Backscattering of Antiprotons

The figure shows the energy and number backscattering coefficients of hydrogen ions incident on thick layer of aluminum as a function of incident kinetic energy [1], and can be used to estimate the backscattering coefficients of antiprotons. (You can see the real size image by clicking on the image).

In the previous essay I wrote about the backscattering (also called reflection) of antiprotons from aluminum wall found by Italian physicists. The ratio of the number of backscattered particles to the number of particles incident on a layer of material is called number backscattering coefficient R_N.

R_N of antiprotons are almost the same as that of protons, because the basic formula of the phenomena related to the passage of particles through matter, i.e., the energy loss per unit pass length and the Rutherford scattering cross section, are independent of the sign of the incident particles. Therefore, R_N of antiprotons can be estimated by the use of a universal empirical equation we published for R_N of light ions, which include the hydrogen ion, i.e., the proton, incident on different absorber materials [1].

The equation for the hydrogen ion incident on the thick aluminum layer is given by the upper curve in the above figure. The energies of the antiprotons referred to by the Italian physicists are from 1 to 10 keV. For these energies, R_N of the hydrogen ion and accordingly that of the antiproton can be seen to decrease from about 13%, a considerable fraction, at 1 keV to 2.5% at 10 keV.

Reference

1. R. Ito, T. Tabata, N. Itoh, K. Morita, T. Kato and H. Tawara, "Data on the backscattering coefficients of light ions from solids (A revision)," Institute of Plasma Physics Nagoya University Report IPPJ-AM-41 (1985).

No Surprise to a Radiation Physicist

The e-mail note for "Physical Review Focus 11 August 2008" carried the news story entitled "Antimatter Bounces off Matter" (online Focus story is given in [1]). The story tells that in the August Physical Review A, a team of Italian researchers reports that a good fraction of a low energy antimatter beam directed at a normal matter wall will bounce right back.

Fans of science fiction know that the meeting of matter and antimatter results in the annihilation of both, accompanied by a release of a tremendous amount of energy. Therefore, the above result, based on a new analysis of 12-year-old data, is reported to be "surprising even to most physicists, though it is explained by basic textbook principles."

I have read the full story [1] and the abstract of the paper [2] written by the Italian physicists. The antimatter in their experiment was a beam of low energy antiprotons, and they observed a large fraction of the beam was reflected by an aluminum wall at the end of the apparatus. They made a Monte Carlo simulation of the antiproton path in aluminum, and found that the observed reflection occurred primarily via a multiple Rutherford scattering on Al nuclei.

The phenomenon is the backscattering of antiprotons from a thick layer of matter. Radiation physicists well know the same phenomenon for the beams of electrons, protons, ions and positrons (note that positrons are the antiparticles of electrons). I'm one of those physicists, and especially studied the backscattering of electrons and ions. Thus the story came as no surprise to me.

The abstract concludes with the sentence, "These results contradict the common belief according to which the interactions between matter and antimatter are dominated by the reciprocally destructive phenomenon of annihilation." I would like to say, "It is rather surprising that such a common belief has been held not only by fans of science fiction but also by high-energy physicists."

Antiprotons having some energy and passing far from nuclei behave like ordinary particles with a unit negative charge, and suffer the same amount of Rutherford scattering as that protons do; they annihilate with protons only when they come quite close to nuclei. This should be a common belief about antiproton beams instead of the one described by the Italian team.

1. Antimatter Bounces Off Matter, Physical Review Focus, 11 August 2008.
2. A. Bianconi, et al., Experimental evidence of antiproton reflection by a solid surface, Phys. Rev. A Vol. 78 (issue of August 2008).

Got a Good Message

Yesterday I got an e-mail message from the provider of my second Web site to notify the arrival of a comment at the guestbook page. Recently I got only spam comments there. So I opened the provider's page for deleting the guest comment newly arrived. However, I found a good message there. It was from the late Dr. Lewis Spencer's youngest daughter. She found my mention [1] of her father's passing by Google search, and expressed thanks to me. This was one of the pleasant things of my life these days.

As I wrote on the Web page [1], I saw Dr. Spencer only twice; once in Kyoto, and once at the former National Beureau of Standards, USA. On both of these occasions, however, he was very kind to me. So I'll never forget his hospitality. I also learned much from his excellent publications on the penetration of fast electrons through matter for doing my research work. When I saw him for the first time, I was surprised to find the person who did wonderful calculations did not have one of the arms (possibly it was the right one). I do not know how he lost his arm.

