for natural radioactivity using a germanium detector prior to assembly.

IV. BABYSCAN CALIBRATION
The BABYSCAN was calibrated using a Monte Carlo N-Particle Transport Code (MCNP) [7] for a wide variety of weight and height combinations for each of the 3 bed-height positions. These  In the "universal" phantom, polyethylene blocks and 137 Cs-containing rods are combined to make six different age and anthropometric types. We used 2-and 6-year-old phantoms to further check the BABYSCAN calibration, and also to compare the BABYSCAN's characteristics with those of FASTSCAN. Table II shows the results of these validation measurements at the 137 Cs energy range. The Phantom was counted and analyzed in the same manner that a child with that height would be analyzed. The results are consistent with the accuracy of the FASTSCAN. The Cs-region background count of BABYSCAN is about 3.5 times higher than that of FASTSCAN, which is 13% smaller than the factor 4 expected from the differences in both the number of detectors and the measurement time. This 13% background reduction (despite a rather large opening at the top) is the result of the reinforced shielding.

V. INITIAL OPERATING EXPERIENCE OF THE BABYSCAN
The first BABYSCAN unit, installed at the Hirata Central Hospital in Fukushima Prefecture, started operation on December 2, 2013. We here demonstrate its performance based on the data of first 100 subjects, whose age distribution (minimum 3.8 months old, maximum 10 year old, mean 4.2 year old) is shown in Fig. 3, and their anthropometric parameters are plotted in Fig. 4    Tokyo.
We are happy to report that radiocesium was not detected in any of the 100 subjects. Nevertheless, as expected, 40 K was detected in all subjects. To demonstrate this, we show typical gamma-ray energy spectra in Fig. 5; the spectra shown in black dots were taken with subjects (4 minutes),  The minimum detectable activity (MDA) for 137 Cs (Bq/body), calculated for each subject, is plotted in Fig. 7 against weight (kg). Here again, data taken with 20-cm/25-cm/30-cm beds are shown in open circles/filled circles/open squares. As the bed-to-detector distance decreases, the solid angle increases and hence the MDA decreases. This plot clearly shows that our initial goal of achieving a detection limit lower than 50 Bq/body has been met.

VI. CONCLUSION
BABYSCAN, a whole body counter for small children was developed, and the first unit has been installed at a hospital in Fukushima. The radiocesium detection limit of BABYSCAN is better than 50 Bq/body, which has been realized by a careful ergonomic design, optimized detector geometry and reinforced shielding. Even with this low detection limit, radiocesium was not detected in any of the first 100 Fukushima children, while, as expected, 40 K was detected in all subjects. The results of larger-scale measurements with the BABYSCAN will be reported in our forthcoming publications.