5 yen coin
|Value||5 Japanese yen|
|Center hole diameter||5 mm|
|Composition||c. 65% Cu
c. 35% Zn
|Years of minting||1959–present|
|Catalog number||KM 72, 72a, 96.1 and 96.2|
|Design||Rice ,water and gear|
The 5 yen coin (五円硬貨 Go-en kōka ) is one denomination of Japanese yen. The current design was first minted in 1959 using Japanese characters known as the "new script", and were also minted from 1949-1958 using "old-script" Japanese characters. Five-yen coins date to 1870 (when, due to the much higher value of the yen, they were minted in gold).
The front of the coin depicts a rice plant growing out of the water, with "five yen" written in kanji; the back is stamped with "Japan" and the year of issue, also in kanji, separated by sprouts of a tree. The three graphic elements of the coin represent agriculture and fisheries, the key elements of the Japanese first-sector economy. Around a hole, there is a gear that represents industry. It is the only Japanese coin in circulation to lack Arabic numerals on either face.
Cultural significance 
The Japanese for "five yen," go en (五円) is a homophone with go-en (御縁), "en" being a word for causal connection or relationship, and "go" being a respectful prefix. As a result, five-yen coins are commonly given as donations at Shinto shrines with the intention of establishing a good connection with the deity of the shrine, and is widely believed it is best to insert a single five-yen coin into a new wallet before inserting any other money.
Use in nuclear accident investigation 
Following the nuclear accident at Tokai, Ibaraki in 1999, physicists Masuchika Kohno and Yoshinobu Koizumi showed how this coin could be used to estimate neutron dosage to the surrounding population, by measuring its zinc isotope ratios. They write:
- The Japanese 5-yen coin is about 22 millimeters in diameter and 1.5 mm thick, weighs 3.75 grams and has a central hole 5 mm wide. We chose this coin for monitoring neutron exposure because it is widely circulated, the zinc content is precisely controlled, and the generated has a convenient half-life (244.1 days) and gamma ray energy emission (1,115.5 keV). To obtain a record of the dosage of neutrons released as a result of the accident, we collected exposed coins from people's houses at distances 100–550 m from the facility.
They concluded that the coin could offer information about the total neutron effect during the accident, and about shielding by modern Japanese houses, given that the coins were recovered from indoors.
- Kohno, Masuchika; and Yoshinobu Koizumi (2000). "Tokaimura accident: Neutron dose estimates from 5-yen coins". Nature 406 (6797): 693. doi:10.1038/35021138. PMID 10963586.
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