1. Lewis V. Spencer, IDEA & ISAAC Web site (2005).

Joseph Rotblat, 1908–2005

On August 31, 2005, the physicist Joseph Rotblat, one of the founders of the Pugwash peace movement in 1957, passed away. Rotblat and Pugwash shared the Nobel Peace Prize in 1995 "for their efforts to diminish the part played by nuclear arms in international politics and, in the longer run, to eliminate such arms" [1]. I received four e-mail messages about the passing of Rotblat.

The first e-mail message I received was from Tatsujiro Suzuki, the organizer of Peace Pledge Japan (PPJ) [2], to all the signers of the pledge advocated by PPJ. He wrote about these facts: Rotblat joined the Manhattan Project, but learning the impossibility of Germany's making an atomic bomb, he resigned from the Project. Rotblat gave warm encouragement to PPJ. Suzuki's message also included copies of articles published in the Yomiuri and the Mainichi both reporting the death of Rotblat.

The second was a message addressed to David Bradley from an American friend of mine John Hubble. The latter kindly forwarded it to some of his friends including me. John was a former president of the International Radiation Physics Society (IRPS), and asks Bradley to write an obituary in the next issue of the IRPS Bulletin, describing Rotblat's contribution to IRPS Meetings in 1982 and 1985.

The third was an e-mail notification of the PhysicsWeb news published by the Institute of Physics. From the piece of news on the death of Rotblat [2], I learned that in 1950 he had established a new career for himself as a medical physicist, that he had been professor of physics at Bartholomew's Hospital in London from 1950 until his retirement in 1976 and that he had also served as editor of the journal Physics in Medicine and Biology. I was also a little interested in medical physics. However, it was after the middle of the 1980th, and I did not know such a career of Rotblat.

The fourth was a reply message sent to John and all the addressees in the "Cc" line of his message, from Prof. Mohamed Ahmed Gomaa in Cairo, Egypt. This message tells us the followings: Gomaa attended the postgraduate course at London University from 1964 to 1965. Rotblat was an External examiner for his Ph D in 1967. He studied Evans's textbook, The Atomic Nucleus [4], for years together with other students under Rotblat's guidance. — It was nice to hear about Evans's book; I also studied part of it. I have a copy of a reprint edition of this book made in India in 1967. I remember this: When I got that copy, there were some dead bodies of tiny insects among its pages. This seems to represent the condition of printing offices in India those days. —

I heartily wish that Rotblat's final aim of eliminating nuclear arms from the world be realized as soon as possible.

References

1. The Nobel Peace Prize 1995, Nobelprize.org (1995).
2. Peace Pledge (Note added later: Link aborted).
3. Joseph Rotblat dies, PhysicsWeb News (September 2, 2005).
4. R. D. Evans, The Atomic Nucleus, Reprint edition (Krieger Publishing, 1982; originally published by McGrow-Hill in 1955).

References added later

• J. P. Holdren, Joseph Rotblat (1908–2005), Science Vol. 310, p. 633 (2005).
• F. Calogero and R. Hinde, Joseph Rotblat, Physics Today. Vol. 59, No. 4, p. 85 (2006).
• M. Underwood, Joseph Rotblat: A Man of Conscience in the Nuclear Age (Sussex Academic Press, 2009).
• A. Brown, Keeper of the Nuclear Conscience: The Life and Work of Joseph Rotblat (Oxford University Press, to be published April 1, 2012).

"He Wants to Dazzle Voters"

Recently the British newspaper The Financial Times published an editorial comment [1] on the election to be held in Japan on September 11. It keenly criticizes the leader of a party for wanting to dazzle voters, by describing the following facts:

He has kept voters' attention relentlessly focused on himself and his attempts to reform the party and Japan Post. Foreign policy is glaringly absent from the campaign, although Japan faces several pressing international challenges. They include rising oil prices, the unnervingly bad relationship with China, and the negotiations over US military bases with Washington.

This is a correct view. We, Japanese voters, should not be deceived by such a style of campaign.

Reference

1. "Japan's small world," Financial Times (August 29, 2005).